PERCEPTUAL COMPLETION:

A CASE STUDY IN PHENOMENOLOGY AND COGNITIVE SCIENCE

Evan Thompson
Department of Philosophy
York University
4700 Keele Street
North York, Ontario
Canada M3J 1P3
Electronic mail: evant@yorku.ca

Alva Noë
Department of Philosophy
University of California, Santa Cruz

Santa Cruz, CA
U.S.A. 95064

Electronic mail: anoe@cats.ucsc.edu

Luiz Pessoa
Department of Computer and Systems Engineering
Federal University of Rio de Janeiro
Centro de Tecnologia, Bloco H, Sala H-319
Ilha do Fundao, Rio de Janeiro, RJ
21945-970 Brazil
Electronic mail: pessoa@cos.ufrj.br

To appear in: J. Petitot, J-M Roy, B. Pachoud, & F.J. Varela (eds.), Naturalizing Phenomenology: Issues in Contemporary Phenomenology and Cognitive Science (Stanford University Press).

Cognitive science, in its aim to be the science of the mind, cannot avoid taking account of how human beings experience themselves and the world. Yet precisely how should cognitive science go about taking account of human experience? One proposal, growing out of the work of Daniel C. Dennett (1991), advises cognitive scientists to construct a preliminary description of human experience from the third-person perspective, and then to interpret and evaluate this description in relation to an account of the brain. Within this proposal there is no place for the autonomous investigation of human experience, that is, for the conceptual and reflective examination of human experience as it is lived, independent of scientific accounts of the brain. We think that this lack of concern for the autonomy of human experience is unacceptable: cognitive science must include reflective examinations of human experience in addition to causal-explanatory theories (Varela et al. 1991). In this essay, we shall argue that proposals such as Dennett's rest on a mistaken conception of the relation between the person or experiencing subject and the person's brain--the brain is treated as if it were the proper subject of experience, when the proper subject is rather the person. As a result of this misconception, Dennett and others are led not only to misdescribe the character of perceptual experience, but also to misjudge the conceptual and empirical significance of issues in visual science about perceptual completion. Furthermore--and this is striking in the context of Dennett's dismissal of phenomenology--these mistakes were long ago identified as such within the phenomenological tradition. By drawing on the resources of this tradition, we hope to show the importance of phenomenological reflection and conceptual clarification for empirical research in cognitive science.

1. PERCEPTUAL COMPLETION: THE BASIC ISSUES

Figure 1 illustrates the so-called neon color illusion: a red diamond is seen where there is only a lattice of red line-segments. The term "neon color spreading" (van Tuijl 1975) is often used to describe this phenomenon: the figure seems to result from the color having spread between the line-segments into the background. The color is said to "fill in" the background, thereby forming the figure. Nevertheless, one does not see any spreading or filling-in process; one sees only the figure. What line of reasoning, then, lies behind the description of such figures in the terminology of "filling-in"?

Visual scientists use the terms "filling-in" and "perceptual completion" to refer to situations where subjects report that something is present in a particular region of visual space when it is actually absent from that region, but present in the surrounding area. The idea is easiest to understand in the case of the blind spot. We have a blind spot in each eye corresponding to the region where the optic nerve leaves the retina and there are no photoreceptors (Figure 2). In everyday perception we are never aware of the blind spot. The blind spots of the two eyes do not overlap, and something that falls on the blind spot of one retina will fall outside the blind spot of the other retina. Even under monocular viewing the blind spot is not easily revealed. Close one eye and fixate a point on a uniformly colored piece of paper: there is no experience of any gap or discontinuity in the visual field. Now follow the instructions for the blind spot demonstration in Figure 3. The left dot disappears and one perceives a uniform expanse of brightness and color. This is an example of perceptual completion or visual filling-in: the color and brightness surrounding the area corresponding to the blind spot are said to "fill in" that area so that a uniform expanse is perceived.

The existence of such perceptual completion phenomena at the subject-level is uncontroversial. The term "filling-in," however, is often used in a controversial sense that goes beyond what subjects report. In this controversial sense, the term "filling-in" suggests that certain kinds of subject-level, perceptual completion phenomena are accomplished by the brain's providing something to make up for an absence--by the brain's actively filling-in the missing information. Whether there is neural filling-in, however, is a matter of great debate in visual science. We will argue later that there is considerable evidence for neural filling-in, but that great care needs to be taken in thinking about the relation between neural filling-in and subject-level perceptual completion.

To appreciate the debates about filling-in it is necessary first to review certain facts about vision and second to discuss certain conceptual and methodological points.

Consider that many of the objects we perceive have roughly uniform regions of surface color and lightness. Now consider two facts. First, neurons in the visual system do not respond strongly to uniform regions, but rather to luminance discontinuities (Hubel & Wiesel 1962, 1968). In other words, most neurons respond strongly only to boundaries (or edges), and do not produce vigorous responses to regions or surfaces (large expanses of stimulation) (Figure 4). Second, psychophysical experiments--for example, with stabilized images--have shown the importance of boundaries for proper surface perception (Krauskopf 1963; Yarbus 1967). In the classic study by Krauskopf (1963), an inner green ring was surrounded by a red annulus (Figure 5). When the red-green boundary was stabilized on the retina (so that it always maintained a fixed position on the eye), subjects reported that the central disk disappeared and the whole target, disk plus annulus, appeared red. (This is another case of visual filling-in: Krauskopf's observers perceived the central area as having the red of the surround even though "green" light was striking the corresponding region of the retina.) These and other results (e.g., Land 1977) suggest that even under natural viewing conditions the perceived color of a surface depends not only on the light reflected from the surface but on the change in light across the boundary of the surface.

If boundaries are so important, how is the brain able to determine the color and lightness of continuous regions? In other words, if continuous regions carry weak (or zero) physical signals, how do we see object surfaces (and not only boundaries)? Is there an active filling-in process at the neural level that provides for the immediate lack of physical signals at certain image regions (away from surface borders)? Some visual scientists have developed theories and models based on the idea of a neural filling-in process that involves activity-spreading, diffusion, or other forms of neural completion. Others have argued against this idea, suggesting, for example, that contrast measures at borders can be used to assign surface feature. We argue later that in the case of brightness perception there is good evidence for neural filling-in that involves spatially propagating activity.

The filling-in controversy is not only empirical, however, for it involves fundamental conceptual and methodological issues about the proper form of explanation in visual science, as well as deeper philosophical issues about the nature of visual perception, issues especially familiar to philosophers and psychologists in the phenomenological tradition.

To appreciate the methodological issues it is necessary to introduce the concepts of the bridge locus and neural-perceptual isomorphism. The best way to do this is with an example. Figure 6 presents another case often discussed in connection with filling-in, the Craik-O'Brien-Cornsweet effect (Craik 1940; O'Brien 1958; Cornsweet 1970). Two largely uniform regions of different brightness are seen while most of the corresponding stimulus regions have exactly the same luminance. In fact, the two regions differ only in the luminance distribution at the "cusp" edge separating the two regions. Why, then, do we see a brightness step?

Here is one route leading to an answer to this question that appeals to neural filling-in (Todorovic´ 1987). Suppose one assumes that activity of a particular type in a specific set of neurons is necessary and sufficient for the occurrence of the Craik-O'Brien-Cornsweet effect. These neurons would form the "immediate substrate" for the perceptual effect. Visual scientists use the term bridge locus to refer to this idea of a particular set of neurons "whose activities form the immediate substrate of visual perception" (Teller & Pugh 1983, p. 581). Now suppose one also assumes that there must be a one-to-one correspondence between the perceived spatial distribution of brightness in the effect and the neural activity at the bridge locus. In other words, just as the perceptual content consists of two uniform regions with a brightness step, so too the immediate neural substrate must consist of spatially continuous activity and a step difference. In short, suppose one assumes that there has to be an isomorphism between the perceived brightness distribution and the neural activity at the bridge locus (Todorovic´ 1987). One would then arrive at the following sort of explanation of the effect: the brain takes the local edge information and uses it to fill-in the two adjacent regions so that the region with the luminance peak (left) becomes brighter than the region with the luminance trough (right). The end result is the perception of a brightness step in the absence of any corresponding luminance step.

Two basic ideas are involved here. The first is that the way things seem to the subject must be represented neurally in the subject's brain. The second is the idea that, in analyzing visual perception, one must arrive at a "final stage" in the brain--a bridge locus--where there is an isomorphism between neural activity and how things seem to the subject. [The isomorphism can arise at some earlier stage of visual processing, as long as it is preserved up to the bridge locus (see Teller & Pugh 1983, p. 586; Teller 1984, p. 1242; Todorovic´ 1987, p. 550).] We will refer to this idea as analytic isomorphism. When applied to perceptual completion phenomena, such as the Craik-O'Brien-Cornsweet effect, analytic isomorphism entails that there must be neural filling-in to make up the difference between how things are and how they seem to the subject.

