email: m.velmans@gold.ac.uk
URL: http://www.gold.ac.uk/academic/ps/velmans.htm
KEYWORDS: Consciousness, Brain, Reflexive, Perception, Dualism,
Reductionism, Physical, Mind, Representationalism, Idealism, Realism.
ABSTRACT
Dualist and Reductionist theories of mind disagree about whether or not consciousness can be reduced to a state of or function of the brain. They assume, however, that the contents of consciousness are separate from the external physical world as-perceived. According to the present paper this assumption has no foundation either in everyday experience or in science. Drawing on evidence for perceptual projection in both interoceptive and exteroceptive sense modalities, the case is made that the physical world as-perceived is a construct of perceptual processing and, therefore, part of the contents of consciousness. A finding which requires a Reflexive rather than a Dualist or Reductionist model of how consciousness relates to the brain and the physical world. The physical world as-perceived may, in turn be thought of as a biologically useful model of the world as described by physics. Redrawing the boundaries of consciousness to include the physical world as-perceived undermines the conventional separation of the 'mental' from the physical', and with it the very foundation of the Dualist-Reductionist debate. The alternative Reflexive model departs radically from current conventions, with consequences for many aspects of consciousness theory and research. Some of the consequences which bear on the internal consistency and intuitive plausibility of the model are explored, e.g. the causal sequence in perception, representationalism, a suggested resolution of the Realism versus Idealism debate, and the way manifest differences between physical events as-perceived and other conscious events (images, dreams, etc.) are to be construed.
In the present paper I wish to challenge some of our most deeply-rooted assumptions about what consciousness is, by re-examining how consciousness, the human brain, and the surrounding physical world relate to each other.
The current debate surrounding these issues was, in part, framed by Descartes in his Treatise of Man (1644). According to Descartes, external (physical) reality is mapped in point to point fashion onto an image on the retina, and this produces motions of the "animal spirits" contained in the nerves, which are transmitted in hydraulic fashion to the pineal gland. Motions of the animal spirits within the pineal gland, in turn, produce representations (of the external reality) within the soul (in the form of conscious experiences).
20th Century Dualists adopt very different neurophysiological models (see, e.g. Eccles, 1980,1987; Gazzaniga & Le Doux, 1978; Penfield, 1975; Sherrington, 1942) but, in essence, the relationship between consciousness, the brain, and the external world remain the same. Figure 1, taken from Gazzaniga & Le Doux (1978) for example, incorporates recent findings regarding the way left and right visual fields are mapped onto the contralateral and ipsilateral hemispheres. Nonetheless, brain states continue to represent states of affairs out-there in the world and conscious experiences appear (within the cloud in the figure) quite separate from the brain.
----------------
Figure 1. A Dualist model of how the conscious mind relates to the brain
and the external world (taken from Gazzaniga & Le Doux, 1978).
----------------
Given the success of physical science, it is hardly surprising that this "cloud" model of consciousness has come under much attack. If consciousness is something quite unlike the material substance of the brain and the surrounding physical world, then what is it? According to Descartes it is a "substance which thinks" (res cogitans) but what kind of substance is that? And if consciousness is something non-material, how could it interact with the material stuff of the brain in the ways that Descartes and other Dualists (e.g. Eccles, 1980,1987) propose? Given such serious, obvious problems, it is worth turning briefly to the intuitive appeal of Dualism as a philosophy of mind.
Theological considerations may, to some extent, have fostered the view that consciousness is separate from brain (e.g. via Descartes' preoccupation with the nature of the soul). At heart, however, Dualism is an attempt to make sense of everyday, human, conscious experience - a phenomenon that cannot be easily accommodated within reductionist, materialist science. As Eccles(1980) notes, the existence of consciousness can hardly be doubted, yet one could not hope to find it by any conceivable histological examination of the brain. Nor does materialist science (in its present form) give any account of how consciousness evolved.
The notion that consciousness is something other than brain substance, furthermore, is implicit in some of our everyday assumptions about how the contents of consciousness differ from the external physical world.
Consider, for example, the following propositions, and decide which
of them are true:
Propositions 3, 4 and 5, which deal with some of the qualities of conscious experience also command widespread assent (if my straw poll of psychology students is any guide) and they too, are consistent with Dualism. Such qualities, however, pose various problems for Reductionism.
Functionalism, in viewing consciousness as a brain process or mode of functioning attempts to sidestep such problems. Modes of brain functioning need not have a precise location or extension in that they may be emergent properties of the entire, interconnected brain. Furthermore, as Putnam(1975) (and Aristotle before him) points out, such functions are not physical properties in the normal sense. They describe the form rather than the matter of the brain, thereby capturing something of the "insubstantiality" of consciousness without needing to postulate the existence of some added, non-material substance or entity. However, properties 3,4 and 5 refer not to how consciousness functions but rather to its contents (often referred to as qualia in modern philosophy of mind). More to the point, there is reason to doubt that consciousness has a function (let alone is a function) in the activities of brain (see e.g. Kihlstrom,1987; Velmans,1989).
Central-state Identity theory, which attempts to identify consciousness with the neurophysiological structure of the brain faces similar problems. If the contents of consciousness are insubstantial, and have no location or extension in space, then how can they be ontologically identical to brain states which undoubtedly do have physical properties, and do have both location and extension in space? Identity theorists have responded by challenging the legitimacy of our everyday intuitions about consciousness, likening them to a primitive theory which is destined for replacement by the advance of neurophysiological science (Armstrong,1968; Churchland, 1988). Alternatively, according to Searle(1987) we need to extend our concept of the "physical", accepting, for example, that physical systems such as the brain have subjective properties (which account for the "privacy" of conscious experience). At the present time, however, the promise of enlightenment by neurophysiological advance remains nothing more than a promise (which may never be fulfilled). The proposal that physical systems have subjective properties, furthermore, raises as many problems as it solves.
All this is familiar territory, and needless to say, there have been many other, ingenious attempts to make sense of our everyday intuitions regarding consciousness without returning to the mysteries of Dualist theory (see, e.g. Blakemore & Greenfield, 1987 and Marcell & Bisiach, 1988 for a range of recent views). It is unnecessary for the purposes of the present paper to review these proposals or for that matter to cover the defence of, or the attack on Dualism in any greater depth. My purpose is rather more insidious.
