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Phenomenology overflows accessibility George Sperling (1960) showed subjects arrays of alphanumeric characters; for example, three rows of four characters, for 50 msec, followed by a blank field. Subjects said that they could see all or almost all of the characters and this has also been reported in replications of the experiment (Baars 1988, p. 15). The phenomenology of a version of the experiment was described by William James in his Principles of Psychology: “If we open our eyes instantaneously upon a scene, and then shroud them in complete darkness, it will be as if we saw the scene in ghostly light through the dark screen. We can read off details in it which were unnoticed whilst the eyes were open” (James 1890); and it may be what Aristotle was talking about when he said, “even when the external object of perception has departed, the impressions it has made persist, and are themselves objects of perception” (Aristotle in Ross 1955, 460b). When Sperling asked subjects to say what letters they had seen, subjects were able to report only about 4 of the letters, less than half of the number of letters they said they could see. (This result was first reported by Cattell [1885]– I am indebted to Patrick Wilken [2001]). Did the subjects really see all or almost all the shapes as they said? Sperling’s clever idea was to test whether people really did see all or almost all of the characters and whether the phenomenology persists after the stimulus was turned off by playing a tone soon after the array was replaced by a blank. Subjects were to report the top row if the tone was high, the bottom row if the tone was low, and the middle row in case of an intermediate tone. The result was that subjects could report all or almost all the characters in any given row. Versions of this type of experiment have been done with as many as 32 alphanumeric characters with similar results (Sligte et al. 2008). An attractive picture of what is going on here– and one that I think makes the most sense of the data– is that although one can distinctly see all or almost all of the 9–12 objects in an array, the processes that allow one to conceptualize and identify the specific shapes are limited by the capacity of “working memory,” allowing reports of only about 4 of them. That is, the subject has experiences as of specific alphanumeric shapes, but cannot bring very many of them under specific shape or alphanumeric concepts (i.e., representations) of the sort required to report or make comparisons. The subject can bring them under a general concept like “alphanumeric character”– which is why the subjects can report that they have seen an array of alphanumeric characters– but not under the more specific concepts required to identify which alphanumeric character. Interestingly, Sperling found the same results whether he made the exposure of the grid as short as 15 msec or as long as 500 msec. Sperling’s experiment is often described as showing that a “visual icon” persists after the stimulus is turned off. However as Max Coltheart (1980) notes, this term is used ambiguously. In my terms, the ambiguity is between (1) phenomenal persistence and (2) persistence of accessible information concerning the stimulus. Since these are the very notions whose empirical separation is the topic of this article, the term icon is especially unfortunate and I will not be using it further.2 The idea that one does in fact phenomenally register many more items than are (in a sense) accessible and that the phenomenology persists beyond the stimulus is further tested in a combination of a change “blindness” paradigm with a Sperling-like paradigm (Landman et al. 2003). First, I will sketch the change “blindness” paradigm. In these experiments, a photograph is presented briefly to subjects, followed by a blank, followed sometimes by an identical photograph but other times by a similar but not identical photograph, followed by another blank. Then 487 BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh Figure 1. Compare this with Figure 4 without looking at the two figures side by side. There is a difference between the two pictures that can be hard to be aware of, a fact that motivates the appellation (a misnomer in my view) “Change Blindness.” the cycle starts over. One can get the idea by comparing Figure 1 and Figure 4 (p. 494) without placing them side by side. When the two photographs differ, they usually differ in one object that changes color, shape, or position, or appears or disappears. The surprising result is that subjects are often unaware of the difference between the two pictures, even when the changed region takes up a good deal of the photographic real estate. Even with 50 repetitions of the same change over and over again, people are often unaware of the change. It is widely agreed that the phenomenon is an attentional one. The items that change without detection have been shown to be items that the subjects do not attend to. But the controversial question– to be discussed later– is whether the phenomenon is one of inattentional blindness or inattentional inaccessibility.3 Now for the experiment by Landman et al. (2003). The subject is shown 8 rectangles for half a second as in (a) of Figure 2. There is a dot in the middle which the subject is supposed to keep looking at. (This is a common instruction in visual perception experiments and it has been found, using eye-tracking, that subjects have little trouble maintaining fixation.) The array is replaced by a blank screen for a variable period. Then another array appears in which a line points to one of the objects which may or may not have changed orientation. The subject’s task is to say whether the indicated rectangle has changed orientation. In the example shown in Figure 2, there is an orientation change. Using statistical procedures that correct for guessing, Landman et al. computed a standard capacity measure (Cowan’s K; see Cowan 2001) showing how manyrectangles the subject is able to track. In (a), subjects show a capacity of 4 items. Thus, the subjects are able to deploy working memory soastoaccess only half of the rectangles despite the fact that in this as in Sperling’s similar task, subjects’ reported phenomenology is of seeing all or almost all of the rectangles. This is a classic “change blindness” result. In (b), the indicator of the rectangle that may or maynot change comes on inthe first panel. Not surprisingly, subjects can get almost all of the orientations right: their capacity is almost 8. The crucial manipulation is 488 Figure 2. Landman et.al.’s (2003) paradigm combining change “blindness” with Sperling’s (1960) experiments on iconic memory. The rectangles are displayed here as line drawings but the actual stimuli were defined by textures. (From Lamme 2003.) the last one: the indicator comes on during the blank after the original rectangles have gone off. If the subjects are continuing to maintain a visual representation of the whole array– as subjects say they are doing– the difference between (c) and (b) will be small, and that is in fact what is observed. The capacity measure in (c) is between 6 and 7 for up to 1.5 seconds after the first stimulus has been turned off, suggesting that subjects are able to maintain a visual representation of the rectangles. This supports what the subjects say, and what William James said, about the phenomenology involved in this kind of case. What is both phenomenal and accessible is that there is a circle of rectangles. What is phenomenal