Analytic isomorphism is essentially a conceptual or methodological doctrine about the proper form of explanation in cognitive neuroscience. The doctrine is that it is a condition on the adequacy of an explanation that there be a bridge locus where an isomorphism obtains between neural activity and the subject's experience. Furthermore, the isomorphism is typically taken to hold for spatial or topographic properties, thus suggesting that vision involves representations having the form of an "internal screen" or "scale model" that preserves the metric properties of the external world (O'Regan 1992).

In this essay, we argue that analytic isomorphism should be rejected. Nevertheless, we believe that the empirical case for neural filling-in remains strong.

Enter the philosophers. Dennett (1991, 1992) has tried to brand "filling-in" the "F-word" in cognitive science. He thinks that the sort of reasoning epitomized by analytic isomorphism, and hence the idea that there must be neural filling-in, depend on a fundamentally mistaken conception of consciousness. Dennett calls this conception "Cartesian materialism." In the stereotypical version of Cartesian materialism, there is a place in the brain--a "Cartesian theater"--where contents become conscious as a result of being presented to an inner "audience" or homunculus--a viewer of the panoramic "internal screen" (O'Regan 1992). Everybody agrees that this idea is totally wrong. Nevertheless, most scientists and philosophers do not correctly understand exactly why it is wrong. As Dennett observes, the real mistake is a conceptual one: the mistake is to assume that consciousness is a property of individual contents in the way that truth can be considered a property of individual sentences. Given this concept of consciousness, it would seem that there must be a determinate spatio-temporal point in the brain where a content "enters consciousness." Dennett thinks that this concept of consciousness is incoherent and offers in its place the idea that "consciousness is a species of mental fame" (Dennett forthcoming). Just as it is impossible to be famous for a second, or to become famous in a second, or to be famous when there are no other people around, so it is impossible for a single, momentary, isolated content to become conscious: "Those contents are conscious that persevere, that monopolize resources long enough to achieve certain typical and 'symptomatic' effects--on memory, on the control of behavior and so forth" (Dennett forthcoming). In short, for Dennett, consciousness is constituted through the joint interaction of spatially and temporally distributed information-processing systems.

Dennett argues that to claim that there is neural filling-in "is a dead giveaway of vestigial Cartesian materialism" (1991, p. 344). Like a number of visual scientists, he believes that there is no reason to suppose that the brain fills in the regions; the brain simply represents the fact that regions are filled-in without itself doing any filling in. For Dennett, perceptual completion is a case of the brain's "finding out" or "judging" that certain features are present, without the brain's having to "present" or fill in those features.

Dennett has done a service by showing how the filling-in idea often depends on Cartesian materialism and analytic isomorphism (see also O'Regan 1992). But we disagree with Dennett's positive view that perceptual completion is always just a matter of the brain's "finding out." In the course of this essay, we will argue for three main points: (1) The idea of neural filling-in has to be separated from Cartesian materialism and analytic isomorphism. Dennett is right to reject the latter but wrong to reject the former. (2) There is evidence in visual science to support the idea of neural filling-in. Contrary to Dennett, filling-in is not always just finding out. (3) Dennett misunderstands the relation between perceptual content and brain processes. The misunderstanding leads him to neglect precisely what phenomenologists routinely practice--the careful conceptual and phenomenological study of lived experience--and to offer instead statements about perceptual experience motivated entirely by subpersonal considerations about the brain. As a result, Dennett seriously distorts the conceptual and phenomenological character of visual perception. In contrast, we argue for the need to pursue careful conceptual and phenomenological studies of perceptual experience as a complement to scientific research.

2. FILLING-IN VERSUS FINDING OUT: DENNETT ON FILLING-IN

In this section, we present the details of Dennett's position on filling-in. Dennett makes two kinds of points about filling-in, one conceptual and the other empirical.

The conceptual points depend on distinguishing clearly between the content of a representation and the vehicle or medium of representation. Suppose one sees a colored region. This is one's perceptual content. Dennett assumes that there must be states or processes in the brain that bear this very content. (As we will see later, this assumption is problematic.) But he observes that this could be accomplished by the brain in a number of different ways. First, there could be a representation of that region as colored, or a representation of that region could be absent, but the brain ignores that absence. The point here is to distinguish between the presence of a representation and ignoring the absence of a representation (Dennett 1992, p. 48). Second, suppose there is a representation of that region as colored. This too could be accomplished by the brain in different ways: for example, the representation could be spatially continuous or pictorial, or it could be symbolic.

These conceptual points show up the main mistake made by analytic isomorphism. Analytic isomorphism holds that there must be an isomorphic neural representation for each conscious perceptual content. As Dennett correctly observes, however, there need be no isomorphism between perceptual contents and neural representations, for some perceptual contents might correspond to neural processes that ignore the absence of neural representations, or they might correspond to symbolic representations.

Take the blind spot, for example. From the fact that one has no awareness of a gap in one's visual field, it does not follow that there must be a neural representation of a gapless visual field, for the brain might simply be ignoring the absence of receptor signals at the blind spot. Nor does it follow that the blind spot must be completed with spatially continuous representations, for the region might simply be designated by a symbol.

Dennett's conceptual points still leave open the empirical matter of just what the brain does to accomplish perceptual completion. Here Dennett is not entirely clear about what he means when he says that the brain "jumps to a conclusion."

In the case of the blind spot, Dennett asserts that the visual cortex has no precedent of getting information from that retinal region, and so it simply ignores the absence of signals from that area. The moral of this story is that the brain does not need to provide any representation for perceptual completion to occur; completion can be accomplished by ignoring the absence of a representation (cf. Creutzfeld 1990, p. 460). The contrast, then, is between providing a representation and jumping to a conclusion, in the sense of ignoring the absence of a representation. According to this story, the brain does not need to fill in the blind spot in the sense of providing a roughly continuous spatial representation, nor does it need to label the blind-spot region--the absence of any representation for the blind spot region is simply ignored or not noticed in subsequent visual processing. Notice that this is a case of providing content; the point is that there is no representational vehicle specifically devoted to the blind spot.

On the other hand, Dennett sometimes contrasts providing a roughly continuous spatial representation with labeling a region. And he says that "filling in" means the former. Here the contrast is between, on the one hand, providing a spatial representation of each sub-area within a region--filling-in--and, on the other hand, jumping to a conclusion, in the sense of attaching a label to the region all at once. In this story the brain provides both content for the blind-spot region and a representational vehicle devoted to that region, namely, a label.

Dennett's slogan is: "The brain's job is not 'filling in.' The brain's job is finding out" (1992, p. 47). The principle of brain function being assumed here Dennett calls "the thrifty producer principle": "If no one is going to look at it, don't waste effort providing it." For example, to see a region as colored, all the brain needs to do is to arrive at the judgement that the region is colored. Whether Dennett thinks that the brain accomplishes this by ignoring the absence of a representation or by providing a label ("color by number"), he clearly thinks that filling in the color of each sub-area ("color by bit map") is not the thriftiest way to do it.

3. DENNETT'S CRITICISM AND VISUAL SCIENCE

In visual science there has been a great deal of debate about filling-in in relation to the broader topic of neural-perceptual isomorphism. Our main aim in this section is to establish a connection between these long-standing debates in visual science and Dennett's more recent treatment of filling-in.

3.1 Linking propositions and isomorphism

In 1865 Ernst Mach stated what has since become known as "Mach's principle of equivalence":

Every psychical event corresponds to a physical event and vice versa. Equal psychical processes correspond to equal physical processes, unequal to unequal ones. When a psychical process is analyzed in a purely psychological way, into a number of qualities a, b, c, then there corresponds to them just as great a number of physical processes a, b, g. To all the details of psychological events correspond details of the physical events (Mach 1865/1965, pp. 269-70).

Thirteen years later in 1878 Ewald Hering (1878/1964) asserted that the neural-perceptual parallelism was a necessary condition of all psychophysical research. G. E. Müller (1896) then gave a more explicit description of the neural-perceptual mapping. He proposed five "psychophysical axioms" that postulated a one-to-one correspondence between neural and perceptual states (see Teller 1984; Scheerer 1994). In particular, his second axiom stated that perceptual equalities, similarities, and differences correspond to neural equalities, similiarites, and differences. This axiom was not offered as a solution to the so-called mind-body problem, but rather as a methodological principle that could be a guide in inferring neural processes from perceptual experiences (Sheerer 1994, p. 185).

Wolfgang Köhler accepted this idea, but thought that Müller's axioms were not comprehensive enough because they did not include occurrent perceptual states, but covered only the logical order between neural and perceptual states (Sheerer 1994, p. 185). In 1920 he proposed what he would later call the principle of isomorphism (Köhler 1920), building on Müller's earlier formulation, as well as Max Wertheimer's (1912). In his 1947 book Gestalt Psychology, Köhler wrote: "The principle of isomorphism demands that in a given case the organization of experience and the underlying physiological facts have the same structure" (Köhler 1947, p. 301).

