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When all's said and done, more is said than done.The main purposes of this review are to set out for neuroscientists one
possible approach to the problem of consciousness and to describe the relevant
ongoing experimental work.We have not attempted an exhaustive review of other
approaches.While most neuroscientists acknowledge that
consciousness exists, and that at present it is something of a mystery, most of
them do not attempt to study it, mainly for one of two reasons:
(1) They consider it to be a philosophical problem, and so best left to
philosophers.We have taken exactly the opposite point of view.We can state bluntly the major question that neuroscience must
first answer: It is probable that at any moment some active neuronal
processes in your head correlate with consciousness, while others do
not; what is the difference between them?What is special
(if anything) about their connections?And what is special (if
anything)about their way of firing?If one could understand the
mechanism for one aspect, then, we hope, we will have gone most of the way
towards understanding them all.We made the personal decision (Crick and Koch, 1990) that several topics
should be set aside or merely stated without further discussion, for experience
had shown us that otherwise valuable time can be wasted arguing about them
without coming to any conclusion.So much is now known about genes that any simple definition
is likely to be inadequate.How much more difficult, then, to define a
biological term when rather little is known about it.For this reason, appropriate experiments on such animals may
be relevant to finding the mechanisms underlying consciousness.It is not profitable at this stage to argue about whether simpler
animals (such as octopus, fruit flies, nematodes) or even plants are conscious
(Nagel, 1997).It is probable, however, that consciousness correlates to some
extent with the degree of complexity of any nervous system.When one clearly
understands, both in detail and in principle, what consciousness involves in
humans, then will be the time to consider the problem of consciousness in much
simpler animals.Nor
will we spend time discussing whether a digital computer could be
conscious.There are many forms of consciousness, such as those associated with
seeing, thinking, emotion, pain, and so on.Visual Consciousness
How can one approach consciousness in a scientific manner?Consciousness
takes many forms, but for an initial scientific attack it usually pays to
concentrate on the form that appears easiest to study.We chose visual
consciousness rather than other forms, because humans are very visual animals
and our visual percepts are especially vivid and rich in information.In
addition, the visual input is often highly structured yet easy to control.The visual system has another advantage.There are many experiments that,
for ethical reasons, cannot be done on humans but can be done on animals.Fortunately, the visual system of primates appears fairly similar to our own
(Tootell et al.Other neuroscientists
might prefer one of the other sensory systems.Very light anesthesia may not make much difference to
the response of neurons in macaque V1, but it certainly does to neurons in
cortical areas like V4 or IT (inferotemporal).That is, in some cases, a person uses the
current visual input to produce a relevant motor output, without being able to
say what was seen.Milner and Goodale (1995) point out that a frog has at least
two independent systems for action, as shown by Ingle (1973).These may well be
unconscious.We suggest that such an arrangement is inefficient when very many such
systems are required.This, in our view,
is what seeing is about.As pointed out to us by Ramachandran and Hirstein
(1997), it is sensible to have a single conscious interpretation of
the visual scene, in order to eliminate hesitation.The latter is
conscious, while the former, acting more rapidly, is not.In a similar way, a sprinter is believed to start to run before he
consciously hears the starting pistol.The Nature of the Visual Representation
We have argued elsewhere (Crick and Koch, 1995a) that to be aware
of an object or event, the brain has to construct a multilevel, explicit,
symbolic interpretation of part of the visual scene.By multilevel, we mean, in
psychological terms, different levels such as those that correspond, for
example, to lines or eyes or faces.In neurological terms, we mean, loosely,
the different levels in the visual hierarchy (Felleman and Van Essen, 1991).By an
explicit representation, we mean a smallish group of neurons which employ
coarse coding, as it is called (Ballard et al.If all such groups of neurons (there may be several of them, stacked one
above the other) were destroyed, then the person would not see a face, though
he or she might be able to see the parts of a face, such as the eyes, the nose,
the mouth, etc.There may be other places in the brain that explicitly
represent other aspects of a face, such as the emotion the face is expressing
(Adolphs et al.How many neurons are there likely to be in such a group?Much neural activity is usually needed for the brain to construct a
representation.Most of this is probably unconscious.It may prove useful to
consider this unconscious activity as the computations needed to find the best
interpretation, while the interpretation itself may be considered to be the
results of these computations, only some of which we are then
conscious of.In one
extreme form this would mean that, at one time or another, any neuron in cortex
and associated structures could express the NCC.It would
be a pity to miss the simpler one if it were true.As a rough analogy, consider
a typical mammalian cell.The way its complex behavior is controlled and
influenced by its genes could be considered to be largely global, but its
genetic instructions are localized, and coded in a relatively straightforward
manner.The conscious visual representation is likely to be distributed
over more than one area of the cerebral cortex and possibly over certain
subcortical structures as well.We have argued (Crick and Koch, 1995a) that in
primates, contrary to most received opinion, it is not located in cortical area
V1 (also called the striate cortex or area 17).This is not to say
that what goes on in V1 is not important, and indeed may be crucial, for most
forms of vivid visual awareness.This hypothesis is still very speculative.When one is actually looking at a visual scene, the
experience is very vivid.This should be contrasted with the much less vivid
and less detailed visual images produced by trying to remember the same scene.It is possible that our dimmer visual
recollections are mainly due to the back pathways in the visual hierarchy
acting on the random activity in the earlier stages of the system.This probably explains many of our
fleeting memories when we drive a car over a familiar route.If we do pay