but in a sense not accessible, is all the specific shapes of the rectangles. I am taking what subjects say at face value (though of course I am prepared to reject what subjects say if there is evidence to that effect). Whether that is right will be taken up in the section 11. Subjects are apparently able to hold the visual experience for up to 1.5 seconds– at least “partial report superiority” (as it is called) lasts this long– considerably longer than in the Sperling type experiments in which the production of 3 to 4 letters for each row appears to last at most a few hundred msecs. The difference (Landman et al. 2003) is that the Sperling type of experiment requires a good enough representation for the subjects to actually volunteer what the letters were, whereas the Landman et al. methodology only requires a “same/different” judgment. Yang (1999) found times comparable to Landman et al.’s, using similar stimuli. In one variation, Landman and colleagues did the same experiment as before but changed the size of the rectangles rather than the orientation, and then in a final experiment, changed either the size or the orientation. The interesting result is that subjects were no worse at BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh detecting changes in either orientation or size than they were at detecting changes in size alone. That suggests that the subjects have a representation of the rectangles that combines size and orientation from which either one can be recovered with no loss due to the dual task, again supporting the subjects’ reports. There is some reason to think that the longest lasting visual representations of this sort come with practice and when subjects learn to “see” (and not “look”). Sligte et al. (2008) found long persistence, up to 4 seconds in a paradigm similar to that of Landman with lots of practice. Others (Long 1980; Yang 1999) have noted that practice in partial report paradigms makes a big difference in subjects’ ability to make the visual experience last. These experiments are hard for the subjects and there are large differences among subjects in the ability to do the experiment (Long 1985, Yang 1999); in some cases experimenters have dismissed subjects who simply could not perform the tasks (Treisman et al. 1975). Snodgrass and Shevrin (2006) also find a difference (in a different paradigm) between “poppers” who like to relax and just see and “lookers” who are more active visual seekers. The main upshot of the Landman et al. and the Sligte et al. experiments (at least on the surface– debunking explanations will be considered later) is along the same lines as that of the Sperling experiment: The subject has persisting experiences as of more specific shapes than can be brought under the concepts required to report or compare those specific shapes with others. They can all be brought under the concept “rectangle” in the Landman et al. experiment or “letter” in the Sperling experiment, but not the specific orientation-concepts which would be required to make the comparisons in Landman et al. or to report the letters in Sperling. Why are subjects able to gain access to so few of the items they see in the first condition of the Landman et al. experiment (i.e., as described in [a] of Figure 2) and in the Sperling phenomenon without the tones? I am suggesting that the explanation is that the “capacity” of phenomenology, or at least the visual phenomenal memory system, is greater than that of the working memory buffer that governs reporting. The capacity of visual phenomenal memory could be said to be at least 8 to 32 objects– at any rate for stimuli of the sort used in the described experiments. This is suggested by subjects’ reports that they can see all or almost all of the 8 to 12 items in the presented arrays, and by the experimental manipulations just mentioned in which subjects can give reports which exhibit the subjects apprehension of all or almost all of the items. In contrast, there are many lines of evidence that suggest that the “working memory” system– the “global workspace”– has a capacity of about 4 items (or less) in adult humans and monkeys and 3 (or less) in infants. When some phenomenal items are accessed, something about the process erases or overwrites others, so in that sense the identities of the items are not all accessible. However, any one of the phenomenal items is accessible if properly cued, and so in that sense all are accessible. Another sense in which they are all accessible is that the subject knows that he sees them all (or almost all). The upshot is that there is phenomenology without accessibility (Block 1995a), in one sense of the term but not another (Chalmers 1997; Kobes 1995). Of course, there is no point in arguing about which sense of the word “accessibility” to use. The argument of this target article is thus importantly different from that of earlier works (Block 1995b; 1997), in which I claimed that the Sperling experiment directly shows the existence of phenomenal states that are not cognitively accessible– a conclusion that is not argued for here. In this article, I use the fact of overflow to argue for the conclusion that the machinery of phenomenology is at least somewhatdifferent fromthemachineryofcognitive accessibility. I then argue that there is a neural realization of the fact of phenomenological overflow– if we assume that the neural basis of phenomenology does not include the neural basis of cognitive access to it as a constituent, and that is a reason to accept that assumption. The neural argument suggests that the machinery of cognitive access is not included in the machinery of phenomenology. What does it mean to speak of the representational capacity of a system as a certain number of objects? Working memory capacity is often understood in terms of “slots” that are set by the cognitive architecture. One capacity measure that is relevant to phenomenology is the one mentioned above in connection with the Landman et al. (2003) and Sligte et al. (2008) experiments– Cowan’s (and Pashler’s) K, which I will not discuss further. Another capacity measure relevant to phenomenology is what subjects say about seeing all of a number of items. There are two significant remaining issues: 1. How do we know that the Sperling, Landman et al., and Sligte et al. effects are not retinal or otherwise pre-phenomenal? 2. How do weknow we can believe subjects’ reports to the effect that they experience all or almost all of the objects in the Sperling and the Landman et al. experiments? Perhaps subjects confuse potential phenomenology with actual phenomenology just as someone may feel that the refrigerator light is always on because it is on when he looks. 10. Is the effect retinal or otherwise pre-phenomenal? The persistence of phenomenology is based in the persistence of neural signals. But some neural persistence may feed phenomenology rather than constitute it, and that creates a difficulty for the view that the capacity of phenomenology is larger than the capacity of working memory. It is no news that the “representational capacity” of the retina is greater than 4! Activations of the retina are certainly not part of the minimal neural basis of visual phenomenology. Rather, activations of the retina are part of the causal process by which that minimal supervenience base is activated. A minimal neural basis is a necessary part of a neural sufficient condition for conscious experience. We know that the retina is not part of a minimal core neural basis, because, for one thing, retinal activation stays the same even though the percept in the binocular rivalry experiments mentioned earlier shift back and forth. It may be said that phenomenological capacity is no greater than that of working memory, the appearance to the contrary deriving from tapping large capacity preconscious representations. 