There are several points about Köhler's principle of isomorphism that deserve mention. First, by the phrase "have the same structure" Köhler had in mind structural properties that are topological. Although the concept of neural-perceptual isomorphism has often been taken to mean a geometrical one-to-one mapping, Köhler clearly intended the isomorphism concept to have a topological sense. For example, he argued that spatial relationships in the visual field cannot correspond to geometrical relationships in the brain; they must correspond rather to functional relationships among brain processes (Köhler 1929, pp. 136-141; 1930, pp. 240-249).

Second, Köhler did not hypothesize that neural-perceptual isomorphism obtained for all properties of perceptual experience. In particular, he did not extend the principle of isomorphism to sensory qualities, such as brightness and color (Köhler 1969, pp. 64-66, as quoted in Kubovy & Pomerantz 1981, p. 428). The principle was restricted to "structural properties" of the perceptual field, that is, to characteristics of perceptual organization, such as grouping and part-whole relationships.

Finally, it is not clear whether Köhler espoused what we are calling analytic isomorphism. Two considerations suggest that he did not. First, Köhler upheld a non-localizationist view of brain function, in which field physics was the main analogy for the underlying physiology; hence the notion of a privileged site of perceptual experience in the brain seems foreign to his way of thinking about the neural-perceptual relation. Second--and this is more telling--he seems to have held (at least according to one interpreter) that the isomorphism principle "is not an a priori postulate, but 'remains an hypothesis which has to undergo one empirical test after the other'" (Sheerer 1994, p. 188).

In recent years Davida Teller has reintroduced some of these issues into visual science (Teller 1980, 1984, 1990; Teller & Pugh 1983). According to Teller, acceptable explanations within visual science have the following form:

If the question is, what is it about the neural substrate of vision that makes us see as we do, the only acceptable kind of answer is, we see X because elements of the substrate Y have the property Z or are in the state S (Teller 1990, p. 12).

This formulation leaves open another question about form: what is the relation between the form of a given neural response and the form of the corresponding visual appearance? Answers to this question invoke linking propositions--propositions that relate neural states to perceptual states. By analyzing how visual scientists reason, Teller (1984) formulated five families of linking propositions called Identity, Similarity, Mutual Exclusivity, Simplicity, and Analogy.

The Analogy family is the one that concerns us here. It is a "less organized" family of propositions whose form is as follows

F "Looks Like" Y -> F Explains Y

where "F" stands for physiological terms, and "Y" stands for perceptual terms. The arrow-connective "->" has a conditional sense, thus the formulation reads: If the physiological processes (events, states,) "look like" the perceptual processes (events, states), then the physiological processes explain the perceptual processes.

The arrow is not the connective of logical entailment. It is heuristic, and is meant to guide one to the major casual factors involved in a given perceptual phenomenon. Thus the term "explains" on the right-hand side is really too strong--the idea is that "F" is the major causal factor in the production of "Y": "... if psychophysical and physiological data can be manipulated in such a way that they can be plotted on meaningfully similar axes, such that the two graphs have similar shapes, then that physiological phenomenon is a major causal factor in producing the psychophysical phenomenon" (Teller 1984, p. 1240).

The Analogy family of linking propositions is similar to Köhler's principle of isomorphism but more general. Isomorphism in Köhler's sense can be seen as a particular instance of the Analogy idea, one in which "looks like" is taken in the sense of structural correspondence. In visual science today, this idea of a one-to-one structural correspondence is often taken to mean a spatial correspondence, so that, for example, spatial variations of brightness in the visual field are explained by analogous spatial variations of neural activity (Todorovic´ 1987, p. 548). As we discussed in Section 1, it is this assumption of spatial isomorphism that typically lies behind the appeal to neural filling-in in visual science.

3.2 Criticisms of isomorphism and filling-in

When Dennett says that the brain doesn't really fill in, it only "finds out," he is implicitly rejecting the principle of isomorphism applied to perceptual completion. In other words, he is rejecting the hypothesis that perceptual completion depends on neural completion processes that are structurally isomorphic to the perceptual phenomena. In visual science there has been a great deal of debate about such hypotheses, and the debates all pre-date Dennett's treatment (Ratliff & Sirovich 1978; Grossberg 1983; Bridgeman 1983; Todorovic´ 1987; Kingdom & Moulden 1989; see also O'Regan 1992). In fact, in 1978 Ratliff and Sirovich argued against the need for a neural filling-in process in a way similar to Dennett. They argued that to assume that there must be neural filling-in to account for the homogeneous appearance of bounded regions is to misinterpret Mach's principle of equivalence as requiring that there be an isomorphic mapping from the form of the neural process to the form of the perceptual response. But such an isomorphism is not logically necessary. Therefore, neither is a neural filling-in process (see also Bridgeman 1983; and Kindgom & Moulden 1989).

Ratliff and Sirovich went on to make some remarks that are interesting in relation to Dennett's discussion of Cartesian materialism:

The neural activity which underlies appearance must reach a final stage eventually. It may well be that marked neural activity adjacent to edges [rather than neural filling-in betwen the edges]... is, at some level of the visual system, that final stage and is itself the sought-for end process. Logically nothing more is required (1978, p. 847).

This point is similar to Dennett's that, once discriminations have been made, they do not need to be re-presented to some central consciousness system--a "Cartesian theater" (1991, p. 344). But there is a dissimilarity too: as Dennett's critique of Cartesian materialism and his alternative "multiple drafts" model of consciousness makes plain, the notion of a "final stage" may have no application at all. In fact, given the dense connectivity of the brain, with reciprocal forward and backward projections, it is not clear what "final stage" could mean in any absolute sense (see Section 5.1). For this reason, Dennett's discussion of filling-in represents an advance over Ratliff and Sirovich's.

Although neural filling-in may not be logically necessary, whether there is neural filling-in has to be an empirical question. Ratliff and Sirovich admitted this: "we cannot by any reasoning eliminate a priori some higher-order stage or filling in process... But parsimony demands that any such additional stage or process be considered only if neurophysiological evidence for it should appear" (1978, p. 847). Dennett too admits this (1991, p. 353; 1992, pp. 42-43). What sort of evidence is there, then, for neural filling-in?

4. EVIDENCE FOR NEURAL FILLING-IN

Dennett subjects a rather large variety of visual phenomena to his "thrifty producer principle"--blind spot completion, apparent motion, peripheral vision, neon color spreading--as well as non-visual phenomena such as the phoneme-restoration effect. The conceptual point he wishes to make is well taken, but the phenomena have to be considered in a case-by-case way.

We would like to draw attention to three cases here-- the blind spot and motion aftereffects, illusory contours, and the temporal dynamics of brightness and color induction. All three provide vivid counterexamples to Dennett's assertion that perceptual completion is accomplished by the brain's ignoring an absence. Furthermore, for the temporal dynamics of brightness at least, and very likely also for illusory contours, the brain does not seem to be providing content by assigning a label, but rather by making use of a spatially organized representational medium.

4.1 The blind spot and motion aftereffects

If one visually adapts to a screen containing moving dots, and then views a screen containing stationary dots, one will typically perceive the stationary dots to be moving in the opposite direction. This is known as the motion aftereffect. What happens if you adapt monocularly to moving dots that traverse your blind spot? Does the blind spot region--which contains no photoreceptors and so is not stimulated--also generate an aftereffect?

Murakami (1995) addressed this question in an intriguing study. Instead of directly assessing whether a regular aftereffect is produced, he assessed the interocular transfer of the effect, that is, whether the motion aftereffect could be measured at the corresponding visual field of the other eye. It is well known that a standard motion aftereffect transfers interocularly. Murakami found that in the blind spot case the aftereffect also transfers interocularly. In other words, adaptation to filled-in motion at the blind spot of one eye can cause a motion aftereffect at the corresponding visual field of the other eye. This result provides evidence for the perception of real motion and the perception of filled-in motion sharing a common neural pathway early on in the visual system.

This study is directly relevant to Dennett's position. Why would the motion aftereffect transfer if the brain had just "ignored the absence" of stimulation? If the brain treated perceptually completed motion at the blind spot and real motion differently, then one would not expect the motion aftereffect to transfer. Murakami's study thus provides a measurable effect of what appears to be the brain's having taken the trouble to fill in the motion at the blind spot (though not necessarily in a topographic manner).

4.1 Illusory contours

Figure 7 shows the famous "Kanizsa triangle" (Kanizsa 1955, 1979). In the figure there are illusory contours--clear boundaries where there is no corresponding luminance gradient--and a brightening within the figure. The illusory contours and the central brightening are said to be "modal" in character (Michotte et al. 1964): they are perceptually salient and appear to belong to the figure rather than the ground.