I propose to show that the debate between Dualists and Reductionists has its roots in misleading assumptions that they share, e.g. I take issue with propositions 1 and 2 above. Along with the Reductionists, I also take issue with propositions 3,4 and 5, but contrary to the Reductionists, I argue that they need to be replaced by an expanded rather than a reduced view of conscious experience. Furthermore, far from questioning the legitimacy of our everyday experience, I argue that propositions 1 to 5 do not correspond to our everyday experience, nor, for that matter, to any scientific findings. Rather, they are a misconstrual of everyday experience that has its origins in an inappropriate model of how consciousness, brain, and physical world relate to each other. This is the "cloud" model shown in Figure 1....a model which forms the departure point for both Dualists and Reductionists, which therefore provides the grounds for their dispute.
This dispute, of course, is over whether or not the "cloud" can be reduced to a state of, or function of the brain - whether the Dualist model in Figures 1 and 2 should be replaced by the Reductionist model shown in Figure 3. I argue, by contrast, that both Dualism and Reductionism need to be replaced by the Reflexive model shown in Figure 4.
-----------------
Figure 2. A Dualist model of the causal sequence in visual perception. Light rays from a cat (as-perceived by an Experimenter) impinge on the Subject's eye. Impulses travelling up the optic nerve produce a neural representation of the cat within S's central nervous system. CNS activity, in turn, has a causal influence on S's mind, resulting in a percept of a cat. It is central to this model that the percept (of a cat) in the mind of S is quite separate both from the neural representation (of a cat) in S's brain and the cat (as-perceived by E) out-there in the world.
Figure 3. A Reductionist model of the causal sequence in visual perception. Light rays from a cat (as-perceived by an Experimenter) impinge on the subjects eye. Impulses travelling up the optic nerve produce a neural representation of the cat within S's central nervous system. This CNS activity is subjectively experienced as a percept of a cat (in the mind of S) but neurophysiological discoveries will show this subjective experience to be nothing more than a state of or function of S's brain.
Figure 4. A Reflexive model of the causal sequence in visual perception.
Light rays from a cat (as-perceived by an Experimenter) impinge on the
Subject’s eye. Impulses travelling up the central nervous system produce
a neural representation of the cat within S's central nervous system. Information
within this neural representation is incorporated within an 'experiential
model' of the cat produced by the brain in the form of a cat as-perceived
by S. This is 'projected' by the brain to the judged location of the initiating
stimulus, out-there in the world. As in the Dualist and Reductionist models
the the neural representation of a cat in S's brain is separate from the
cat (as-perceived by E) out there in the world. Contrary to these models,
however, S's percept of a cat and the cat as-perceived (by S) are
one and the same. Indeed what S experiences is similar to what E experiences,
viz. a cat out there in the world, but viewed from S's perspective rather
than from the perspective of E.
-----------------
So what does this mean?
Propositions 1 to 5 imply that the contents of consciousness are quite
separate from the physical world we see around our bodies, i.e. from the
physical world as-perceived. As shown in Figures 2 and 3, they are
in different places. For Dualists, percepts of the world
are "in the mind," whereas for Reductionists, they may be thought of as
being "in the brain". The physical world as-perceived, by contrast, is
out-there in space, beyond the body surface.
Note, to begin with, that all the models (the Reflexive one included) agree that for each experience there will be a corresponding brain event and that such brain events are separate from the external world as-perceived. Propositions 1 to 5, however, do not refer to brain events but rather to what is experienced. Where and what, we need to ask, are these experiences?
Now there are some "inner" experiences that are relatively insubstantial. Subjectively, they do not appear to have a clear location, or, at best, to be located somewhere inside the head. Included amongst these are visual images (with one's eyes closed) thoughts (when manifest as "inner speech") and dreams. Proposition 1, however, refers not to images, thoughts or dreams but to percepts of physical objects. Where, when our eyes are open, are these?
Figures 2,3 and 4, for example, deal with the experience of a cat, and from the subjective point of view, it is easy to determine which of them is true. When one gazes at a cat, out-there in the world, there is no duplicate experience of a cat "in the mind" (Figure 2) or for that matter, "in the brain" (Figure 3). Rather, all that one experiences is one cat out-there in the world (Figure 4). Asked to describe what one experiences, the only thing to describe is the physical cat as-perceived....fat, furry, and friendly, beyond the body surface. In this sense, one's percept of the cat and the cat as-perceived are one and the same. A similar argument applies to one's own physical body and to all other physical objects out-there in the world. Asked to describe one's experiences of one's body, one can do no other than describe one's body as-perceived - it looks fat or thin and extended down towards the floor, it feels hard or soft, and so forth.
The initial drift of my argument should, at this point, be clear. In an important sense, one's percepts of physical objects and physical objects as-perceived are one and the same....and this requires us to abandon proposition 1. It also suggests what is wrong with propositions 2 to 5. Propositions 2 to 5 contrast the "contents of consciousness" with "physical objects as-perceived". But, as noted above, the contents of consciousness include not only "inner" experiences such as images, thoughts and dreams but also the body as-experienced, and experiences of physical objects beyond the body surface, which are, in a sense, none other than physical objects as-perceived. Physical objects as-perceived, therefore, cannot be contrasted with the contents of consciousness, for they are included amongst the contents of consciousness. In short, we are dealing here with a "category error" (but one of a very different kind to that described by Ryle, 1949).
The consequences for how one thinks about consciousness are profound. While some contents of consciousness are relatively insubstantial ("inner experiences") other contents of consciousness are very substantial indeed....for they are none-other than the experienced physical world.
To those of a Dualist or Reductionist persuasion, this way of thinking about the contents of consciousness may not be easy to grasp and I return to it, from the perspective of Psychological science, below. Before doing so, however, it is worth noting that this 'radical' claim is far from new and appears in various guises in the work of George Berkeley (1710) Immanuel Kant (1781) C.H. Lewes (1877) W.K. Clifford (1878) Ernst Mach (1885) Morton Prince (1885) William James (1890, 1904) A.N. Whitehead (1932) Charles Sherrington (1942) Bertrand Russell (1948) Lord Brain (1966) Wolfgang Kohler (1966) and Karl Pribram (1971, 1974, 1979).
William James (1904) for example, attempts to convince the reader on this point by suggesting that he -
"... begin with a perceptual experience, the 'presentation', so called, of a physical object, his actual field of vision, the room he sits in, with the book he is reading as its centre, and let him for the present treat this complex object in the commonsense way as being 'really' what it seems to be, namely, a collection of physical things cut out from an environing world of other physical things with which these physical things have actual or potential relations. Now at the same time it is just those self-same things which his mind, as we say, perceives, and the whole philosophy of perception from Democritus's time downwards has been just one long wrangle over the paradox that what is evidently one reality should be in two places at once, both in outer space and in a person's mind. 'Representative' theories of perception avoid the logical paradox, but on the other hand they violate the reader's sense of life which knows no intervening mental image but seems to see the room and the book immediately just as they physically exist".