489 BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh The experimental literature points to activations at all levels of the visual system during phenomenal persistence (Coltheart 1980; Di Lollo 1980). However, there are clear differences between the effects of retinal persistence and persistence higher in the cortex. One of the neatest dissections of phenomenal persistence in low level-vision from that in high-level vision comes from an experiment by G. Engel (1970). (The discussion of this experiment in Coltheart [1980] is especially useful.) Engel used as stimuli a pair of “random dot stereograms” of the sort invented by Bela Julesz in 1959. I can’t show you an example that would allow you to experience the effect because the kind Engel used requires a stereo viewer. I can best explain what they are by telling you how they are made. Youstart with a grid of, say, 100 by 200 tiny squares. You place a dot in a random location within each square in the grid. The result is something that looks a bit like snow on an old black and white TV– like one of the rectangles in Figure 3. Then you copy the grid dot by dot, but you make a certain change in the copy. You pick some region of it and move every dot in that region slightly to the left, leaving the rest of the dots alone. The right rectangle in the picture is the result of moving a square-shaped region in the middle horizontally. The resulting figure looks to the untrained naked eye just like the first rectangle, but since the visual system is very sensitive to slight disparities, if each eye is presented with one of the two rectangles in a stereo viewer (or if you can “free fuse” the images), the viewer sees a protruding square. The illusion of depth requires the two rectangles to be presented to the two eyes, but if the presentations to the two eyes are at different times, there will be no loss of the experience of depth so long as the two presentations are not separated by too much time. Suppose the left stereogram is presented to the left eye for 10 msecs, and then the right stereogram is presented to the right eye 50 msecs later. No problem: there is an experience of depth. Indeed, one can present the second stereogram up to 80 msecs later and still get the experience of depth. Monocular persistence, persistence of the signal to a single eye, lasts 80 msecs. Think about the left eye getting its stereogram and the right eye then getting its stereogram 50 msecs later. If there is no independent stereo persistence, the depth experience would expire in another 30 msecs, when the monocular persistence to the left eye runs out. But that does not happen. Depth Figure 3. Random-dot stereograms. (Thanks to Ju´lio M. Otuyama.) 490 experience goes on much longer. Engel considered the question of how long one can wait to present another pair of stereograms before the subject loses the experience of depth. He presented sequences of the left stimulus, then the right, then the left, then the right, and so on. If the initial left was followed by a right within 80 msecs, he found that the next left had to come within 300 msecs in order for the subject’s experience of depth to be continuous. That is, the experience of depth lasts 300 msecs. The retina is of course completely monocular: each retinal activation depends on input to just one eye. Indeed, substantial numbers of binocular cells are not found in early vision. The conclusion: Depth requires processing in areas of the visual system higher than early vision. Sothis experiment shows two kinds of persistence, monocular persistence lasting 80 msecs and binocular persistence lasting 300 msecs; and the binocular persistence is clearly phenomenal since it is a matter of the subject continuing to see depth. Here is another item of evidence for the same conclusion. There is phenomenal persistence for visual motion, which cannot be due merely to persistence of neural signals in the retina or in early visual areas. Anne Treisman used a display of 6 dots, each of which moved in a circular pathway, either clockwise or counterclockwise (Treisman et al. 1975). Subjects were asked to report whether the motion was clockwise or counterclockwise. Treisman et al. found a partial report advantage much like that in Sperling’s experiment. (See also Demkiw & Michaels 1976.) Whycan’t this phenomenon be accounted for by neural persistence in the retina or early vision? The point can be understood by imagining a moving dot that goes from left to right on a TV screen. Suppose the screen is phosphorescent so that the images leave a ghost that lasts 1.5 seconds (inspired by the Landman et al. experiment) and suppose that the initial moving dot moves across the screen in 100 msecs. What the viewer will then see is a dot on the left that expands into a line towards the right over a 100 msec period. The line will remain for 1,300 msecs and then it will shrink towards the right to a dot on the right over another 100 msec. The idea of the analogy is to point to the fact that retinal persistence of the signals of a moving object cannot be expected to create phenomenal persistence of the experience of that moving object. Phenomenal persistence has to be based in neural persistence that is a good deal higher in the visual system. As will be discussed later, there is an area in the visual system (V5) that is an excellent candidate for the neural basis of the visual experience of motion. Perhaps the strongest evidence for cortical persistence comes from the Sligte et al. paper mentioned earlier. There is evidence that the persisting visual experience can be divided into two phases. In the first phase, it is indistinguishable from visual perception. This phase typically lasts at most a few hundred msecs, and often less than 100 msecs(unless subjects are dark-adapted, in which case it lasts longer). The persistence of the experience can be tested by many methods, for example, asking subjects to adjust the timing of a second visual stimulus so that what the subject experiences is a seamless uninterrupted visual stimulus. (See Coltheart [1980] for a description of a number of converging experimental paradigms that BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh measure visible persistence.) In the second phase, the subject has a fading but still distinctly visual experience. The first two phases are of high capacity and disturbed if the test stimulus is moved slightly, and easily “masked” (Phillips 1974, Sligte et al. 2008) by stimuli that overlap in contours with the original stimulus. (Such a mask, if presented at the right lag, makes the stimulus hard to see.) Sligte et al. (2008) used dark adaptation to increase the strength of the first phase, producing what could be described as a positive afterimage. They also introduced a further variable, two kinds of stimuli: a black/white stimulus and a red/gray isoluminant stimulus in which the foreground and background have the same level of luminance. The idea was to exploit two well-known differences between rods and cones in the retina. Rods are color blind and also have an extended response to stimulation, whereas cones have a brief burst of activity. Rods react to isoluminant stimuli as to a uniform field. The black and white stimulus in dark adaptation will