Several researchers have suggested cognitive theories of illusory contour perception, most notably Gregory (1972) and Rock (Rock & Ansen 1979). In these theories, illusory contour formation is largely the result of a cognitive-like process of postulation. Illusory contours are viewed as solutions to a perceptual problem: "What is the most probable organization that accounts for the stimulus?" Although there is ample evidence for the role of cognitive influences in illusory contours, current studies point to the importance of relatively low-level processes in the formation of illusory contours.

Two lines of evidence point to an early neural mechanism for illusory contour completion: (1) neurophysiological data, and (2) psychophysical studies of the similiarities between real and illusory contours.

4.1.1 Neurophysiological evidence

In an influential paper, Rudiger von der Heydt and colleagues (von der Heydt et al. 1984) presented results from single cell recordings in alert macaque monkeys that suggest neural correlates of illusory contours in area V2. Among other things, they studied neural responses to notch stimuli--dark rectangles with parts missing, forming an illusory rectangle. Cellular activity fell off with increasing notch separation and was greatly reduced when only a single notch was present, in parallel with the perceptual disappearance of the illusory figure. In all, the cellular recordings of von der Heydt and colleagues revealed cells whose responses to illusory contour variations resembled human psychophysical responses to similar variations (see also von der Heydt & Peterhans 1989; Peterhans & von der Heydt 1989; for similar results in cat visual cortex see Redies et al. 1986). Although some have described these findings as the discovery of "illusory contour cells" (Lesher 1996), von der Heydt et al. (1984) tried to draw a clear distinction between the stimulus-response relationship, on the one hand, and perceived entities, on the other. For instance, they used the term "illusory contour stimuli," rather than "illusory contour cells," and they introduced the term "anomalous contours" to define a stimulus property without reference to perception. Although making a link between single cell activities and perceptual phenomena is problematic (see Pessoa et al., in press), the evidence here nevertheless seems to suggest that the perceptual completion of boundaries involves the neural completion of a presence, rather than "ignoring an absence." Moreover, the neural completion appears to be spatially organized, instead of symbolic (constituted by a label).

4.1.2 Psychophysical evidence

Many psychophysical studies have provided evidence for a common early treatment of both real and illusory contours by the visual system (see Lesher 1996; Spillman & Dresp 1995). For example, Smith and Over (1975, 1976, 1977, 1979) have revealed similarities between the two types of contours in the realm of motion aftereffects, tilt aftereffects, orientation discrimination, and orientation masking.

Tilt aftereffects are particularly interesting. A tilt aftereffect will occur if one adapts for a few seconds by looking at lines oriented counterclockwise from the vertical, and then one is exposed to a test stimulus of vertical lines. The latter will appear to be tilted clockwise, away from the adapting orientation. There is compelling evidence from recent studies showing that tilt aftereffects cross over between real and illusory contours (Berkeley et al. 1994; Paradiso et al. 1989). Thus adaptation with real lines can affect the perception of illusory contour orientation and vice-versa.

An important question concerns the level at which real and illusory contours have similar status. Motion and tilt aftereffects are often attributed to short term habituation in early visual cortex (Barlow and Hill 1963; Movshon et al. 1972). Thus the evidence from psychophysics is that real and illusory contours share internal processes at an early level of the visual system. In fact, there is considerable evidence pointing to the functional equivalence of real and illusory contours in the operation of the visual system (see Table 1 of Lesher 1996; Spillman & Dresp 1995, p. 1347). Hence psychophysical evidence reinforces the point that, in the case of illusory contours, the brain does not appear to be "ignoring an absence," but rather completing a presence in a way similar to the case of real contours.

4.2 Temporal dynamics of brightness and color induction

There is an enormous literature on the spatial variables determining brightness and color induction. In contrast, there are considerably fewer studies investigating temporal variables (Boynton 1983; Kinney 1967; see Heinemann 1972). But there are a few studies with results that speak directly to the question of evidence for filling-in.

Paradiso and Nakayama (1991) used a visual masking paradigm to investigate two issues--first, the role of edge information in determining the brightness of homogeneous regions, and second the temporal dynamics of perceptual filling-in. They reasoned that if the filling-in process involves some form of activity-spreading, it may be possible to demonstrate its existence by interrupting it. If boundaries interrupt filling-in, what happens when new borders are introduced? Is the filling-in process affected before it is complete?

Figure 8 shows the paradigm they used as well as the basic result. The target is presented first and is followed at variable intervals by a mask. For intervals on the order of 50-100msec the brightness of the central area of the disk is greatly reduced. If the mask is presented after 100msec, the brightness of the central region is largely unaffected. The most striking result was that the brightness suppresion depended on the distance between target and mask. In particular, for larger distances maximal suppression occurred at later times.

Paradiso and Nakayama's results are consistent with the hypothesis that brightness signals are generated at the borders of their target stimuli and propagate inward at a rate of 110-150deg/sec (6.7-9.2 msec/deg). The idea that contours interrupt the propagation is perhaps clearest for the case where a circular mask is introduced, resulting in a dark center, for the brightness originating from the target border seems to be "blocked." Paradiso and Nakayama discuss several alternative accounts, such as lateral inhibition processes, but do not consider them to be plausible explanations of their findings.

In this context, it is worth pointing out that the filling-in model of brightness perception proposed by Grossberg and Todorovic´ (1988) has been shown by Arrington (1994) to produce excellent fits to the data from Paradiso and Nakayama (1991). This sort of close link between psychophysics, neurophysiology, and modeling seems especially promising for investigating the mechanisms responsible for perceptual completion.

Another relevant study comes from De Valois, Webster, and De Valois (1986). They employed center-surround standard (reference) and matching (variable) stimuli, similar to the ones used in classic contrast studies. They compared the results of direct changes in brightness (or color) where the center of the standard pattern was modulated (as was the matching pattern), to the changes that occurred when the surround was modulated sinusoidally while the center was kept constant at the mean level. (In other words, they compared the perception of direct changes in luminance to the perception of changes in brightness produced by luminance changes in a surrounding area--this is a temporal version of the classical simultaneous contrast effect.) Their studies revealed two main findings: (1) The temporal frequencies studied had little effect on the apparent brightness change in the former, direct condition (color variations were present but small). (2) In the latter, induced condition, the amount of brightness change fell drastically as the temporal frequency increased (see Pessoa et al. in press for further discussion). These results can be interpreted in terms of a spreading mechanism of induction that occurs over time, one that would provide a spatially continuous representation for filling-in. Brightness and color signals would be generated at the edges between center and surround, and would propagate inside the center region determining the appearance.

Rossi and Paradiso (1996) have replicated the brightness induction results of De Valois et al. (1986) and have studied the role of pattern size on the effect by varying the spatial frequency of the inducing pattern. The correlation found between spatial scale, degree of induction, and cutoff frequency indicates that there is a limited speed at which induction proceeds and that larger areas take more time to induce. Rossi and Paradiso conclude that the limits on the rate of induction are consistent with an active filling-in mechanism initiated at the edges and propagated inward.

The studies discussed in this section provide strong evidence for the sort of featural filling-in contrary to Dennett's position. In brightness filling-in the brain seems to be providing something, and it seems to be doing so through a roughly continuous propagation of signals, a process that takes time. On the other hand, ignoring a region by jumping to the conclusion that it has the same label as its surround doesn't take time in the same way: although labeling would involve brain processes with their own temporal limitations, there seems no reason to suppose that it would be subject to the same kind of temporal constraints as those involved in signals having to propagate through some spatially extended area. As Dennett himself has said: "you can't assign a label to a region gradually, and it is hard to think of a reason why the brain would bind the 'gradually' label to a process that wasn't gradual" (1993, p. 209).

5. FILLING-IN WITHOUT ANALYTIC ISOMORPHISM AND CARTESIAN MATERIALISM

We now return to the conceptual issues surrounding the neural-perceptual relation. We have seen that there is considerable evidence for neural filling-in. The main point of this section is that the existence of neural filling-in does not entail analytic isomorphism or Cartesian materialism.

Discussions of neural filling-in have been closely tied to the doctrine of analytic isomorphism. Visual scientists sometimes interpret the evidence for neural filling-in within the framework of analytic isomorphism (see Section 5.1). On the other hand, Dennett rejects Cartesian materialism and with it neural filling-in: although he appears to concede that neural filling-in is an "empirical possibility," and says that he does not wish "to prejudge the question" (1991, p. 353), he nevertheless asserts that the "idea of filling in... is a dead giveaway of vestigial Cartesian materialism" (p. 344).