And Whitehead (1932) puts the same point in somewhat anthropocentric fashion -
"The mind in apprehending also experiences sensations which, properly speaking, are projected by the mind alone. These sensations are projected by the mind so as to clothe appropriate bodies in external nature. Thus the bodies are perceived as with the qualities which in reality do not belong to them, qualities which in fact are purely offsprings of the mind. Thus nature gets credit which should in truth be reserved for ourselves; the rose for its scent; the nightingale for its song; and the sun for its radiance. The poets are entirely mistaken. They should address their lyrics to themselves, and should turn them into odes of self-congratulation on the excellency of the human mind. Nature is a dull affair, soundless, scentless, colorless, merely the hurrying of material, endless, meaningless".
The claim that some experiences are projected by the brain thereby 'clothing' Nature to form the physical world as-perceived is supported, furthermore, by a variety of clinical observations and experimental findings.
PAIN IN THE BODY
Consider, to begin with, the case of pain. Philosophers of mind, often regard pain as a paradigm case of a conscious experience, taking it for granted that it is "in the mind" or "in the brain" - and that it lacks physical qualities such as extension, and location in the three-dimensional physical world. The notion that pain has neither spatial location nor extension (or at best a location and extension within the brain) has never, however, impressed the medical profession, who regard the location and distribution of pain in different regions of the body as self-evident, a useful indicator of the nature of disease, and who tell us, furthermore, that there are no pains located in the brain!
It is important to note, furthermore, that for each pain as-
experienced in the body there is no duplicate experience of pain 'in the mind'. There will, of course, be neural causes of that pain within the brain. These causes, however, should not be confused with their effects....the pain as-experienced within the body.
If one wishes to retain pain as a paradigm case of what is a conscious experience, therefore, one must accept that the contents of consciousness may, at times, be located and distributed throughout the body as well as being 'in the head' or without a clear location.
PAIN PROJECTED BEYOND THE BODY SURFACE
Indeed, there are occasions when pain is experienced to be out-there
in the world, in "phantom limbs" beyond the body surface .
The following case history, reported by Livingston (1943) is
instructive:
"In 1926, a physician, who had long been a close friend of mine, lost his left arm as a result of gas bacillus infection ... The arm was removed by a guillotine type of amputation close to the shoulder and for some weeks the wound bubbled gas. It was slow in healing and the stump remained cold, clammy, and sensitive ... In spite of my close acquaintance with this man, I was not given a clear impression of his sufferings until a few years after the amputation, because he was reluctant to confide to anyone the sensory experiences he was undergoing. He had the impression, that is so commonly shared by layman and physician alike, that because the arm was gone, any sensations ascribed to it must be imaginary. Most of his complaints were ascribed to his absent hand. It seemed to be in a tight posture with the fingers pressed closely over the thumb and the wrist sharply flexed. By no effort of will could he move any part of the hand ... The sense of tenseness in the hand was unbearable at times, especially when the stump was exposed to cold or had been bumped. Not infrequently he had a sensation as if a sharp scalpel was being driven repeatedly, deep into ... the site of his original puncture wound. Sometimes he had a boring sensation in the bones of the index finger. The sensation seemed to start at the tip of the finger and ascend the extremity to the shoulder, at which time the stump would begin a sudden series of clonic contractions. He was frequently nauseated when the pain was at its height. As the pain gradually faded, the sense of tenseness in the hand eased somewhat, but never in a sufficient degree to permit it to be moved. In the intervals between the sharper attacks of pain, he experienced a persistent burning in the hand. The sensation was not unbearable and at times he could be diverted so as to forget it for short intervals. When it became annoying, a hot towel thrown over his shoulder or a drink of whisky gave him partial relief". (cited in Melzack, 1973)
By way of treatment, Livingston administered a novocaine injection of the upper thoracic sympathetic ganglia of both sides. This removed the pain (for a number of months) but not the phantom limb. Rather -
"To our mutual surprise, he (now) felt that he could voluntarily move each of his phantom fingers". (Ibid).
PROJECTED TACTILE SENSATIONS
It is crucial to note, furthermore, that the ability of the brain to project pain, applies equally to aspects of events as-experienced more often thought of as "physical" than "mental". "Hardness" and "solidity", for example, are often thought of as physical qualities. Nevertheless, the hardness and solidity we experience results from the way mechanical deformations of the skin are interpreted by the brain and projected to the location of the stimulated sense organs. Direct electrical stimulation of the somatosensory cortex, for example, produces feelings of numbness and tingling not in the brain but in different regions of the body (Penfield and Rassmussen, 1950). As with pain, furthermore, such tactile sensations are, on occasion, projected beyond the body surface. There is a sense, for example, in which one can 'feel' the pressure of the point of a pen against paper - and butter 'feels' soft when cut with a knife. Melzack (1973) in his review of phantom limb experiences, reports that -
"Most amputees report feeling a phantom limb almost immediately after amputation of an arm or a leg.....The phantom limb is usually described as having a tingling feeling and a definite shape that resembles the real limb before amputation. It is reported to move through space in much the same way as the normal limb would move when the person walks, sits down, or stretches out on a bed. At first, the phantom limb feels perfectly normal in size and shape - so much that the amputee may reach out for objects with the phantom hand, or try to get out of bed by stepping onto the floor with the phantom leg. As time passes, however, the phantom limb begins to change shape. The arm or leg becomes less distinct and may fade away altogether, so that the phantom hand or foot seems to be hanging in mid-air.
Sometimes the limb is slowly 'telescoped' unto the stump until only the hand or foot remain at the stump tip".
In addition to such tingling and kinaesthetic sensations, amputees report a variety of other sensations including pins-and-needles, itching, sweating, warmth or coldness and heaviness in their phantom limbs (Melzack, 1973; Craig, 1978).
Intriguingly, even vibrations applied to the skin of normal limbs
can, under special circumstances, be projected beyond the body surface. Von Bekesy (1967) for example, placed a vibrator on each forearm of his subjects and studied the effects of manipulating their frequency and phase relationships. By adjusting the phase relations, the two vibrators could be made to appear as a single point source which 'jumped' from one limb to the other. Moreover, after a period of exposure (usually several hours) subjects began to localise the source of stimulation at a point in space between the limbs!
