The Contents Question

The role that conscious experience can play in vision science depends on our understanding of what such experience reveals. Our first impression is that what conscious experience reveals is both private and obvious to the person who experiences it: if you see something or other, then what the experience reveals is just what you see. But if you try to say what that is, you find it is far from straightforward. Indeed there is a long chapter in the history of psychology of the late nineteenth and early twentieth centuries, where that question was at the forefront of discussions of what psychology is and what it should be (Titchener 1912;Washburn 1922).1 The introspective method was taught as an objective way to study the contents of conscious experience by turning the mind on itself in a disciplined analytical manner, freeing it to report its own conscious contents, as opposed to reporting properties of the object of our perception (inadvertently being influenced by what one knew about the objects of perception was known as the stimulus error). In the end the method failed to provide the foundations for a science of conscious experience, although conscious experience itself continues to be a growing concern in both psychology and philoso-phy.2 It is not my purpose here to discuss introspection or to look at the fascinating history of the study of consciousness in psychology. I wish only to point out some of the problems raised by the use of conscious contents as a source of evidence for building theories of perception.

There are two sets of questions about our conscious awareness. One is what might be called the objective scientific question: What are we entitled to conclude about perception from certain perceptual experiences we have? The other, logically prior question is what the content of our perceptual experience is: What is the thing about which questions of interpretation can be raised? This question is independent of methodological issues concerned with how one should interpret reports of ''how something looks.'' The question even applies to one's understanding of the content of one's own conscious experiences. The question—what do I experience when I look at this stimulus?—is fraught with problems. One might reasonably take the position that to ask what we experience is already to take a theoretical stand, namely that the content of the experience is transparent to the person who experiences it; it is part of what Sellars (1956) called ''the given.'' There has been a considerable amount of philosophical discussion of this question. The assumption that one is the infallible arbiter of the content of one's conscious experience has serious problems, particularly if one takes

1. For original writings from this era, and earlier, see the interesting website ''Classics in the History of Psychology'' maintained by Christopher D. Green at York University, Toronto, Canada: http://psychclassics.yorku.ca/index.htm/.

2. The Association for the Scientific Study of Consciousness (ASSC) has become a major scholarly society with a large annual meeting where, among other things, the relation between brain and consciousness is discussed and neuropsychologists report various fascinating brain damage syndromes in which patients exhibit disconnects between behavior and conscious contents. See http://assc.caltech.edu/index.htm/.

it to have conceptual content—that is, to be the experience as of something or other.

Consider what we experience when we look at a scene. Suppose I look at the wall of my room;what is the content of my conscious experience? If I try to describe what I am experiencing I find myself describing the things in the room and their visible properties (such as color, texture, or location). My conscious experience is the experience of the things that I perceive ( possibly also what these things remind me of and what feelings they may arouse, but let's confine ourselves to the perceptual experience itself). How exactly can I describe what I experience? Does it consist of all the properties that are in the scene, or only those that I notice? Do I experience only what is present in the incoming information, or does my experience also include what the visual system ''fills in'' and what I infer? For example, do I experience the uniformity of the color and lightness of the wall which, as it happens, I know is not in fact uniformly illuminated? Is the uniformity of lightness and color constancy that I am describing an inference or a direct experiential content? The lightness you perceive is known to depend on your perception of the location and arrangement of the surfaces in question (Gilchrist 1977). Also in a typical scene it is rare that I see all of any object because most things will be at least partially occluded by others, even though I do not notice these occlusions unless they are brought to my attention. It thus appears that the content of my experience includes postconstancy and post-filling-in information, and therefore, my experience relies on more than just the information coming from my eyes.