We agree with Dennett that a particular conception of consciousness--"Cartesian materialism"-- motivates analytic isomorphism and its argument for neural filling-in. We also agree that any argument for neural filling-in based on Cartesian materialism should be rejected. (We too think that Cartesian materialism is fundamentally misguided, though not for the reasons that Dennett thinks: see Section 6.) But the empirical case for neural filling-in as discussed above is entirely separate from such philosophical considerations about consciousness. This means that theories and models in visual science that appeal to neural filling-in need not be motivated by Cartesian materialism. One must distinguish sharply between the existence of neural filling-in as an empirical matter and Cartesian materialist interpretations of filling-in. Visual scientists are mistaken when they interpret the evidence for neural filling-in within the framework of analytic isomorphism, but Dennett is equally mistaken when he says that talk of filling-in reveals a commitment to Cartesian materialism.

5.1 Isomorphism and the bridge locus

As we discussed in Section 3, the term "isomorphism" first gained prominence in visual science through the work of Wolfgang Köhler. Although the isomorphism concept has often been interpreted to mean a spatial or topographic correspondence, Köhler held that neural-perceptual isomorphism should be thought of as topological or functional. Our view is that there is nothing conceptually wrong with these sorts of isomorphism as such. Whether there are either spatial/topographic or topological/functional neural-perceptual isomorphisms in any given case is an empirical question for cognitive neuroscience to decide.

What we find problematic is the doctrine of analytic isomorphism, which holds that cognitive neuroscientific explanation requires the postulation of a "final stage" in the brain--a bridge locus--where there is an isomorphism between neural activity and how things seem to the subject. There are two critical points to be made here, one concerning the role played by the concept of the bridge locus and the other concerning the concept of isomorphism.

Teller and Pugh (1983) introduced the term "bridge locus" in their framework for mapping between the neural and the perceptual domains:

Most visual scientists probably believe that there exists a set of neurons with visual system input, whose activities form the immediate substrate of visual perception. We single out this one particular neural stage, with a name: bridge locus. The occurence of a particular activity pattern in these bridge locus neurons is necessary for the occurence of a particular perceptual state; neural activity elsewhere in the visual system is not necessary. The physical location of these neurons in the brain is of course unknown. However, we feel that most visual scientists would agree that they are certainly not in the retina. For if one could set up conditions for properly stimulating them in the absence of the retina, the correlated perceptual state would presumably occur (Teller & Pugh, 1983; p. 581.)

This passage expresses a number of different ideas that need to be disentangled. First, Teller and Pugh state explicitly that a particular pattern of activity at the bridge locus is necessary for the occurence of a particular perceptual state. But at the end of the passage they also explicitly state that retinal stimulation is probably not necessary (assuming one could stimulate the bridge locus neurons directly), thereby suggesting that the bridge locus activity pattern is sufficient for the perceptual state. Therefore, it seems that part of what they mean by the "bridge locus" is a particular set of neurons having a particular pattern of activity that is necessary and sufficient for a particular perceptual state. Second, in calling the bridge locus a particular "neural stage," and in saying that this stage is not likely to be found in the retina, Teller and Pugh seem to be conceiving of the bridge locus in a localizationist manner as a particular cortical region or area.

Analytic isomorphism relies on the concept of the bridge locus. Consider the following statement by Todorovic´ (1987, p. 549): "A logical consequence of the isomorphistic approach is that a neural activity distribution not isomorphic with the percept cannot be its ultimate neural foundation." By "ultimate neural foundation" Todorovic´ indicates that he means the bridge locus. The doctrine of analytic isomorphism states that it is a condition on the adequacy of cognitive neuroscientific explanation that there be an ultimate neural foundation where an isomorphism obtains between neural activity and the subject's experience.

We are suspicious of this notion of the bridge locus. Why should there have to be one particular neural stage whose activity forms the immediate substrate of visual perception? Such a neural stage is not logically necessary; moreover--to borrow Ratliff and Sirovich's point about neural filling-in--parsimony demands that any such stage be considered only if neurophysiological evidence for it should appear. On this score, however, the evidence to date does not seem to favor the idea. First, brain regions are not independent stages or modules; they interact reciprocally due to dense forward and backward projections, as well as reciprocal cross-connections (Zeki & Shipp 1988). There is ample evidence from neuroanatomy, neurophysiology, and psychophysics of the highly interactive, context-dependent nature of visual processing (DeYoe & Van Essen 1995). Second, cells in visual areas are not mere "feature detectors," for they are sensitive to many sorts of attributes (Martin 1988; Schiller 1995). One of the main ideas to emerge from neuroscience in recent years is that the brain relies on distributed networks that transiently coordinate their activities (Singer 1995; Vaadia et al. 1995), rather than centralized representations. Finally, Dennett and Kinsbourne (1992) have argued that the notion of a single neural stage for consciousness hinders our ability to make sense of neural and psychophysical data about temporal perception.

Some of these critical points could perhaps be met by relying on a less localizationist conception of the bridge locus, which, as Todorovic´ (1987, p. 550) observes, is probably an "oversimplified notion," for "there is no compelling reason to believe that the bridge locus is confined to neurons of a single type within a single cortical area." Although this is a step in the right direction, the term "bridge locus"--defined as "the location [our emphasis] at which the closest associations between F [physiological] and Y [psychological] states occur" (Teller & Pugh 1983, p. 588)--does not strike us as particularly useful for thinking about the distributed neural correlates of perceptual experience. For example, such correlates might involve neural assemblies where membership is defined through a temporal code, such as response synchronization (Singer 1995; Varela 1995). For this reason, we think that the concept of the bridge locus should be abandoned.

To abandon the concept of the bridge locus means rejecting analytic isomorphism, for analytic isomorphism depends on this concept. Some visual scientists, however, reject analytic isomorphism while nevertheless adhering to the concept of the bridge locus. For example, Ratliff and Sirovich (1978) denied analytic isomorphism, but asserted that the neural processes involved in perception "must reach a final stage eventually." The notion of a "final stage" seems equivalent to the notion of the bridge locus. We would reject any framework that depends on the concept of the bridge locus, whether isomorphic or nonisomorphic.

We now return to the concept of isomorphism. A good example of what we object to in analytic isomorphism can be found in a statement made by Todorovic´ (1987) in his discussion of "isomorphistic" versus "nonisomorphistic" theories of the Craik-O'Brien-Cornsweet effect (see Figure 6). Todorovic´ admits that any mapping from neural to perceptual states "is an aspect of the notorious mind-body problem," but then goes on to say: "conceptually the idea of an isomorphism between certain aspects of neural activity and certain aspects of percepts may be more acceptable [than a nonisomorphic mapping], at least within a general reductive stance that assumes that, at some level of description, perceptual states are neural states" (1987, p. 550). We disagree. On the one hand, as Todorovic´ recognizes, and as Köhler himself observed over thirty years ago (Köhler 1960, pp. 80-81), the thesis of neural-perceptual isomorphism does not logically entail mind-brain identity. On the other hand, suppose one does assume that "at some level of description, perceptual states are neural states." Still, neural-perceptual analytic isomorphism would be plausible only if perceptual states are strictly identical to neural states (so that each type of perceptual state is identical to a particular type of neural state). But isomorphism would not be plausible if the identity is weak, that is, if perceptual states are multiply realizable with respect to neural states (so that, although every perceptual state is identical to some neural state, one and the same type of perceputal state can be realized in many different types of neural states, or in many different types of non-neural physical states for that matter). This issue of strong (or type) identity versus weak (or token) identity is indeed "an aspect of the notorious mind-body problem," and nothing that Todorovic´says favors the strong identity thesis. Hence no basis has been given for the a priori claim that isomorphism is conceptually preferable to nonisomorphism in cognitive neuroscientific explanation.

6. BEYOND DENNETT'S ELIMINATIVISM: RECOVERING THE PERSONAL IN COGNITIVE SCIENCE

The final matters we wish to discuss concern the conceptual and phenomenological issues about perception raised by Dennett's treatment of filling-in.

Dennett thinks that the notion of filling-in falls into the trap of Cartesian materialism. We agree that the analytic isomorphism argument for filling-in that we have rejected does involve Cartesian materialism: analytic isomorphism holds that filling-in has to take place so that there can be a neural-perceptual isomorphism at the bridge locus; Cartesian materialism holds that brain contents taken on their own are either conscious or not conscious, and for a neural content to be conscious it has to arrive at a privileged site in the brain. But we disagree about the consequences of denying analytic isomorphism and Cartesian materialism. Dennett thinks it follows that filling-in is unnecessary and that the brain jumps to a conclusion instead. As we have seen, however, although neural filling-in is not logically necessary, there is nonetheless plenty of evidence for it in visual perception.

Dennett's rejection of neural filling-in is motivated by general philosophical considerations in addition to his wish to dispel Cartesian materialism. In our assessment, Dennett recognizes an important conceptual point about perceptual content, but then misconstrues its significance, especially for understanding the relation between the perceptual experience of the person and brain processes.