The apparent location of tactile sensations and vibrations beyond the body surface does not, of course, alter the fact that the source of stimulation is located on the surface of or within the body. These examples do illustrate, however, that experienced hardness, solidity, vibration, etc., thought of as "physical" qualities by virtue of the fact that they represent aspects of the material world are nevertheless as much aspects of experience produced by brain activity as are pains, images and dreams.
PROJECTED AUDITORY SENSATIONS
The tendency to regard physical events as-experienced as somehow independent of the brain (and, therefore, outside the contents of consciousness) is also found in the area of audition. Sounds, for example, are usually thought of as physical events out-there in space, which must be distinguished from experiences of sound 'in the head' or 'in the mind'. All that auditory sensors detect, however, are patterns of vibration at the eardrums. In analogous fashion to vibrations applied to the forearms studied by Von Bekesy (above) the experiences which result from such vibrations are projected by the brain out-there in space (to the judged location of the source).
Studies of sound localization, furthermore, provide a particularly clear example of how an event as-perceived may be located either "in the head" or "out-there in the world" without otherwise altering in quality, and of the implausibility, therefore, of defining what is "in consciousness" in terms of what is experienced to be "in the head" (as opposed to "out-there in the world").
A symphony orchestra played through stereo speakers, for example, appears to be distributed in the space outside one's body. If the same music is played through stereo headphones, however, the instruments appear to be distributed around the space inside one's head. As with the images, dreams, etc., mentioned earlier, it is difficult to envisage that while the orchestral music is located inside one's head, there is some additional perception of the music "inside one's mind". Rather, the perception of music "inside one's mind" is the music as-experienced inside one's head. Equally, if one switches from headphones to stereo speakers, it seems absurd to propose that an additional conscious percept of music appears at the precise moment that the music as-perceived switches from being in the head to being out-there in the world. Nor does it seem plausible to argue that the music as-perceived is transformed from being an "experience" to being something "physical" as it moves from being in the head to being part of the world as-perceived outside one's body, for apart from its changed location, it undergoes no other change in "quality".
In any case, studies of "inside the head locatedness" suggest a far simpler explanation. Laws (1972) for example, investigated the acoustic differences between white noise presented through headphones (perceived to be inside the head) and white noise presented through a speaker at a distance of 3 metres (perceived to be out-there in the world) using probe microphones positioned at the entrance to the auditory canal. This revealed spectral differences (produced largely by the pinna of the ear) between the white noise presented through the speaker and white noise presented through the headphones.
Ingeniously, Laws then constructed an electrical "equalising" circuit to simulate these spectral differences and inserted this into the headphone circuit. With the headphones "unequalised", white noise appears to be inside the head irrespective of loudness. With the headphones "equalised", the white noise not only appears outside the head but actually recedes as the loudness is decreased.
Again, it can hardly be claimed that the insertion of an "equalising" circuit suffices to convert an "experience" to something "physical". Rather, the experiment establishes that spectral distortions produced by the pinnae (or their absence) inform the brain whether or not the source of sound lies beyond the pinnae (see Blauert, 1983, for a review). The experiential model of the source produced by the brain, i.e. the sound as-perceived, is correspondingly located in the head or beyond the pinnae. Whichever is the case, both the "quality" of the sound as-perceived and its perceived location would seem to be a product of perceptual processing in the brain.
EVENTS AS-PERCEIVED AND EVENTS AS DESCRIBED BY PHYSICS
At the same time, studies of sound localization make it clear that we need to distinguish an event (or object) as-perceived from the event (or object) itself, as described, say, by Physics. Laws (1972) for example, found that the perceived distance of white noise produced by a speaker at a distance of 25 cms depended almost entirely on loudness, receding from under 1 metre (on average) at 8 sones, to just over 2 metres (on average) at 1 sone.
The average perceived distance of white noise produced by a speaker at a distance of 3 metres was similarly dependent on loudness, i.e. for a given loudness the perceived distance of a sound was only slightly further away than that produced by the speaker at 25 cm (a noise of 8 sones had a perceived distance of just over 1 metre, etc.). The experienced distance of a sound, therefore, relates only in approximate fashion to the measured distance of the source that produces it (see Blauert, 1983, for a review). We consider the way the physical world as-perceived relates to the world as described by Physics in more depth below. Before we do so, however, let us return to "projection" in the domain of vision, i.e. to the way the visual world as-perceived may be thought of as a projection of the brain.
THE PROJECTED VISUAL WORLD
In the visual modality, perhaps more than any other, the apparent separation of the world as-perceived from what is 'in the mind' seems most clear. Visually perceived objects extended in the three-dimensional space around our bodies seem to have very different qualities, for example, to visual images of those objects. If visual images exemplify the "contents of consciousness", then how could objects as-seen do likewise?
The analysis presented below does not seek to minimize these differences in how objects and images are experienced, for in all probability, they represent discontinuities that from the point of view of human interaction with the world, are as important as they are real. Nevertheless, the fact that objects are experienced to be different from images does not alter the fact that both objects and images are experienced - that they are equally, in a sense, products of processing in the brain.
Now the dependence of visual imagery on cerebral processing is widely accepted - it is consonant, after all, with the conventional assumption that images are 'in the mind'. By contrast, the very spatial separation of objects as-perceived from brains as-perceived makes their interdependence more difficult to imagine. Nevertheless, the evidence for cerebral involvement in the "construction" of objects as-seen, including their seen location in 3D space, is compelling.
It is well known, for example, that as an object recedes, its perceived size decreases far less than its optical projection on the retina would suggest (the phenomenon of "size constancy") i.e. perceived size varies not only with the projected retinal image but also with perceived distance - and the perceived distance of an object is itself influenced by interpretive cerebral processes operating on retinal size, binocular disparity, ocular convergence, textural gradients, the interposition of other objects, motion parallax and so forth. Indeed, three-dimensional space (as-perceived) can itself be shown to be, at least in part, a "construct" of the brain.
One demonstration of such constructive processes is the perception of three-dimensionality produced by visual cues suitably arranged on a two-dimensional surface, as in a stereoscope. Artists achieve a similar effect by the judicious use of aerial perspective, linear perspective, gradients of size and texture, and so forth. As Shepard (1983) points out, the perception of depth produced by such techniques is normally muted by the counteracting information provided by binocular disparity, which tells us that we are in fact looking at a two-dimensional surface. He reports, however,
"... that if I look at the same painting with one eye, from a distance that precludes resolution of the microstructure of the painted surface, and through a reduction tube (formed, e.g. by rolling up a sheet of paper) so as to hide the surrounding frame, I can obtain an experience that seems as vividly three-dimensional as if I were looking with both eyes at the corresponding real scene".