Let's continue with this example. There is a picture on the wall. Do I experience it as a picture or do I experience what the picture depicts—or both? There is also a calendar on the wall which I see and which is therefore part of my experience. Since I forgot to change the page at the end of last month it shows the wrong month. Do I experience it as a calendar, and do I experience it as showing the wrong month? Ordinary informal talk is unclear on such issues. If I do experience it as a calendar showing the wrong month, then other people looking at the wall are unlikely to have the same experience as I do unless they know what I know about the calendar. I look down at my desk and see a sculpture that serves as a paperweight (or perhaps vice versa, depending on how you feel about such pieces). I see it as a three-dimensional object that has not only a front but also a back and sides and parts that are hidden or occluded by other parts. Do I experience only the front, or only the parts that are not occluded, or do I experience the back and the hidden parts as well? I do not see the back in the sense that I receive no optical information from it, so how can it be part of my experience? Some writers in the Gibson "direct perception'' tradition claim that we see the back of 3-D objects as well as the front because both are part of the experience of what Alva Noe (2004) calls the perceptual presence, which is different from our knowledge of the back of a perceived object. This may actually be the more common view. For example, Block (1995) says that if you are looking at a row of buildings and then find out that they are mere fronts of a movie set, the content of your conscious experience changes. According to Block, the visual experience as of a facade is different from the visual experience as of a building. If that is the case, then conscious content is clearly cognitively penetrable, which affects the role it can play in a perceptual theory. It cannot, for example, serve as the imput to vision, as what some philosophers call the ''given.''

Many also say that what we see, our perceptual experience, is viewpoint independent, which implies that we represent it as a solid without giving special status to the surface that faces the viewer. By contrast others insist that what we see is just the front of 3-D objects, and thus that what we see clearly depends on our viewpoint. David Marr's theory of vision explicitly provides a middle ground by proposing what he called a "2|-D sketch,'' which is a representation in depth of only the visible surfaces. Which of these is the content of our experience? For Gaetano Kanizsa (Kanizsa and Gerbino 1982), the perceptual reconstruction of occluded contours is an automatic and cognitively impenetrable stage in the process of seeing (I have also defended this view in Pylyshyn 1999). According to this view, what we experience when we see is not the incoming information but is a complex output of our early vision system together with some inferences, perhaps from other parts of the scene or perhaps from our knowledge and expectations of what is in the scene. Is all this part of our conscious perceptual content? It is certainly what we mean when we report what something ''looks like,'' so at least in the everyday sense it is part of our conscious content. Where do we draw the line? In Pylyshyn 2003 (chap. 1) I give examples to illustrate that the everyday nontechnical sense of ''what something looks like'' is very broad and includes visual puns of the sort popularized by Roger Price in what he called ''droodles'' (see http://www.droodles.com/).

In several thoughtful essays, Fred Dretske (e.g., Dretske 1993, 2006) adds to the perplexity for those who would appeal to the content of conscious experience in building theories of vision, by arguing that we may not always be aware of the content of our experience. That's because, according to Dretske, there is a difference between being conscious of things and being conscious of facts. That one is conscious of something is itself a fact of which we may or may not be conscious. Dretske gives the example of looking at a wall made of hundreds of orange bricks. Given enough time to scan the wall, does our experience include the experience of each of the bricks? Dretske claims it does, because if asked whether there was a blue brick among the orange ones we can confidently answer no. Dretske claims that this implies that we saw (and experienced) each of the bricks since the information that there was no blue brick depends on having been conscious of the properties of each brick.3 Yet if asked we might, quite reasonably, claim that we were not conscious of each of the bricks. According to Dretske that just shows that we need not be aware of the conscious content of our perception. Other philosophers have also spoken about the difference between phenomenal and nonphenomenal consciousness (e.g., Lormand 1996), thus further complicating the problem of using conscious contents for theory construction.

There are many examples of our being unaware of information that was readily perceivable and that, by other criteria, was in fact perceived. The question one might ask of each of them is whether they are cases in which we are not conscious of the information, or cases in which we are conscious of the information but were unaware that we were conscious of it. Examples include various cases of apparent functional ''blindness.'' One of the best-known examples is referred to as change blindness. In these demonstrations subjects are unable to report the change between two alternating briefly presented pictures even though the difference between the two pictures is clearly visible when attention is drawn to it (Simons and Levin 1997;Simons and Rensink 2005). Another example is inattentional blindness, in which subjects fail to see a clearly visible feature that occurs at precisely the point where they were visually fixated while they are attending to a more peripheral item (Mack and Rock 1998). Another such example that is extremely persuasive and puzzling involves watching a movie with several players who are passing a ball around while the subject is required to count the number of passes. In this example many subjects are unaware of a person dressed in a gorilla suit who walks right through the middle of the scene (Simons and Chabris 1999). These types of blindness appear to