however maximize rod stimulation, producing longer visible persistence without affecting the later working memory representation (Adelson 1978). Sligte et al. found, not surprisingly, that the black and white stimuli produced very strong visible persistences, much stronger than the isoluminant red and gray stimuli when the cue was given just after the array of figures was turned off. (In arrays with 16 items, the subjects had a capacity of 15 for the black and white stimuli but only 11 for the red and gray stimuli.) Here is the very significant result for the issue of retinal persistence versus cortical persistence. A brief flash of light just after the array wiped out this difference. However, when the flash of light was given later after about 1,000 msecs after the array stimulus, it had no effect. Further, a pattern mask did have a huge effect at 1,000 msecs, lowering the capacity to the level of working memory. The flash of light right after the stimulus interferes with retinal persistence, whereas the pattern mask after 1,000 msecs interfered with cortical persistence. As I mentioned, Sligte et al. used as many as 32 items instead of the 8 of Landman et al. The capacity for the black/white stimulus was close to 32 for the early cue, the capacity of the red/gray stimulus was about 17 and both fell to about 16 for the cue late in the blank space. And both fell further to somewhat over 4– as in Landman et al.– once the new stimulus came on. If the cue was presented 10 msecs after the first stimulus (the analog of [c] in Figure 2), the black/white stimulus produced greater retention, but if the cue was presented late in the blank period (or once the new stimulus came on as in [a]), the black/white and red/grey stimuli were equivalent. The upshot is that the first phase is very high capacity and is over by 1,000 msecs; the second phase is high capacity and lasts up to 4 seconds; and the third phase has a similar capacity to the working memory phase in Sperling and in Landman et al. The results mentioned earlier in connection with the Sperling and the Landman et al. experiments are likely to be based in central parts of the visual system, and so are not due to something analogous to “looking again” as in the imaginary dialog presented earlier. However, the question of exactly which central neural activations constitute phenomenology, as opposed to constituting input to phenomenology, is just the question of what phenomenology is in the brain; and of course the topic of this article is whether that can be empirically investigated. So it may seem that I have unwittingly shown the opposite of what I am trying to show, namely, that every attempt to give an empirical answer ends up presupposing an answer. So how can my argument avoid begging the question? I have three responses. First, the evidence suggests neural persistence at all levels in the visual system. There is no reason to think the phenomenal level is an exception. Second, it appears as if the activations of lower-level vision are relatively brief as compared with the activations of higher-level vision. Third, as mentioned earlier, there is evidence to come that a certain kind of activation of V5 is the core neural basis of the experience of motion. We can see how experimental evidence from phenomenal persistence could dovetail with the evidence outside of memory for V5 as the neural basis for the visual experience of motion. If some version of Treisman’s experiment were done in a scanner, my point of view would predict persisting V5 activations of the aforementioned kind. So the issue is not beyond investigation. 11. The Refrigerator Light Illusion The argument of this article depends on the claim that subjects in the Sperling and the Landman et al. experiments have phenomenal experiences of all or almost all of the shapes in the presented array. One objection is that subjects’ judgments to that effect are the result of an illusion in which they confuse potential phenomenology with actual phenomenology. In order to explain this allegation and defend against it, I will first have to say more about cognitive accessibility. The dominant model of cognitive accessibility in discussions of consciousness– and one that is assumed both in this target article and by Stan Dehaene and his colleagues, the critics who I will be talking about in this section– is a model of broadcasting in a global workspace that started with the work of Bernard Baars (1988; 1997) The idea is closely related to my notion of access consciousness and Dan Dennett’s (1993; 2001) notion of “cerebral celebrity” or fame in the brain.4 Think of perceptual mechanisms as suppliers of representations to consuming mechanisms which include mechanisms of reporting, reasoning, evaluating, deciding, and remembering. There is empirical evidence that it is reasonable to think of perceptual systems as sending representations to a global active storage system, which is closely connected to the consuming systems. Those representations are available to all cognitive mechanisms without further processing. (That’s why blindsight “guesses” don’t count as cognitively accessible in this sense; further processing in the form of guessing is required to access the representations.) This workspace is also called “working” memory– the word “memory” being a bit misleading because, after all, one can report an experience while it is happening without having to remember it in any ordinary sense of the term. Dehaeneandcolleagues (Dehaene et al. 1998; Dehaene & Nacchache 2001) have given impressive evidence that our ability to report our phenomenal states hinges on such a global workspace and that the connection between perception and the workspace lies in long-range neurons in sensory areas in the back of the head which feed forward 491 BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh to the workspace areas in the front of the head. In past publications, I argued for phenomenology without cognitive accessibility (Block 1995a; 1995b; 2001) on the basis of the Sperling experiment. Dehaene and Naccache (2001) replied, making use of the global workspace model: Some information encoded in the nervous system is permanently inaccessible (set I1). Other information is in contact with the workspace and could be consciously amplified if it was attended to (set I2). However, at any given time, only a subset of the latter is mobilized into the workspace (set I3). We wonder whether these distinctions may suffice to capture the intuitions behind Ned Block’s (Block 1995b; see also Block 2001) definitions of phenomenal (P) and access (A) consciousness. What Block sees as a difference in essence could merely be a qualitative difference due to the discrepancy between the size of the potentially accessible information (I2) and the paucity of information that can actually be reported at any given time (I3). Think, for instance, of Sperling’s experiment in which a large visual array of letters seems to be fully visible, yet only a very small subset can be reported. The former may give rise to the intuition of a rich phenomenological world– Block’s P-consciousness– while the latter corresponds to what can be selected, amplified, and passed on to other processes (A-consciousness). Both, however, would be facets of the same underlying phenomenon. (Dehaene & Naccache 2001, p. 30) The distinction between I1,I2, and I3 is certainly useful, but its import depends on which of these categories is supposed to be phenomenal. One option is that representations in both categories I2 (potentially in the