Furthermore, the notion that the world as-seen is in its own way as much a construct of the brain as are visual images, has been fortified in recent years by evidence of functional similarities between visual perception and visual imagery. Indeed, there are grounds for believing that the processes which produce visual percepts and visual images are, to some extent, the same.
Under conditions of poor illumination, for example, it can be very difficult to decide whether what one sees in space is an object, a visual image or an hallucination. Perky (1910) found, for example, that objects faintly projected onto a screen were often judged by subjects to be the result of their own imagination and Kulpe (1902) found that dimly lit, fluctuating stimuli were elaborated by subjects into hallucinations which they, nevertheless, judged to be "real".
Even where subjects are in no doubt that they are imaging or hallucinating, they may report that what they see is located out-there in space (as opposed to "inside the head"). This is particularly clear with eidetic imagers who typically report their visual images to be projected onto surfaces in front of their eyes and which seem to them to be quite distinct from visual memories which they report as being "inside their heads". When they describe such images, furthermore, they describe what they see as opposed to what they have seen (Leask, Haber and Haber, 1969; Haber, 1979).
Such abilities, when they occur, are usually found in children. Spanos, Ham and Barber (1973) however, report that 1 to 2% of adults appear to have the ability to hallucinate an object in a room when asked to do so (without the object being present). Barber (1979) points out that "Subjects who insist that they can see the object that is not present also typically insist that they can touch it, smell it, move it, manipulate it, and so on".
Such subjects also have
"... an amazingly vivid memory of objects and events that they have encountered during their life. They can, for example, vividly recall the way a childhood doll looked, felt, and smelled. When they are asked now to see a doll sitting in a chair in front of them, their memory of the doll is so total that they literally feel that they see it, touch it, and smell it again".
The most convincing demonstration not only of the "psychological reality" of such imagery but also of its functional relatedness to visual perception is reported by Stromeyer and Psotka (1971), using random dot stereograms of the kind shown in Figure 5, below.
-------------------
Figure 5. Random 100 x 100 cell stereograms, used by Stromeyer &
Psotka (1971) to demonstrate eidetic imagery.
-------------------
The stereogram patterns shown consist of 100 x 100 cell matrices in which each cell is either filled or left blank in random fashion. The patterns are, furthermore, identical, except that a central, square region of one pattern is shifted laterally by a few elements relative to that of the other pattern. When viewed through a stereoscope, binocular fusion of the two patterns enables one to see this square, which seems to hover in space above the picture plane (Julesz, 1971,1979).
Stromeyer and Psotka, in their investigation of a particularly gifted eideticer, presented the right-hand pattern to the right eye (for four periods of 3 minutes each, separated by rest intervals of 1 minute) and then, after a period of 24 hours, presented the left-hand pattern to the left eye. Remarkably, the subject was able to fuse her eidetic image of the right-hand pattern with the perceived left-hand pattern and reported seeing the central square, hovering around 26 cm above the picture plane. The subject was able to perform similar feats in double-blind trials using 100 x 100 matrices with different patterns over delays as long as 3 days, and with 1000 x 1000 dot matrices over delays up to 3 hours.
It seems clear, given this evidence, that the clarity and detail of an eidetic image projected out-there in space can under exceptional circumstances rival that of visual perception and that the brain can, furthermore, treat such visually imaged information as if it were information given in perception.
Such cases are, however, rare. It is worth noting, therefore, that in recent years evidence has continued to mount for functional similarities between visual perception and normal visual imagery, e.g. that visual imagery and visual perception compete for the same processing mechanisms, that performance on a range of tasks using visual stimuli is very similar to performance on the same tests using visual images of these stimuli, and so forth. This evidence has been extensively reviewed elsewhere and we need not, therefore, consider it in detail here (see Finke, 1979, 1980; Finke and Kosslyn, 1980; Finke & Shepard, 1986; Segal, 1971; Shepard, 1983; Shepard and Cooper, 1982; Shepard and Podgorny, 1978).
Rather, as a final illustration of the way the world as-seen is constructed, let us consider "apparent motion", the experience of objects in motion under conditions where visual information is reduced.
Apparent motion was initially investigated by Exner in 1875 and Wertheimer in 1912 using two laterally separated dots presented in alternation. Provided the timing is right, this is experienced as a single dot moving in a straight line between the two positions. Later studies with more complex shapes (presented in alternation) have made it apparent that the brain's ability to fill in missing perceptual information applies not only to lateral trajectories of motion but also to rotation, to motion in depth and to complex deformations of shape which may accompany such motions (see Bundesen, Larsen and Farrell, 1981; Farrell, 1983; Farrell and Shepard 1981; Shepard and Judd, 1976).
Shepard and Judd, for example, have explored the complex movements and transformations apparently undergone by a complex object, when two views of it are presented in alternation within the same circular field. A number of transformations are illustrated in Figure 6, below.
-------------------
Figure 6. Polygon pairs used to demonstrate apparent movement and apparent
transformation of shape (taken from Shepard, 1983).
-------------------
Provided that the rate of alternation is not too fast, the polygon in panel (a) appears to move to the left or to the right, in (b) to move closer or to recede and in (c) to rotate. In panels (d) and (e) the above motions are combined. Thus in (d) the polygon either approaches and moves to the right or recedes and moves to the left, whereas in (e) it either approaches and turns clockwise or recedes and turns counterclockwise. Remarkably, the alternating shapes in (i) tend to be seen as a two-dimensional polygon flapping in and out of the picture plane (through 90 degrees) in spite of the fact that the edge of the polygon is nothing more than a straight line!
The perception of such intervening motions and transformations of shape which accompany them in the absence of corresponding visual stimulation provides a convincing demonstration of the brain's ability to construct experiences of objects (in motion) out-there in the world. When combined with evidence of the constructed nature of perceived size, perceived distance and perceived shape (see above) such experiments provide a clear illustration of the manner in which the world as-seen around our bodies may be as much a product of brain processing as are "inner" events such as images and dreams.
THE BOUNDARIES OF CONSCIOUSNESS CANNOT BE SPATIALLY DEFINED
In short, the above examples demonstrate that whether we consider pains, tactile sensations, kinaesthetic sensations, the sounds we hear or the objects we see, being "in the head" or "out-there in space" is irrelevant to whether or not an event as-experienced is produced by the brain. Rather, the brain has the option of producing events as-experienced that have no clear location or a location in the head, or of projecting events as-experienced to different locations in the body or the three-dimensional world surrounding it. Furthermore, events may be experienced either as being spatially extended or as having no spatial extension. The boundary between inner (non-extended) events and outer (extended) events, therefore, does not divide that which is "in consciousness" from that which is not.