The important conceptual point is the following. Suppose one has a perceptual experience that there is something red in front of one. It is normally assumed that the brain represents that there is something red there. But nothing whatsoever follows about the nature of the neural representation of this situation. For example, it does not follow on logical, conceptual, or methodological grounds, that there is a spatial or pictorial representation of the red region in one's brain. In general, one cannot infer anything about the nature of the neural representational medium from the character of the perceptual content.

One of Dennett's favorite examples for making this point is parafoveal vision (1991, pp. 354-355; 1992). Suppose someone walks into a room covered with wallpaper whose pattern is a regular array of hundreds of identical images of Marilyn Monroe. The person would report seeing that the wall is covered with hundreds of identical Marilyns. But the person can foveate only a few Marilyns at a time and the resolution of parafoveal vision is not good enough to discriminate between Marilyns and colored shapes. One can conclude that the brain represents that there are hundreds of identical Marilyns, but not that there is a spatial or pictorial representation of each identical Marilyn (see also O'Regan 1992, pp. 474-475, 481). Once again, there is no logical route from the content of perception to the representational vehicles of perception in the brain.

We think that the full significance of this point has to do with an important distinction that Dennett himself has emphasized in his writings--the distinction between the personal and the subpersonal. For Dennett, the personal is discerned from "the intentional stance," whereas the subpersonal is discerned from "the design stance" (Dennett 1987). When one adopts the intentional stance toward an animal or system, one is interested in it as a whole interacting in its environment, but when one adopts the design stance one is interested in its internal functional organization.

In visual science, the importance of drawing this kind of distinction between the personal and the subpersonal has been emphasized in ecological and active approaches to vision (Gibson 1979; Ballard 1991; Thompson et al. 1992; Thompson 1995). For example, many years ago J.J. Gibson wrote: "In my theory, perception is not supposed to occur in the brain but to arise in the retino-neuro-muscular system as an activity of the whole system" (1972, p. 217). "Perceiving is an achievement of the individual, not an experience in the theatre of consciousness" (1979, p. 239). One does not have to agree with Gibson's specific hypotheses about how visual perception works to see the main point being made here: the proper subject of perception is not the brain, but rather the whole embodied animal interacting in its environment.

Despite his own personal/subpersonal distinction, Dennett does not embrace wholeheartedly the point that it is the person or animal that is the proper subject of perception. In saying that there is no logical route from personal-level perceptual content to neural representation, Dennett acknowledges one side of the point, the side facing from the personal to the subpersonal. But he does not embrace the other side, the side facing from the subpersonal to the personal. Dennett holds that although there is no logical route from personal-level content to subpersonal representation, there is a logical route in the other direction, from subpersonal representation to personal-level content. Indeed, the route could not be more direct, for Dennett treats personal-level content as if it were entirely constituted by subpersonal content, with the result that there can be no difference in kind between perceptual content and neurally represented content. This assumption is fundamental to his approach to consciousness, according to which the truth-values of third-person descriptions of perceptual experience are ultimately to be evaluated in relation to the scientific account of brain processes.

We think that this way of thinking leads Dennett to misdescribe certain perceptual situations that demand careful conceptual and phenomenological study. This is best seen in his discussion of parafoveal vision.

In the Marilyn wallpaper case, after asserting that the brain just "jumps to the conclusion that the rest are Marilyns, and labels the whole region 'more Marilyns' without any further rendering of Marilyns at all," Dennett writes: "Of course it does not seem that way to you. It seems to you as if you are actually seeing hundreds of identical Marilyns" (1991, p. 355). Dennett clearly implies that he thinks there is a sense in which you are not seeing hundreds of identical Marilyns, that in some way your seeing hundreds of identical Marilyns is an illusion (see also Blackmore et al. 1995). But what exactly does he mean?

The first thing to be noticed about these remarks is their indifference with respect to the personal/subpersonal distinction. When Dennett says "Of course it does not seem that way to you" what way does he have in mind? The immediately preceding sentence is a description of how the brain jumps to a conclusion "without any further rendering of Marilyn at all." Does he mean, then, that it does not seem to you that your brain jumps to a conclusion? Of course, this is true: it does not seem to you, the person, that your brain labels the whole region "more Marilyns." But so what? Suppose your brain does propagate a high-resolution, foveated Marilyn image "across an internal mapping of an expanse of wall." Still it would not seem that way to you either. After all, one of Dennett's points is that one cannot read the character of the neural representations off of one's experience as a perceptual subject.

On the other hand, the sentence immediately following the remark "Of course it does not seem that way to you" does refer to your perceptual experience: "It seems to you as if you are actually seeing hundreds of identical Marilyns." But in what sense does Dennett deny that you are actually seeing all those Marilyns? Here is what he goes on to write:

in one sense you are: there are, indeed, hundreds of identical Marilyns out there on the wall, and you're seeing them. What is not the case, however, is that there are hundreds of identical Marilyns represented in your brain. Your brain just somehow represents that there are hundreds of identical Marilyns, and no matter how vivid your impression is that you see all that detail, the detail is in the world, not in your head (1991, p. 355).

This passage misdescribes the perceptual experience of the person on two counts. First, Dennett describes your perceptual experience as if it involved seeming to see all the details in your head. As he says several pages later: "The hundreds of Marilyns in the wallpaper seem to be present in your experience, seem to be in your mind, not just on the wall... But why should your brain bother importing all those Marilyns in the first place?" (pp. 359-360). This description is completely unfaithful to the character of perceptual experience: the Marilyns do not seem to be present in your experience or in your mind (whatever that might mean); they seem to be present there on the wall.

Second, Dennett says that, in looking at the Marilyns, it seems to you that you see all the detail. This too is mistaken. Although you do seem to see all the detail in the sense that the wall seems to you to be covered with hundreds of identical Marilyns, you do not seem to see each Marilyn equally well. At any given moment of your perception, the Marilyns straight ahead seem clear as day, while those off to the side appear less distinct, and those in the periphery seem barely noticeable. Of course, at any time you can turn your head to look at the others: in the example as Dennett describes it, "you walk into a room" and notice the wallpaper; thus you are an active, moving perceiver, able to scan the environment, not an experimental subject being instructed to fixate a point in your visual field while sitting still. Nevertheless, in turning your head, some Marilyns will appear to your sight and others will become indistinct, though still seen as present on the wall. Thus Dennett's account conflates two different things--your seeming to see that there are hundreds of identical Marilyns on the wall and your seeming to see all of them in full detail so that each is equally distinct. Not only can you seem to see that there are hundreds of identical Marilyns without your seeming to see each Marilyn in full detail, but there is a sense in which it is necessarily the case that your visual experience of the Marilyns will be vague or indistinct in just this way. The reason is that this kind of vagueness, blurredness, or indistinctness in the visual field is an ineliminable feature of visual experience. Hence Dennett, in presupposing that the visual experience of the Marilyns could have presented each Marilyn clearly and distinctly, seems to be disregarding the very nature of the phenomenon.

This sort of indeterminacy in perceptual experience was the subject of extensive philosophical investigation earlier in this century, not only by the phenomenologists Edmund Husserl and Maurice Merleau-Ponty, but also by Ludwig Wittgenstein during his "middle period" (1930/1975). (It was discussed even earlier by William James.) In Wittgenstein's case, one important point of departure was the recognition that one cannot see, for example, a gaggle of geese flying overhead as a gaggle of, say, one hundred and twenty-seven geese, for one cannot visually distinguish the latter from a gaggle of one hundred and twenty-five geese. It can make no sense (at least in normal contexts) to say that someone noticed one hundred and twenty-seven geese fly by, where what is meant is that the person's visual experience represented just that number of geese and no other. According to Wittgenstein, this kind of consideration reveals something essential about the nature of visual experience; in particular, it shows that intrinsic to visual experience is a particular kind of vagueness or indeterminacy. To use one of Wittgenstein's examples, because one cannot by looking distinguish a row of thirty identical pencils marks from a row of thirty-one, it follows that there is no such thing as a visual image of precisely thirty identical pencil marks (1930/1975, pp. 258, 268). Consider another of his examples: it would make no sense to say of a visual figure (i.e., a figure in visual space) that although it seemed to be a circle, it was in fact a thousand-sided figure. The reason is that there is no visual criterion that would enable one to distinguish a circle from an appropriate thousand-sided figure (1930/1975, p. 266). Therefore, there is no difference between these two visual figures. For Wittgenstein, each of these cases exemplifies a respect in which there is an ineliminable, irreducible lack of determinacy to visual experience:

The moment we try to apply exact concepts of measurement to immediate experience, we come up against a peculiar vagueness in this experience. But that only means a vagueness relative to these concepts of measurement. And, now, it seems to me that this vagueness isn't something provisional, to be eliminated later on by more precise knowledge, but that this is a characteristic logical peculiarity...