workspace) and I3 (in the workspace) are phenomenal. That is not what Dehaene and Naccache have in mind. Their view (see especially section 3.3.1 of their paper) is that only the representations in I3 are phenomenal. They think that representations in the middle category (I2) of potentially in the workspace seem to the subject to be phenomenal but that this is an illusion. The only phenomenal representations are those that are actually in the workspace. But in circumstances in which the merely potential workspace representations can be accessed at will, they seem to us to be phenomenal. That is, the subjects allegedly mistake merely potential phenomenology for actual phenomenology. Importantly, the workspace model exposes a misleading aspect of talk of cognitive accessibility. What it is for representations to be in the workspace (I3) involves both actuality (sent to the workspace) and potential (can be accessed by consuming mechanisms without further processing). The representations that are actually in the workspace are in active contact with the consuming systems, and the consuming systems can (potentially do) make use of those representations. We might speak of the representations in I3 (in the workspace) as cognitively accessible in the narrow sense (in which consuming mechanisms make use of what is already there), and representations in the union of I3 and I2 as cognitively accessible in the broad sense. It is narrow cognitive accessibility that Dehaene et al. identify with phenomenology. When I speak of phenomenology overflowing cognitive accessibility, I mean that the capacity of phenomenology is greater than that of the workspace– so it is narrow accessibility that is at issue. In the rest of this article, I will be using “cognitive accessibility” in the narrow sense. The thesis of this article is that phenomenology does not include 492 cognitive accessibility in the narrow sense. Here we see that as theory, driven by experiment, advances, important distinctions come to light among what appeared at first to be unified phenomena (See Block & Dworkin 1974, on temperature; Churchland1986;1994;2002,onlifeandfire). But what is wrong with the broad sense? Answer: The broad sense encompasses too much, at least if a necessary and sufficient condition of phenomenology is at stake. Representations in I2 can be “amplified if …attended to”, but of course uncontroversially unconscious representations can be amplified too, if one shifts attention to what they represent (Carrasco 2007). So including everything in I2 in consciousness would be a mistake, a point I made (Block 1997) in response to the claim that consciousness correlates with a certain functional role by Chalmers (1997). No doubt a functional notion that is intermediate between narrow and broad could be framed, but the challenge for the framer would be to avoid ad hoc postulation. An experimental demonstration that shifting attention affects phenomenology to a degree sufficient to change a sub-threshold stimulus into a supra-threshold stimulus is to be found in a series of papers by Marisa Carrasco (Carrasco et al. 2004) in which she asked subjects to report the orientation of one of a pair of gratings which had the higher contrast. Carrasco presented an attention-attracting dot on one side of the screen or the others that subject was supposed to ignore, slightly before the pair of gratings. She showed that attention to the left made a grating on the left higher in contrast than it would otherwise have been. In subsequent work (Carrasco 2007), Carrasco has been able to show precisely measurable effects of attentional shifts on contrast and color saturation, but not on hue. This alleged conflation of potential phenomenology with actual phenomenology could be called the Refrigerator Light Illusion5 (Block 2001), the idea being that just as someone might think the refrigerator light is always on, confusing its potential to be on with its actually being on, so subjects in the Sperling and the Landman et al. experiments might think that all the items register phenomenally because they can see any one that they attend to. In the rest of this section, I argue against this allegation. Let us begin by mentioning some types of illusions. There are neurological syndromes in which cognition about one’s own experience is systematically wrong; for example, subjects with anosognosia can complain bitterly about one neural deficit while denying another. And cognitive illusions can be produced reliably in normals (Piattelli-Palmarini 1994). To take a famous example, doctors are more reluctant to recommend surgical intervention if they are told that a disease has a mortality rate of 7% than if they are told it has a survival rate of 93%. Moving to a cognitive illusion that has a more perceptual aspect, much of vision is serial but subjects take the serial processes to be simultaneous and parallel (Nakayama 1990). For example, G. W. McConkie and colleagues (McConkie & Rayner 1975; McConkie & Zola 1979) created an eye-tracking setup in which subjects are reading from a screen of text but only the small area of text surrounding the fixation point (a few letters to the left and 15 to the right) is normal– the rest is perturbed. Subjects have the mistaken impression that the whole page contains normal text. Subjects suppose that the impression BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh of all the items on a page is a result of a single glance, not realizing that building up a representation of a whole page is a serial process. These illusions all have a strong cognitive element. Are the results from experiments like those of Sperling and Landman et al. the result of cognitive illusions? One reason to think they are not is that the phenomena which the Sperling and the Landman et al. experiments depend on do not require that subjects be asked to access any of the items. It is a simple matter to show subjects arrays and ask them what they see without asking them to report any specific items (as was done first in Gill & Dallenbach 1926). This suggests that the analysis of subjects’ judgments in the partial report paradigms as based on cognition– of noticing the easy availability of the items– is wrong. A second point is that cognitive illusions are often, maybe always, curable. For example, the framing illusion mentioned above is certainly curable. However, I doubt that the Sperling and the Landman et al. phenomenology is any different for advocates of the Dehaene view. Third, the sense that in these experiments so much of the perceptual content slips away before one can grab hold of it cognitively does not seem any kind of a cognition but rather is percept-like. Recall, that in the Sperling experiment, the results are the same whether the stimulus is on for 50 msecs or 500 msecs. Steve Schmidt has kindly made a 320 msec demo that is available on my web site at: http://www.nyu.edu/ gsas/dept/philo/faculty/block/demos/Sperling320msec. mov. See for yourself. The suggestion that the putative illusion has a perceptual or quasi-perceptual nature comports with the way Dan Dennett and Kevin O’Regan describe the sparse representations allegedly revealed by change “blindness” (Dennett 1991; O’Regan 1992).6 Their idea is that the way it seems that it seems is– supposedly– not the way it actually seems. They allege not a mismatch between appearance and external reality as in standard visual illusions but rather a mismatch between an appearance and an appearance of an appearance. We could call this alleged kind of illusion in which the