This inclusion of extended objects and events within the contents of consciousness runs directly counter to the fundamental bifurcation of the universe proposed by Descartes into res extensa (the extended substance of which the physical world is composed) and res cogitans (the thinking substance of which consciousness is composed). At the same time, the above analysis undermines Reductionism. For while it may be tempting to reduce some insubstantial thinking substance to something more substantial, i.e. to the material substance of the brain - it seems absurd to propose that the physical world as-perceived is nothing more than a state of, or function of the brain. The brain, after all, is part of the world as-perceived (by some external observer, e.g. a neurophysiologist). Reductionism, therefore, requires one to reduce the whole of the world as- perceived to one of its constituent parts!
The Reflexive model accords with our everyday experience. However, given the way relations between nsciousness, brain and physical world are commonly construed, the model poses many questions. If percepts of the world and the world as-perceived are, in a sense, one and the same, then how can one make sense of the Causal Theory of Perception (the chain of causation resulting in a percept) and what remains in the model for percepts to represent? How, furthermore, does the world as-perceived relate to the very different world described by Physics (the world of Quantum Mechanics, Relativity Theory, and so forth)? And what are the consequences for the Realism versus Idealism debate? On first glance, the Reflexive model seems to commit one to Berkeley's Idealism - that the world only exists insofar as it is perceived to exist. Yet this conflicts with our natural intuitions, bolstered by a wealth of circumstantial evidence, that the external world is real. The solid, material objects of the external world as-perceived, after all, seem very different to the "inner" events we experience.
The resolution of these, and other problems, from the perspective of the Reflexive model requires a major programme of intellectual reconstruction, for, to date, our understanding of these issues has largely been developed in the context of the Dualist or the Reductionist models shown in Figures 2 and 3 - and this reconstruction cannot be completed within a single journal article. Nonetheless, it is important to demonstrate that the Reflexive model not only accords with everyday experience and is consistent with scientific findings, but that it also makes intuitive sense. Below, therefore, I will indicate, in an initial way, how the model can deal with problems of the kind outlined above.
THE REFLEXIVE MODEL
Note, to begin with, that the Reflexive model makes the conventional assumption that causal sequences in exteroception are initiated by events in the external world, and that these events as described by Physics may be very different to the same events as-perceived. Subsequent links in the causal chain are also entirely conventional. Visual perception, for example, involves stimulation of the retina, innervation of the optic
nerve and occipital lobes, and the formation of representations of the
initiating events within the brain. The main departure from the conventional
occurs in the last step - which deals with the experienced effects
of such neural causal sequences. Whereas Dualists locate percepts of
external events 'within the mind', and Reductionists locate these percepts
within the brain, the Reflexive model states that external events as-perceived
are "projected" by the brain to the judged location of the initiating stimulus.
This claim is not a metaphysical claim. While the operation of perceptual
"projection" has not, as yet, received the full attention it deserves in
Psychological science, the evidence for it, in various sense modalities,
is clear (see the review above). While our knowledge of its underlying
mechanisms remains rudimentary, these can be (and, to some extent, already
have been) explored by experimental means.
THE WORLD AS-EXPERIENCED IS A REPRESENTATION
Once one accepts that experiences of external events are projected
to the approximate location of the initiating source one then needs to
face up to the fact that such projected experiences constitute what
we normally think of as the "physical world" (for there is nothing other
than the "physical world" manifest in external experience). As noted above,
however, this physical world as-perceived may be very different to the
world as described by Physics. Again, there is nothing particularly mysterious
about this. Perceptual processes are likely to have developed in response
to evolutionary pressures, and select, attend to, and interpret information
in accordance with human adaptive needs. As such, they provide only a partial,
approximate model of the world. Nevertheless, perceptual models must relate
in some pragmatic way to what the world is 'really like', else human beings
would be unlikely to have survived. According to the above, the world as-experienced
is one such model (produced by perceptual processing). In all likelihood,
therefore, it is a partial, approximate but nonetheless useful representation
of some more fundamental reality (which Physics, and other conceptual systems,
would describe in a very different way).
The world as-experienced by humans, furthermore, is likely, in some
respects, to be peculiar to humans. Other animals, with different
perceptual systems are likely to inhabit correspondingly different experienced
worlds (constructed and "projected" by their perceptual systems) - and
this brings us to the question of Idealism vs Realism.
IDEALISM AND REALISM
If percepts of objects and objects as-perceived are one and the same, then the following conclusion seems inescapable....If our perceptual processes do not operate, then it is not just some ephemeral set of "mental" events that disappears - it is the solid world we experience surrounding our bodies that, for us, ceases to exist. This world may still, of course, exist for other human beings. However, if there were no human beings and there were no other creatures with perceptual processes similar to human beings, then the world as we perceive it would literally cease to be. In this sense, the Reflexive model commits one to Idealism - that the existence of the world as-perceived (by us) depends on the existence of and operation of our own perceptual processes.
It does not follow, however, that if there were no human, or similar sentient creatures, that the world would cease to be.... and it is here that we part company with Berkeley's version of Idealism. As noted above, the world as-perceived may be thought of as a representation of a more fundamental reality which Physics, for example, would describe in a very different way....and we have every reason to believe that such a reality existed prior to the appearance of humans and would continue to exist after their departure. If there were no sentient creatures to perceive that reality, the universe might exist, although it would not be experienced to exist. In this sense, the Reflexive model is consistent with Realism.
This is not, however, a Realism of the conventional kind. If the world as-perceived (by humans) is, in essence, a representation (of a more fundamental reality) then the familiar world that we experience would not be here if we were gone. Without a sense of touch or an ability to feel weight, there would be no hard and heavy objects. Without eyes there would be no appearance of movement or light. And the chatter of birds and clap of thundercloud become silence if there is no one to hear.
In short, the Reflexive model combines elements of both Realism and Idealism, but I have argued that they apply to different things. While the world we experience is a representation which depends for its existence on human perceptual processing, the reality so represented does not.