Admittedly the words 'rough', 'approximate', etc. have only a relative sense, but they are still needed and they characterise the nature of our experience; not as rough or vague in itself, but still as rough or vague in relation to our techniques of representation (1930/1975, p. 263).

Contrary to what Dennett seems to imply, then, the inability to see each Marilyn clearly and distinctly should not be seen as a contingent feature of perceptual experience, one that might have been otherwise. Rather, the indeterminacy of the Marilyns in the periphery of the visual field, compared to those in the center, is essential to the nature of the experience as a visual experience.

In the phenomenological tradition, this kind of indeterminacy has been discussed in connection with the perceptual organization of the visual field and the embodied character of perception. Building on Gestalt psychology, phenomenologists have discussed at great length the figure-ground structure of both static and dynamic perception, in which the figure is typically clear while the ground recedes eventually into an indeterminate background (Husserl 1913/1982, pp. 70-71; Merleau-Ponty 1945/1962, pp. 4-6; Köhler 1947, pp. 173-205; Gurwitsch 1964). As Merleau-Ponty discusses at the very beginning of his Phenomenology of Perception, the basic tenet of Gestalt theory is that the simplest thing that can be given in perceptual experience is not a simple sensation or quale, but rather a figure on a background. But whereas the Gestalt theorists took this to be an empirical, psychological fact, Merleau-Ponty argues that "this [figure on a background] is not a contingent characteristic of factual perception... It is the very definition of the phenomenon of perception, that without which a phenomenon cannot be said to be a perception at all" (1945/1962, p. 4). From the vantage point of phenomenological reflection, the figure-ground structure (in which as a matter of formal necessity the ground becomes increasingly indeterminate in contrast to the figure) is an invariant feature of perception as a type of intentional experience, and is known to be such a priori. The basis for this claim involves a particular aspect of the phenomenological method, known as "ideation through imaginative variation." Very roughly, the idea in the present case is that, no matter how one imagines the perceptual situation to be varied, the figure-ground structure always remains as a formal, constitutive feature of perception, while on the other hand, imagining the figure-ground structure to be absent is tantamount to no longer imagining a case of perception.

Phenomenologists make the same kind of point about the essentially embodied character of perception. Perceptual experience involves the tacit, background awareness of one's body, but this feature is not a contingent one; it is a formal invariant of perception, and is known a priori to phenomenological reflection. For something to be a possible object of perceptual experience it must be the kind of thing that can be experienced as being situated in relation to the spatial vantage point provided by one's body. Indeed, the spatial position of one's body determines in part the figure-ground structure of perception: the figure corresponds to the focus of one's gaze and the ground to what one sees in the surroundings. Furthermore, the perceptual surroundings can be differentiated into the immediate spatial context of the figure, on the one hand, and the indeterminate background of the periphery, on the other, with the indeterminate background including the marginal presence of one's own body. Thus the background awareness of the body turns out to be essential to the figure-ground structure of perceptual experience: the perceptual object not only stands out against an external background; it is situated within an implicit bodily space. In Merleau-Ponty's words: "one's own body is the third term, always tacitly understood, in the figure-background structure, and every figure stands out against the double horizon of external and bodily space" (1945/1962, p. 101).

With respect to the Marilyn example, the same kind of point emerges from these phenomenological considerations as did earlier in our brief discussion of Wittgenstein: there can be no incompatibility between the indistinctness of parafoveal vision and actually seeing that the wall is covered with hundreds of identical Marilyns. The reason, for the phenomenologist, is that the indistinctness of peripheral vision--although empirically explicable in terms of the anatomy and physiology of the retina--is based on something a priori, namely, the figure-ground structure and the embodied character of visual perception. Merleau-Ponty's statement of this point is worth quoting at length:

To see an object is either to have it on the fringe of the visual field and be able to concentrate on it, or else to respond to this summons by actually concentrating upon it. When I do concentrate my eyes on it, I become anchored in it, but this coming to rest of the gaze is merely a modality of its movement: I continue inside one object the exploration which earlier hovered over them all, and in one movement I close up the landscape and open the object. The two operations do not fortuitously coincide: it is not the contingent aspects of my bodily make-up, for example the retinal structure, which force me to see my surroundings vaguely if I want to see the object clearly. Even if I knew nothing of rods and cones, I should realize that it is necessary to put the surroundings in abeyance the better to see the object, and to lose in background what one gains in focal figure, because to look at the object is to plunge oneself into it, and because objects form a system in which one cannot show itself without concealing others (1945/1962, pp. 67-68).

In speaking of the "fringe" of the visual field in the first sentence of this passage, Merleau-Ponty is drawing on an idea that goes back to William James (1890/1981). What James recognized, which Husserl later discussed in great detail, is that one's field of vision is situated within and bounded by a "horizon" (this term is Husserl's), and that the outline or boundary of the visual field is indistinct, having the experiential character of a fringe. For James, as for Husserl and Merleau-Ponty, to neglect the phenomenon of the fringe is to seriously distort the nature of experience: it amounts to the assumption that the concept of determinacy one finds in geometrical representations of space and physical objects is applicable in stating the character of experience. Hence James, when discussing the fringe, writes: "It is, the reader will see, the reinstatement of the vague and inarticulate to its proper place in our mental life which I am so anxious to press on the attention" (1892/1961, p. 32). Merleau-Ponty echoes the idea in the early pages of his Phenomenology of Perception:

The region surrounding the visual field is not easy to describe, but what is certain is that it is neither black nor grey. There occurs here an indeterminate vision, a vision of something or other, and, to take the extreme case, what is behind my back is not without some element of visual presence (1945/1962, p. 6)

"We must recognize the indeterminate as a positive phenomenon," he goes on to say, and this we cannot do as long as "we are caught up in the world and... do not succeed in extricating ourselves from it in order to achieve consciousness of the world" (p. 5). What he means is that the sort of determinateness one finds in physical objects must not be assumed a priori to be applicable to perceptual experience, in particular to the experiential character of the visual field. Dennett, however, does exactly that: he treats the indeterminateness of visual experience as though it were merely contingent, thereby supposing that all the determinate individual Marilyns on the wall could have been present in your field of vision with that very same determinateness.

There is another way of formulating Merleau-Ponty's point about not being able to recognize the indeterminate as a positive phenomenon as long as one is caught up in the world, a formulation that brings us back to the distinction between the personal and the subpersonal levels. The point is that the kind of determinateness that one finds in scientific accounts of the world, in particular in subpersonal accounts of the physiology of vision, is not applicable to the perceptual experience of the person.

Dennett's mischaracterization of the person's perceptual experience in the passage about the Marilyns cited above seems to involve a general confusion about the relation between the personal and the subpersonal levels. Dennett implies that although you seem to see all the detail, you do not actually see it, because all the detail is not represented in your brain. Notice what this means: the Marilyns are actually there on the wall, you seem to see that there are hundreds of identical Marilyns there on the wall, and you correctly judge that this is so on the basis of your visual experience. Nevertheless, you do not really see them because they are not all represented in your brain, the implication being that if they were all represented in your brain, then you would really see them.

We have already discussed how there can be no incompatibility between the indistinctness of parafoveal vision and actually seeing that the wall is covered with hundreds of identical Marilyns. But there is a more general problem with Dennett's position. Dennett appears to be making the following inference: you seem to see, but you are not really seeing, because your brain has merely jumped to a conclusion. This inference is a non sequitur that confuses the subpersonal and the personal levels: there is no reason to suppose that the brain's jumping to a conclusion is incompatible with the person's actually seeing the Marilyns. If Dennett is right that the brain jumps to a conclusion in the Marilyn case, then the proper thing to be said is that the person actually sees that the wall is covered with hundreds of identical Marilyns, and that this is accomplished subpersonally by the brain's jumping to a conclusion. What is striking is that Dennett seems prepared to deny that one sees that the wall is covered with hundreds of identical Marilyns, even though one forms the judgement, on the basis of looking, that this is the case, and even though the judgement happens to be correct.

Why does Dennett adopt such an implausible view? It would appear that there is an unstated assumption at work in Dennett's reasoning, one every bit as contentious as the doctrine of analytic isomorphism. The assumption is that for an experience to be non-illusory a necessary condition is that it depend on a neural state that has the very same content. For example, to be really seeing hundreds of identical Marilyns (or even, for that matter, for it to seem to one that one sees them) there would have to be hundreds of high-resolution Marilyn representations in one's brain. Or, to take another more general example, for consciousness to be really continuous, the subpersonal neural representations would have to be continuous, but they are not, and so the continuity of consciousness is an illusion: "One of the most striking features of consciousness is its discontinuity. Another is its apparent continuity. One makes a big mistake if one attempts to explain its apparent continuity by describing the brain as 'filling in' the gaps" (1992, p. 48). What is striking about this assumption is how close in its logical structure it is to analytic isomorphism: analytic isomorphism assumes that there must be an isomorphism between neural activity and perceptual experience; Dennett assumes that there must be a corespondence of content between neural representations and visual perception, otherwise one is not really seeing, or between neural representations and experience in general, otherwise one's consciousness is not really continuous.