introspective phenomenology does not reflect the phenomenology of the state being introspected a hyper-illusion. But are there any clear cases of hyper-illusions? I don’t know of any. One candidate is the claim, often made, that although the “self” is really a discontinuous stream of experiences, we have the illusion that it is a continuous existent (Strawson 2003). But this alleged hyper-illusion is suspect, being perhaps more a matter of failing to experience the gappiness rather than actually experiencing non-gappiness. Further, subjects’ introspective judgments led to the prediction investigated by Sperling, Landman et al., and Sligte et al. One should have an empirical reason to judge that this experimentally confirmed introspective judgment is wrong. Subjects in the Landman et al. experiment are looking right at the rectangles for half a second, a long exposure, and it is not hard to see the orientations clearly. It does not appear to them as if something vaguely rectangularish is coming into view, as if from a distance. In (c) of Landman et al., they see all the rectangle orientations for up to 1.5 seconds in the Landman et al. version and up to 4 seconds in the Sligte et al. version. It is hard to believe that people are wrong about the appearances for such a long period. Dehaene et al. (2006) revisit this issue. Here are the relevant paragraphs (references are theirs): The philosopher Ned Block, however, has suggested that the reportability criterion underestimates conscious contents (Block 2005). Whenweviewacomplexvisual scene, weexperience a richness of content that seems to go beyond what we can report. This intuition led Block to propose a distinct state of “phenomenal consciousness” prior to global access. This proposal receives an apparent confirmation in Sperling’s iconic memory paradigm. When an array of letters is flashed, viewers claim to see the whole array, although they can later report only one subsequently cued row or column. One might conclude that the initial processing of the array, prior to attentional selection of a row or column is already phenomenally conscious. (Block 2005, Lamme 2003) However, those intuitions are questionable, because viewers are known to be over-confident and to suffer from an “illusion of seeing”. (O’Regan & Noe¨ 2001). The change blindness paradigm demonstrates this “discrepancy between what we see and what we think we see” (Simons & Ambinder 2005). In this paradigm, viewers who claim to perceive an entire visual scene fail to notice when an important element of the scene changes. This suggests that, at any given time, very little of the scene is actually consciously processed. Interestingly, changes that attract attention or occur at an attended location are immediately detected. Thus, the illusion of seeing may arise because viewers know that they can, at will, orient attention to any location and obtain conscious information from it. (Dehaene et al. 2006, p. 210) Dehaene and his colleagues propose to use the change “blindness” results to back up their view of the Sperling result. But the issues in these two paradigms are pretty much the same– our view of one is conditioned by our view of the other. Further, as I mentioned earlier, the f irst form of the Landman et al. experiment (see Fig. 2, Part [a]) is itself an experiment in the same vein as the standard change “blindness” experiments. The subject sees 8 things clearly but has the capacity (in the sense of Cowan’s K) to make comparisons for only 4 of them. And so the Landman et al. experiment– since it gives evidence that the subject really does see all or almost all the rectangles– argues against the interpretation of the change “blindness” experiments given by Dehaene and his colleagues. Dehaene et al. (2006) say, “The change blindness paradigm demonstrates this discrepancy between what we see and what we think we see.” But this claim is hotly contested in the experimental community, including by one of the authors that they cite. As I mentioned earlier (see Note 2), many psychologists would agree that initial interpretations of change “blindness” went overboard and that, rather than seeing the phenomenon as a form of inattentional blindness, one might see it as a form of inattentional inaccessibility (Block 2001). That is, the subject takes in the relevant detail of each of the presented items, but they are not conceptualized at a level that allows the subject to make a comparison. As Fred Dretske (2004) has noted, the difference between the two stimuli in a change blindness experiment can be one object that appears or disappears, and one can be aware of that object that constitutes the difference without noticing that there is a difference. 493 BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh CompareFigure 1 with Figure 4. It can be hard for subjects to see the difference between Figure 1 and Figure 4, even when they are looking right at the feature that changes. The idea that one cannot see the feature that changes strains credulity. Two of the originators of the change “blindness” experiments, Dan Simons and Ron Rensink (see Simons & Rensink 2005a) have since acknowledged that the “blindness” interpretations are not well supported by the “change blindness” experiments. In a discussion of a response by Alva Noe¨ (2005), they summarize (Simons and Rensink 2005b): Weandothersfoundthe“sparse representations” view appealing (and still do), and initially made the overly strong claim that change blindness supports the conclusion of sparse representations (Rensink et al. 1997; Simons 1997). We wrote our article because change blindness continues to be taken as evidence for sparse– or even absent– representations, and we used O’Regan and Noe¨’s influential paper ( O’Regan & Noe 2001) as an example. However, as has been noted for some time …this conclusion is logically flawed. (Simons & Rensink 2005b, p. 219) I have been appealing to what the subjects say in Sperling-like experiments about seeing all or almost all the items. However, there is some experimental confirmation of what the subjects say in a different paradigm. Geoffrey Loftus and his colleagues (Loftus & Irwin 1998) used a task devised by Vincent Di Lollo (1980) and his colleagues using a 5 by 5 grid in which all but one square is filled with a dot. Loftus et al. divided the dots into two groups of 12, showing subjects first one group of 12 briefly, then a pause, then the other group of 12 briefly. The subjects always were given partial grids, never whole grids. Subjects were asked to report the location of the missing dot– something that is easy to do if you have a visual impression of the whole grid. In a separate test with no missing dots, subjects were asked to judge on a scale of 1 to 4 how temporally integrated the matrix appeared to be. A “4” meant that one complete matrix appeared to have been presented, whereas a “1” meant that two separate displays had been Figure 4. Compare this with Figure 1 without looking at the two figures side by side. There is a difference that can be hard to see. 494 presented. The numerical ratings are judgments that reflect phenomenology: how complete the grids looked. The length of the first exposure and the time between exposures was varied. The Loftus et al. experiment probes persistence of phenomenology without using the partial report technique that leads Dehaene and his colleagues (2001; 2006) to suggest the Refrigerator Light illusion. The result is that subjects’ ability to judge which dot was missing correlated nearly perfectly with their phenomenological