EXTENDED REPRESENTATIONALISM
The assumption that consciousness is representational in nature is often made within Philosophy of Mind, as is the assumption that representations must be distinguished from that which they represent. The Reflexive model merely extends these assumptions to all the contents of consciousness, including the physical world as-perceived (a form of "extended representationalism"). Furthermore, if we add the assumption that different experiences represent different aspects of the world (excluding illusions, hallucinations, etc.) then it is easy to make sense of the fact that "inner" experiences such as images, thoughts and dreams, appear so different from the external events we experience.
DIVISIONS WITHIN THE CONTENTS OF CONSCIOUSNESS
It can hardly be denied that the physical world as-perceived has very different qualities to events more traditionally thought of as conscious experiences, viz "inner" events such as images, thoughts and dreams. Such "inner" events are relatively ephemeral, they are not clearly located in space, they seem to have no precise size and shape, and they have no solidity or permanence (when our attention wanders they disappear). Physical objects as-perceived, by contrast, are clearly located in space. They do have a precise size and shape - and they seem as solid and permanent as anything can be! On first glance, therefore, the claim that all these are to be included amongst the contents of consciousness, poses something of a problem.
As noted earlier, the sole (and sufficient) grounds for including such diverse events within the contents of consciousness is that they are all included in what we experience. The fact that we experience all these events, however, in no way diminishes the importance of differences in how we experience these events. There can be little doubt, for example, that for everyday purposes, it is useful to distinguish the operation of and boundaries of our bodies from the surrounding world in which our bodies move, a need that is in no way diminished by the fact that both bodies as-perceived and the surrounding world as-perceived count amongst the contents of consciousness produced by the brain. Further, given the ability of the brain to form representations of its own states and operations, and to construe hypothetical worlds as well as real ones there can be little doubt of the need to distinguish these "inner" representations from representations of the body or of the outside world. It is precisely the apparent absence of clear location and extension, or at best, a location and extension "inside the head" which informs us that an experience is of the workings of the brain itself rather than of the body or the surrounding physical world. Furthermore, "inner" experiences appear relatively insubstantial and easily subject to change, because they represent events within our central nervous systems which, in general, have only a momentary, fleeting existence.
On this argument, visual images usually appear to be "in the head" for the reason that they usually represent events which are located there, i.e. the operation of constructive cerebral processes drawing on information stored in memory. They appear relatively insubstantial, for the reason that when our attention shifts, they, and the cerebral constructs they represent, really do disappear. While visual perception appears, at least to some extent, to utilize similar constructive processes, it is, under normal viewing conditions, so tightly guided by the information in the light picked up by the retina, that the resulting experience, appropriately, gives every appearance of being a "direct perception" of what is out-there in the world. The objects as-perceived which result from such processing, appear solid and permanent for the reason that the entities such percepts represent really are relatively substantial and of relatively long duration.
The importance of being able to distinguish "inner" from "outer" worlds is self-evident, although this judgement is not always a simple matter. The adaptive utility of being able to determine the inner or outer origin of information modelled in experience is made terrifyingly clear, for example, in those circumstances where this discrimination fails to operate, e.g. in psychotic delusions and hallucinations, where potent, threatening inner realities are mistaken for menacing events out-there in the world.
We may surmise, therefore, that major subdivisions within the contents of consciousness have developed in the course of evolution.... that the differences between inner, body and outer events as-experienced are striking because they reflect the organisms need to respond appropriately to functional differences in the event groupings which these experiences represent .... important differences that their joint inclusion within the contents of consciousness in no way denies.
CONCLUSIONS
There are many other aspects of the relationship between consciousness, brain, and physical world that need to be examined (or re-examined) from the perspective of the Reflexive model. The above evidence and arguments deal with only a few of the broader empirical and philosophical issues. But the essential point should, by now, be clear. If we are to understand how consciousness, brain and the physical world relate to each other, then we must begin with a model of consciousness that does full justice to all of its contents - and that requires us to extend these contents to encompass all that we normally experience .... to include not just "inner" experiences and the experienced body, but also external events (and objects) as-perceived. Once the boundaries of consciousness are redrawn in this way, it becomes apparent that there is no unbridgeable dualism forever separating the "mental" from the "physical". Nor does it make sense to reduce the "mental" to the "physical". Conversely, in excluding external events (and objects) as-perceived from the contents of consciousness, Dualists and Reductionists create a divide that has no basis in everyday experience, thereby creating a dispute about how consciousness relates to the physical world, that can never be resolved.
REFERENCES
Armstrong, D.M.(1968) A materialist theory of mind.
Barber, T.X.(1979) Eidetic imagery and the ability to hallucinate at will. The Behavioral and Brain Sciences 2:596-597.
Bekesy, G. von (1967) Sensory inhibition. Princeton University Press.
Berkeley, G.(1710) The principles of human knowledge. Reissued by T. Nelson & Sons: London, 1942.
Blakemore, C. & Greenfield,S.(1987) Mindwaves. Blackwell.
Blauert, J.(1983) Spatial hearing: the psychophysics of human sound localization. M.I.T. Press.
Brain, L.(1966) Science and man. Faber & Faber.
Bundensen, C., Larsen, A. & Farrell, J.E.(1981) Visual transformations of size and orientation: spatial imagery and perceptual processes. In Attention and Performance IX. eds. J.Long & A.Baddely. Lawrence Erlbaum.
Churchland, P.S.(1988) Reduction and the neurobiological basis of consciousness. In Consciousness in contemporary science. eds. A.J.Marcel & E.Bisiach, pp 273-304.
Clifford, W.C.K.(1878) On the nature of Things-in-themselves. In Lectures and Essays by the late William Kingdom Clifford. eds. L.Stephen & F.Pollock. London, Macmillan & Co., 1901.
Craig, K.D.(1978) Social modelling influences on pain. In The Psychology of Pain, ed. R.A. Sternbach, pp 73-109. Raven Press, New York.
Descartes, R.(1644) Treatise on Man. Trans. by T.S.Hall. Harvard University Press, 1972.
Eccles, J.(1980) The human psyche. Springer.
Eccles, J.(1987) Brain and mind, two or one? In Mindwaves. eds. C.Blakemore & S.Greenfield, pp 293-308. Blackwell.
Exner, S.(1875) Ueber das sehen von beuregungen und die theorie des zussammengezetzen auges. Sitzungberichte Academie Wissenschaft Wien 72:156-190.
Farrell, J.E.(1983) Visual transformations underlying apparent movement. Perception and Psychophysics 33:85-92.
Farrell, J.E. & Shepard, R.N.(1981) Shape orientation and apparent rotational motion. Journal of Experimental Psychology: Human Perception and Performance 7:477-486.
Finke, R.A.(1979) The functional equivalence of mental images and errors
of movement. Cognitive Psychology 11:235-264.