3. In this example, however, judging that there were no blue bricks is likely an inference of the form: (1) If there had been one clearly visible blue brick I would have seen it; (2) I did not notice a blue brick; therefore (3) there was no blue brick. Thus it does not entail that information from each brick was perceptually (consciously) available, only that the perceiver believes that if there had been a blue brick he would have seen it. This is known in the computational inference field as negation as failure and is entailed by the closed world assumption that is part of the logic programming language Prolog. (See entry in Wikipedia.org.)

involve a failure of information to reach consciousness even though the information is in some sense clearly taken in since it is located on the fovea directly in view. In fact, Dretske (2006) claims that the lesson we should take from such examples is precisely that one can be conscious of something and at the same time be unaware that one is conscious of it.

The distinction between consciousness of things and consciousness of facts is similar in spirit, and might perhaps even be subsumed under, the distinction between phenomenal consciousness and access consciousness advocated by Ned Block (1995). According to Block, there are two functions of consciousness that should be distinguished: the purely phenomenal function (characterized as ''what it is like to be in that state'') and the access function (characterized as states in which information is ''poised to be used as a premise in reasoning, . . . the rational control of action . . . and speech,'' Block 1995, p. 230). These are referred to as P-consciousness and A-consciousness, respectively. These two functions are not only conceptually distinct but also may involve different neural mechanisms (Block 2005). According to this view it is possible to have a vivid phenomenally conscious experience that does not ''broadcast'' information to other mental processes, and thus it is possible to have functional access to information that is accompanied by little or no phenomenal experience. Although Block often talks as though these were two forms of consciousness, he means a ''phenomenal aspect'' or ''phenomenal content'' in contrast to ''representational aspect'' or ''representational content'' of consciousness. Although these two aspects nearly always occur together they are conceptually separable, and in some cases empirically separated, as when one or the other is damaged by brain lesions. As we have already seen, various types of ''blindness'' demonstrate information access without phenomenal consciousness. Cases of phenomenal content without access (without representational content) are more difficult to find inasmuch as the best evidence for phenomenal content takes the form of verbal reports which ipso facto constitutes evidence for informational access.4 The best example may come from split-brain patients who are able to carry out certain tasks

4. Rosenthal (2005, pp. 191-192) argues that it is hard to reconcile cases of P-consciousness without A-consciousness with the understanding of P-consciousness as ''what it would be like'' to be in that state, since there is no way to be in that state unless one is aware of it—i.e., without there being some A-conscious aspect we can use to identify those states. This debate is of interest for an understanding of the nature of conscious states, but it does not bear on the current point which is concerned with what we can learn about perception if we set ''conscious content'' as an explanandum.

without being aware of why they are making the choices they make (discussed in section 4.3.2 below).

Another quite different view of consciousness is provided by David Rosenthal (2005), who argues that consciousness consists of being the target of ''higher-order thoughts'' (HOTs) or of having noninferential (unme-diated and typically unconscious) thoughts about one's thoughts. This view, like other views about what it is to be conscious, deals with issues that are beyond the scope of the present discussion. I mention the HOT view, however, because although it is certainly very different from those of Dretske or Block, it does have room to encompass the distinction between conscious experience and awareness of conscious experience or between P-consciousness and A-consciousness. As long as you think that thoughts about thoughts are a real possibility and recognize that they (sometimes) underwrite conscious contents, you might consider cases where thoughts about thoughts do not yield conscious states, and also cases in which conscious states can arise from thoughts about other sorts of mental states besides thoughts (e.g., desires, acts of will). Because all three views allow for a certain degree of independence between qualitative experience and information-processing functions, all these options allow the possibility of being conscious of something without being aware of what you are conscious of, or of phenomenal consciousness without access consciousness and vice versa.

My point here is not to advocate a particular way of interpreting the notion of conscious experience, but merely to point out that while the content of experience is important for building theories of perception, it is encumbered with many problems. At the very least the examples above show that whether something is or is not part of the content of our experience is not self-evident, so experiential content is not something we can take at face value merely on the grounds that since it is your experience you alone are the authority on its content. In addition, there is no reason why you should be able to say what the theoretically relevant aspect of the experience is and, even worse, you can also get this wrong—as we will see in other examples I will provide below.

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