judgments of whether there appeared to be a whole matrix as opposed to two separate partial matrices. That is, the subjects reported the experience of seeing a whole matrix if and only if they could pick out the missing dot, thus confirming the subjects’ phenomenological reports. To sum up: (1) the subjects’ introspective judgments in the experiments mentioned are that they see all or almost all of the items. Dehaene and his colleagues (2001; 2006) seem to agree since that is entailed by the claim that the introspective judgments are illusory. (2) This introspective judgment is not contingent on subjects’ being asked to report items as would be expected on the illusion hypothesis. (3) This introspective judgment leads to the prediction of partial report superiority, a prediction that is borne out. (4) The accuracy of the subjects’ judgments is suggested by the fact that subjects are able to recover both size and orientation information with no loss. (5) These results cohere with a completely different paradigm– the Loftus paradigm just mentioned. (6) Dehaene and his colleagues offer no empirical support other than the corresponding theory of the change “blindness” results which raise exactly the same issues. The conclusion of this line of argument is, as mentioned before, that phenomenology overflows cognitive accessibility and so phenomenology and cognitive access are based at least partly in different systems with different properties. I will be moving to the promised argument that appeals to the mesh between psychology and neuroscience after I fill in some of the missing premises in the argument, the first of which is the evidence for a capacity of visual working memory of roughly four or less. 12. Visual working memory At a neural level, we can distinguish between memory that is coded in the active firing of neurons– and ceases when that neuronal firing ceases– and structural memory that depends on changes in the neural hardware itself, for example, change in strength of synapses. The active memory– which is active in the sense that it has to be actively maintained– is sometimes described as “short term”– a misdescription since it lasts as long as active firing lasts, which need not be a short time if the subject is actively rehearsing. In this target article, the active memory buffer is called “working memory”. YoumayhaveheardofafamouspaperbyGeorgeMiller called “The magical number seven, plus or minus two: Some limits on our capacity for processing information” (Miller 1956). Although Miller was more circumspect, this paper has been widely cited as a manifesto for the view that there is a single active memory system in the brain that has a capacity of seven plus or minus two “items.” What is an item? There are some experimental BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh results that fill-in this notion a bit. For example, HuntleyFenner et al. (2002) showed that infants’ visual object tracking system– which, there is some reason to believe, makes use of working memory representations– does not track piles of sand that are poured, but does track them if they are rigid. One constraint on what an item might be comes from some experiments that show that although we can remember only about four items, we can also remember up to four features of each one. Luck and Vogel asked subjects to detect changes in a task somewhat similar to the Landman et al. task already mentioned. They found that subjects could detect changes in four features (color, orientation, size, and the presence or absence of a gap in a figure) without being significantly less accurate than if they were asked to detect only one feature (Luck & Vogel 1997; Vogel et al. 2001). In the 50 years since Miller’s paper, reasons have emerged to question whether there really is a single active memory system as opposed to a small number of linked systems connected to separate modalities and perhaps separate modules– for example, language. Some brain injuries damage verbal working memory but not spatial working memory (Basso et al. 1982), and others have the opposite effect (Hanley et al. 1991). And evidence has accumulated that the capacity of these working memories– especially visual working memory– is actually lower than seven items (Cowan 2001, Cowan et al. 2006). The suggestion of seven items was originally made plausible by experiments demonstrating that people, if read lists of digits, words or letters, can repeat back about seven of them. Of course, they can repeat more items if the items can be “chunked.” Few Americans will have trouble holding the nine letters “FBICIAIRS” in mind, because the letters can be chunked into 3 acronyms. Morerelevant to our discussion, visual working memory experiments also come up with capacities in the vicinity of four, or fewer than four, items. (For work that suggests fewer than four, see McElree 2006). Whether there is one working memory system that is used in all modalities or overlapping systems that differ to some extent between modalities, this result is what is relevant to the experiments discussed above. Indeed, you have seen three examples in this target article itself: the Sperling, Landman et al., and Sligte et al. experiments. I will briefly mention a few other quite different paradigms that have come up with the same number. One such paradigm involves the number of items that people– and monkeys– can effortlessly keep track of. For example, at a rhesus macaque monkey colony on a small island off of Puerto Rico, Marc Hauser and his colleagues (Hauser et al. 2000) did the following experiment: Two experimenters find a monkey relaxing on its own. Each experimenter has a small bucket and a pocket full of apple slices. The experimenters put down the buckets and, one at a time, they conspicuously place a small number of slices in each bucket. Then they withdraw and check which bucket the monkey goes to in order to get the apple slices. The result is that for numbers of slices equal to or smaller than four, the monkeys overwhelmingly choose the bucket with more slices. But if either bucket has more than four, the monkeys choose at random. In particular, monkeys chose the greater number in comparison of one versus two, two versus three, and three versus four, but they chose at random in cases of four versus five, four versus six, four versus eight, and, amazingly, three versus eight. The comparison of the three versus four case (where monkeys chose more) and the three versus eight case (where they chose at random) is especially telling. The eight apple slices simply overflowed working memory storage. Infant humans show similar results, although typically with a limit more in the vicinity of three rather than four (Feigenson et al. 2002). Using graham crackers instead of apple slices, Feigenson et al. found that infants would crawl to the bucket with more crackers in the cases of one versus two and two versus three, but were at chance in the case of one versus four. Again, four crackers overf lows working memory. In one interesting variant, infants are shown a closed container into which the experimenter– again conspicuously– inserts a small number of desirable objects (e.g., M&Ms). If the number of M&Ms is one, two, or three, the infant continues to reach into the container until all are removed, but if the number is more than three, infants reach into the container just once (Feigenson & Carey 2003). I mentioned above that some studies have shown that people can recall about four items including a number of features of each one. However, other studies (Xu 2002) have suggested smaller working memory capacities for more complex items. Xu and Chun (2006) have perhaps resolved this controversy by showing that there are two different systems with somewhat different brain bases. One of these systems has a capacity of about four spatial locations, or objects at four different spatial locations, independent of complexity; the other has a smaller capacity depending on the complexity of the representation. The upshot for our purposes is that neither visual working memory system has a capacity higher than four. This section is intended to back up the claim made earlier about the capacity of working memory– at least visual working memory. I move now to a quick rundown on working memory and phenomenology in the brain with an eye to giving more evidence that we are dealing with at least partially distinct systems with different properties. 