Dennett criticizes those who invoke filling-in for relying on a Cartesian materialist conception of consciousness. But the correspondence of content doctrine Dennett actually shares with Cartesian materialism. According to this doctrine, there is no difference in kind between neural content and perceptual content because the former entirely constitutes the latter. According to Cartesian materialism, neural content at the bridge locus or in the Cartesian theater constitutes the subject's conscious perceptual content. According to Dennett, there is no Cartesian theater; rather, spatially and temporally distributed, subpersonal neural content constitutes perceptual content at the personal level. We think that both the Cartesian materialists and Dennett are mistaken, for we hold that there is a difference in kind between neurally represented content and personal level perceptual content (see below).

Dennnett's doctrine of the correspondence of content follows naturally from his account of the relation between the personal and the subpersonal. For Dennett, the person or animal is no more than a logical construct of subpersonal brain states and processes, just as the British Empire at the time of the War of 1812 was a logical construct of the King, the Members of Parliament, various officials and subjects of the Crown, and so on:

we think that certain sorts of questions about the British Empire have no answers, simply because the British Empire was nothing over and above the various institutions, bureaucracies, and individuals that composed it. The question "Exactly when did the British Empire become informed of the truce in the War of 1812?" cannot be answered. The most that can be said is "Sometime between December 24, 1814, and mid-January, 1815"... Similarly, since You are nothing over and above the various subagencies and processes in your nervous system that compose you, the following sort of question is always a trap: "Exactly when did I (as opposed to various parts of my brain) become informed (aware, conscious) of some event?" Conscious experience, in our view, is a succession of states constituted by various processes occurring in the brain and not something over and above these processes that is caused by them (Dennett & Kinsbourne 1992, p. 236, emphasis in original; see also Dennett forthcoming).

We think that this is a distorted and unacceptable treatment of the relation between the personal and the subpersonal. The person is an embodied being embedded in the world, not a logical construction of brain states (Merleau-Ponty 1945/1962; Varela et al. 1991; Thompson & Varela in preparation). We cannot present the step-by-step argument for this position here, so we shall simply list the main reasons as they apply to perceptual experience (see McDowell 1994; Noë 1995; Sedivy 1995; Thompson 1995, pp. 286-303). First, perception is an ability of the animal, and its causally enabling mechanisms are perceptuo-motor processes occurring in the animal and its environment considered as an interactive unit (Merleau-Ponty 1942/1963; Gibson 1979; Ballard 1991; Thompson et al. 1992; Thompson 1995, pp. 215-242; McClamrock 1995; Noë 1995). Second, although brain processes play the main causal role in enabling perception, they are not the proper bearers of perceptual content; the bearer is the animal as a whole interacting in its environment. Third, subpersonal neural content and personal-level perceptual content are not of the same kind: the former pertains to the functional organization of the nervous system insofar as it causally enables the animal's perceptual abilities; the latter pertains to the animal's dealings with its surroundings, and depends on viewing the animal as a rational or intentional being (an "intentional system" to use Dennett's term). In particular, personal-level perceptual content is constrained by understanding, in the sense that all seeing is seeing-as, that is, all seeing involves conceptual understanding and at least the capacity for judgement (Noë 1995, in press). (One cannot seem to see hundreds of identical Marilyns unless one can understand what an image of Marilyn is.) In short, personal-level perceptual content has a rational bearing on thought and action, whereas the content of subpersonal states does not: brain states have contents that outstrip the conceptual skills of the person, and they have only a causal bearing on thought and action, not a rational/normative one.

We believe that Dennett's account of perceptual experience goes astray because of his neglect of the personal/subpersonal distinction. Whereas the Gestalt and the phenomenological psychologists insisted that careful descriptions of perceptual experience without any preconceptions about causal mechanisms are needed to complement causal-explanatory theories, Dennett forgoes such careful descriptions and in their stead offers statements about perceptual experience motivated entirely by subpersonal considerations, thereby distorting the conceptual and phenomenological character of that experience.

In the end, Dennett's treatment of perception goes astray on a fundamental conceptual point obvious to phenomenologists. To see what we mean consider Mach's attempt to draw his visual field with his right eye shut (Mach 1906/1959, p. 19). From his position lying on a divan, he sketched the outline of his nose, his beard and stomach bulging upward, and his legs stretched out. The striking defect in the drawing is at its outer limits, where he attempted to depict the indeterminacy at the periphery of his visual field by means of fading to white. The visual field is indeterminate in various ways, as we discussed in the Marilyn example. But these indeterminacies are not something that can be depicted in a drawing. In general, it seems to be a conceptual/phenomenological truth that one cannot represent the visual field (Wittgenstein 1930/1975, p. 267; Noë 1994). Any attempt to depict the visual field will be a depiction of what is seen, not of the way it seems. Mach did not draw his visual experience of the room; he drew the room as it looked to him from where he lay. The conceptual point of these considerations is that how things seem to one can never be just a matter of whether one has a picture in one's head corresponding to how things seem. Pictures cannot do that job. But Dennett's criticism of filling-in would appear to rest on the idea that they can, for the reason he gives for why one does not actually see hundreds of identical Marilyns on the wall is that there is no such picture in the brain. It would seem that Dennett has lost sight of the conceptual autonomy of perceptual experience at the personal level in relation to subpersonal neural processes.

To uphold the conceptual autonomy of the personal level means treating our understanding of ourselves as conscious perceptual subjects as a distinctive form of understanding, one that can be brought into "mutual accomodation" (Varela et al. 1991) or "reflective equilibrium" (Flanagan 1992) with cognitive science, but that cannot be reduced to an understanding of ourselves as logical constructions of brain states. Once the conceptual autonomy of the personal level is acknowledged, then Dennett's exclusively third-person approach to experience becomes unacceptable. As we have tried to illustrate here, however, it is possible to pursue the descriptive, conceptual and phenomenological study of personal-level experience in tandem with psychophysical and cognitive neuroscientific research. As phenomenologists have long recognized, whenever we attempt to understand and explain our perception, we do so on the basis of our lived, perceptual experience of the world. Unless this experience has been conceptually clarified and systematically described, our scientific explanations will always be incomplete.

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FIGURE CAPTIONS

FIGURE 1: Neon color illusion

When the segments in the diamond are red and the matrix is black (left), the entire diamond appears reddish even though only the segments are physically colored red (right).

FIGURE 2: The blind spot

(a) Vertebrate lens and retina. The blind spot corresponds to the area where the optic nerve leaves the retina and there are no photoreceptors. From Shepard (1994, p. 354). (b) Distribution of photoreceptors over the extent of the retina of the right eye. Note the complete absence of photoreceptors in the blind spot region. From Sekuler & Blake (1985, p. 56).

FIGURE 3: Blind spot demonstration

Close your right eye while fixating at the cross. Adjust the distance of the page in front of you until the left dot disappears (start at a distance of about 8 inches). A similar procedure should be used with the left eye closed.

FIGURE 4: Schematic representation of cellular responses to boundaries and regions

(a) An edge stimulus elicits strong responses when stimulated by an edge (typically moved back and forth to produce vigorous responses). The circle denotes the receptive field of the cell, indicating the portion of visual space that can potentially affect a cell's firing. (b) A large "surface" stimulus evokes strong responses only at its borders. Weak (or no) responses are produced for internal regions away from the borders. Note that given the receptive field size (circle) it is possible to stimulate the cell without including any stimulus edge.

FIGURE 5: Fading of stabilized images

Left: The boundary between a red ring (gray) and a green disk (hatched) is stabilized on the retina. Right: After stabilization a large red disk is seen.

FIGURE 6: Craik-O'Brien-Cornsweet display

The luminance of the two rectangles is identical, except in the vicinity of the common border, where there is a luminance cusp. For appropriate display conditions, the left and right regions appear to have uniform color and the left appears uniformly brighter than the right. Above the figure we show a one-dimensional cross-section of the luminance distribution of the figure below.

FIGURE 7: The Kanizsa triangle

Illusory contours are seen forming a triangle-shaped region although there are no corresponding luminance changes. Note also that the illusory figure is brighter than the background.

FIGURE 8: Masking paradigm in temporal dynamics of brightness study [Paradiso & Nakayama (1991)]

(a) Brightness suppression of a disk-shaped target by a mask consisting of a grid of thin lines. The target and mask are each presented for 16msec. Optimizing the temporal delay between the stimuli yields a percept in which the brightness in a large central area of the disk is greatly suppressed. (b) Brightness suppression is highly dependent on the arrangement of contours in the mask.