Finke, R.A.(1980) Levels of equivalence in imagery and perception. Psychological Review 87, No.2:113-132.
Finke, R.A. & Kosslyn, S.M.(1980) Mental imagery acuity in the peripheral visual field. Journal of Experimental Psychology: Human Perception and Performance 6, No.1:126-139.
Finke, R.A. & Shepard, R.N.(1986) Visual functions of mental imagery.
In Handbook of Perception and Human Performance II: cognitive processes
and performance. eds. K.R.Boff, L.Kauffman & J.P.Thomas, pp 37-1
to 37-55. John Wiley & Sons.
Gazzaniga, M.S. & Le Doux, J.E.(1978) The integrated mind. Plenum.
Haber, R.N.(1979) Twenty years of haunting eidetic imagery: where's the ghost? The Behavioral and Brain Sciences 2:583-619.
James, W.(1890) The principles of psychology. New York: Dover. (Reprinted 1950).
James, W.(1904) Does 'consciousness' exist? In Body and Mind: readings in philosophy. ed. G.N.A.Vesey, pp 202-208. George Allen & Unwin.
Julesz, B.(1979) Random-dot correlogram test for eidetic imagery. The Behavioral and Brain Sciences 2:607-608.
Julesz, B.(1971) Foundations of cyclopean perception. Chicago: University of Chicago Press.
Kant, I.(1781) Critique of pure reason. Trans. by J.M.D.Meiklejohn. London, Dent, 1969.
Kihlstrom, J.F.(1987) The Cognitive Unconscious. Science 237:1445-1452.
Kohler, W.(1966) A task for philosophers. In Mind, Matter and Method: essays in philosophy of science in honour of Herbert Feigl. eds. P.K.Feyerabend & G.Maxwell, pp 70-90. Minneapolis, University of Minnesota Press.
Kulpe, O.(1902) Uber die objectivirung und subjectivirung von sinneseindruken. Philosophische Studien 19:508-536.
Laws, P.(1972) On the problem of distance hearing and the localization of auditory events inside the head. Dissertation, Technische Hochschule, Aachen. Cited in J.Blauert. Spatial hearing: the psychophysics of human sound localization. M.I.T. Press, 1983.
Leask, J., Haber, R.N. & Haber, R.B.(1969) Eidetic imagery in children: II. Longitudinal and experimental results. Psychonomic Monograph Supplements 3:25-48.
Lewes, C.H.(1877) The physical basis of mind. In Body and Mind: readings in philosophy. ed. G.N.A.Vesey, pp 155-164. George Allen & Unwin.
Mach, E.(1885) Contributions to the analysis of sensations. Trans. by C.M.Williams. The Open Court Publishing Co., 1897.
Marcell, A.J. & Bisiach, E. eds.(1988) Consciousness in contemporary science. Clarendon Press. Oxford.
Melzack, R.(1973) The puzzle of pain. Harmondsworth: Penguin.
Paivio, A.(1986) Mental Representations: A dual coding approach, Oxford, Oxford University Press.
Penfield, W. (1975) The mystery of the mind: a critical study of consciousness and the human brain. Princeton University Press.
Penfield, W. & Rassmussen,T.B.(1950) The cerebral cortex of man. Princeton University Press.
Perky, C.W.(1910) An experimental study of imagination. American Journal of Psychology 21:422-452.
Pollen, D.A. & Trachtenberg, M.C.(1972) Alpha rhythms and eye movements in eidetic imagery. Nature 237:109-112.
Pribram, K.H.(1971) Languages of the brain: experimental paradoxes and principles in neuropsychology. New York: Brandon House.
Pribram, K.H.(1974) How is it that sensing so much can do so little? In The Neurosciences Third Study Program. eds. F.O.Schmitt & F.G.Worden. M.I.T. Press.
Pribram, K.H.(1979) Behaviorism, Phenomenology and Holism in Psychology: A scientific analysis. Journal of Social and Biological Structures 2:65-72.
Prince, M.(1885) The nature of mind and human automatism. In Body and Mind: readings in philosophy. ed. G.N.A.Vesey, pp 187-196. George Allen & Unwin.
Putnam, H.(1975) Philosophical papers. Vol.2: Mind, language and reality. Cambridge University Press.
Russell, B.(1948) Human knowledge: its scope and its limits. Allen & Unwin.
Ryle, G.(1949) The concept of mind. Hutchinson.
Searle, J.(1987) Minds and brains without programs. In Mindwaves. eds. C.Blakemore & S.Greenfield, pp 208-233. Blackwell.
Segal, S.J.(1971) Imagery: current cognitive approaches. Academic Press.
Shepard, R.N.(1983) Ecological constraints on internal representation: Resonant kinematics of perceiving, imagining, thinking, and dreaming. Third James J. Gibson Memorial Lecture given at Cornell University, October 21 (manuscript submitted for publication).
Shepard, R.N. & Cooper, L.A.(1982) Mental images and their transformations. M.I.T. Press.
Shepard, R.N. & Judd, S.A.(1976) Perceptual illusion of rotation of three-dimensional objects. Science 191:952-954.
Shepard, R.N. & Podgorny, P.(1978) Cognitive processes that resemble perceptual processes. In Handbook of Learning and Cognitive Processes. Vol.V. Lawrence Erlbaum.
Sherrington, C.S.(1942) Man on his nature. Cambridge University Press.
Spanos, N.P., Ham, M.H. & Barber, T.X.(1973) Suggested ("hypnotic") visual hallucinations: experimental and phenomenological data. Journal of Abnormal Psychology 81:96-106.
Stromeyer, C.F.III & Psotka, J.(1971) The detailed texture of eidetic images. Nature 237:109-112.
Velmans, M.(1989) The function of consciousness in human information processing. Theme paper given at a colloquium on The Function of Consciousness. The Third Annual Meeting of the History and Philosophy of Psychology Section of the British Psychological Society, Bishop Grossetteste College, Lincoln, March 29 (manuscript submitted for publication).
Walker, N.S., Garrett, J.B. & Wallace, B.(1976) The restoration of eidetic imagery via hypnotic age regression: a preliminary report. Journal of Abnormal Psychology 85:335-337.
Wertheimer, M.(1912) Experimentalle studien uber das sehen von bewegung. Zeitschrift fur psychologie 61:161-265. Trans.in part in Classics in Psychology. ed. T.Shipley. New York: Philosophical Library.
Whitehead, A.N.(1932) Science and the Modern World. Cambridge University Press.