13. Working memory and phenomenology in the brain Correlationism in its metaphysical form (which, you may recall, regards cognitive accessibility as part of phenomenology) would have predicted that the machinery underlying cognitive access and underlying phenomenal character would be inextricably entwined in the brain. But the facts so far can be seen to point in the opposite direction, or so Iargue. In many of the experiments mentioned so far, a brief stimulus is presented, then there is a delay before a response is required. What happens in the brain during the delay period? In experiments on monkeys using this paradigm, it has been found that neurons in the upper sides of the prefrontal cortex (dorsolateral prefrontal cortex) fire during the delay period. And errors are correlated with decreased firing in this period (Fuster 1973; Goldman-Rakic 1987). Further, damage to neurons in 495 BEHAVIORAL AND BRAIN SCIENCES (2007) 30:5/6Block: Consciousness, accessibility, and the mesh this area has been found to impair delayed performance, but not simultaneous performance, and damage to other memory systems does not interfere with delayed performance (except possibly damage to parahippocampal regions in the case of novel stimuli; Hasselmo & Stern 2006). Infant monkeys (1.5 months old) are as impaired as adult monkeys with this area ablated, and if the infant area is ablated, the infants do not develop working memory capacity. It appears that this prefrontal area does not itself store sensory signals, but rather, is the main factor in maintaining representations in sensory, sensorimotor, and spatial centers in the back of the head. As Curtis and D’Esposito (2003, p. 415) note, the evidence suggests that this frontal area “aids in the maintenance of information by directing attention to internal representations of sensory stimuli and motor plans that are stored in more posterior regions.” That is, the frontal area is coupled to and maintains sensory representations in the back of the head that represent, for example, color, shape, and motion. (See Super et al. [2001a] for an exploration of the effect of this control on the posterior regions.) The main point is that, as the main control area for working memory, this prefrontal area is the main bottleneck in working memory, the limited capacity system that makes the capacity of working memory what it is. So the first half of my brain-oriented point is that the control of working memory is in the front of the head. The second half is that, arguably, the core neural basis of visual phenomenology is in the back of the head. I will illustrate this point with the example of one kind of visual experience of motion (typified by optic flow). But f irst a caution: No doubt the neural details presented here are wrong, or at least highly incomplete. We are still in early days. My point is that the evidence does point in a certain direction, and more important, we can see how the issues I have been talking about could be resolved empirically. Here is a brief summary of some of the vast array of evidence that the core neural basis of one kind of visual experience of motion is activation of a certain sort in a region in the back of the head centered on the area known as V57. The evidence includes: Activation of V5 occurs during motion perception (Heeger et al. 1999). Microstimulation to monkey V5 while the monkey viewed moving dots influencedthe monkey’s motion judgments, depending on the directionality of the cortical column stimulated (Britten et al. 1992). Bilateral (both sides of the brain) damage to a region that is likely to include V5 in humans causes akinetopsia, the inability to perceive– and to have visual experiences as of motion. (Akinetopsic subjects see motion as a series of stills.) (Rees et al. 2002a, Zihl et al. 1983). The motion aftereffect– a moving afterimage– occurs when subjects adapt to a moving pattern and then look at a stationary pattern. (This occurs, for example, in the famous “waterfall illusion.”) These moving afterimages also activate V5 (Huk et al. 2001). Transcranial magnetic stimulation (TMS8) applied to V5 disrupts these moving afterimages (Theoret et al. 2002). V5 is activated even when subjects view “implied motion” in still photographs, for example, of a discus thrower in mid-throw (Kourtzi & Kanwisher 2000). 496 TMS applied to visual cortex in the right circumstances causes stationary phosphenes9– brief flashes of light and color. (Kammer 1999) When TMS is applied to V5, it causes subjects to experience moving phosphenes (Cowey & Walsh 2000). However, mere activation over a certain threshold in V5 is not enough for the experience as of motion: the activation probably has to be part of a recurrent feedback loop to lower areas (Kamitani & Tong 2005; Lamme 2003; Pollen 2003; Super et al. 2001a). Pascual-Leone and Walsh (2001) applied TMS to both V5 and V1 in human subjects, with the TMS coils placed so that the stationary phosphenes determined by the pulses to V1 and the moving phosphenes from pulses to V5 overlapped in visual space. When the pulse to V1 was applied roughly 50 msecs later than to V5, subjects said that their phosphenes were mostly stationary instead of moving. The delays are consonant with the time for feedback between V5 and V1, which suggests that experiencing moving phosphenes depends not only on activation of V5, but also on a recurrent feedback loop in which signals go back to V1 and then forward to V5. Silvanto and colleagues (Silvanto et al. 2005a; 2005b) showed subjects a brief presentation of an array of moving dots. The experimenters pinpointed the precise time– call it t–at which zapping V5 with TMS would disrupt the perception of movement. Then they determined that zapping V1 either 50 msecs before t or 50 msecs after t would also interfere with the perception of the moving dots. But zapping V5 a further 50 msecs after that (i.e., 100 msecs after t) had no effect. Silvanto et al. argue that in zapping V1 50 msecs before t, they are intercepting the visual signal on its way to V5, and in zapping V1 50 msecs after t, they are interfering with the recurrent loop. These results suggest that one V1-V5-V1 loop is the core neural basis for at least one kind of visual experience as of motion (and also necessary for that kind of experience in humans). Recurrent loops also seem to be core neural bases for other types of contents of experience (Supe`r et al. 2001a). The overall conclusion is that there are different core neural bases for different phenomenal characters. (Zeki and his colleagues have argued for a similar conclusion, using Zeki’s notion of micro-consciousness [Pins &ffytche 2003; Zeki 2001]).10