attention (e.Rakic, 1995) expands the
time frame of consciousness, it is not obvious that it is essential for
consciousness.Consciousness, then, is enriched by visual attention, though attention is
not essential for visual consciousness to occur (Rock et al.This
is a complicated subject, and we will not try to summarize here all the
experimental and theoretical work that has been done on it.Recent Experimental Results
We shall not attempt to describe all the various experimental
results of direct relevance to the search for the neuronal correlates of visual
consciousness in detail but rather outline a few of them and point the reader
to fuller accounts.Action without seeing
Classical blindsight
This will already be familiar to most neuroscientists.It is discussed,
along with other relevant topics, in an excellent book by Weiskrantz (1997).It
occurs in humans (where it is rare) when there is extensive damage to cortical
area V1 and has also been reproduced in monkeys (Cowey and Stoerig, 1995).The pathways involved have not yet been established.Milner, Perrett and their colleagues (1991).In
spite of this, she is very good at catching a ball.Empty
rhomboids stand for intermediate areas or subareas of the labeled
regions.Notice that there are connections between the two hierarchies
at several levels, not just at the top level.We would therefore like to suggest a general hypothesis:that the
brain always tries to use the quickest appropriate pathway
for the situation at hand.Perhaps there is competition, and the
fastest stream wins.Bistable percepts
Perhaps the present most important experimental approach to finding
the NCC is to study the behavior of single neurons in the monkey's brain when
it is looking at something that produces a bistable percept.Allman suggested a more practical alternative: to study the
responses in the visual system during binocular rivalry (Myerson et al.They trained the monkey to report which of two
rivalrous inputs it saw.The experiments are difficult, and elaborate
precautions had to be taken to make sure the monkey was not cheating.Only the first response was recorded.Surprisingly, half of these responded in the opposite direction to the
one expected.The orientation was chosen in each case to be optimal for the neuron
studied, and orthogonal to it in the other eye.Also, here, but not in V4, none of the cells were anticorrelated with
the stimulus.More recently, Bradley et al.These are all exciting experiments, but they are still in the early
stages.The NCC neurons may be
mainly elsewhere, such as higher up in the visual hierarchy.That is, what type of neurons are they, in
which cortical layer or sublayer do they lie, in what way do they fire, and,
most important of all, where do they project?It is, at the moment,
technically difficult to do this, but it is essential to have this knowledge,
or it will be almost impossible to understand the neural nature of
consciousness.Electrical Brain Stimulation
An alternate approach, with roots going back to Penfield (1958),
involves directly stimulating cortex or related structures in order to evoke a
percept or behavioral act.Libet and his colleagues (Libet, 1993) have used
this technique to great advantage on the somatosensory system of patients.The
difference appears to reside in the amount and type of neurons recruited during
peripheral stimulation versus direct central stimulation.In a series of classical experiments, Newsome and colleagues (Britten et
al.MT cells are arranged in columnar structure for direction of
motion).Our reasons are that at each stage in the visual hierarchy the explicit
aspects of the representation we have postulated is always recoded.We think
that these plans are made in some parts of frontal cortex (see below).The strategy to verify or falsify this and related hypotheses is to relate
the receptive field properties of individual neurons in V1 or elsewhere to
perception in a quantitative manner.In that case, further experiments
need to be carried out to untangle the exact relationship between neurons and
perception.It is well known that the color we
perceive at one particular visual location is influenced by the wavelengths of
the light entering the eye from surrounding regions in the visual field (Land
and McCann, 1971; Blackwell and Buchsbaum, 1988).Schein and Desimone, 1990) that neurons
in V4, but not in V1, exhibit the Land effect.Some Experimental Support
In the last two years, a number of psychophysical, physiological and imaging
studies have provided some support for our hypothesis, although this evidence
falls short of proving it (He et al.Let us briefly discuss two other cases.When two isoluminant colors are alternated at frequencies beyond 10 Hz,
humans perceive only a single fused color with a minimal sensation of
brightness flicker.In other words, neuronal activity in V1 can clearly represent
certain retinal stimulation yet is not perceived.This is supported by recent
fMRI studies on humans by Engel, et al.Data are averaged across four subjects.PET experiments showing that in at least some people V1
is activated during visual imagery tasks (Kosslyn et al.V1 is compatible with visual imagery in
patients (Goldenberg et al.The Frontal Lobe Hypothesis
As mentioned several times, we hypothesize that the NCC must have
access to explicitly encoded visual information and directly project into the
planning stages of the brain, associated with the frontal lobes in general and
with prefrontal cortex in particular (Fuster, 1997).The fMRI study of the blindsight patient G.In particular, does the anatomy reveal any feedback
loops that might sustain activity between IT and prefrontal neurons (Crick and
Koch, 1997)?There is suggestive evidence (Webster et al.IT might terminate in layer 4, but
these need to be studied directly.The
existence of such oscillations remains in doubt in higher visual cortical areas
(Young et al.We remain agnostic with respect to the relevance of these
oscillations to conscious perception.The Problem of Qualia
What is it that puzzles philosophers?Scientists understand the enormous power of Natural Selection.They know the
chemical nature of genes and that inheritance is particulate, not blending.It is entirely possible that the very elaborate nature of neurons and
their interactions, far more elaborate than most people imagine, is misleading
us, in a similar way, about consciousness.He has given philosophical reasons why he thinks it is wrong.Neuroscientists
know only a few of the basics of neuroscience, such as the nature of the action
potential and the chemical nature of most synapses.Most important, there is
not a comprehensive, overall theory of the activities of the brain.Much of these are still lacking.As we see it, the hard problem can be broken
down into several questions, of which the first is the major problem: How do we
experience anything at all?The reason that visual consciousness is
largely private is, we consider, an inevitable consequence of the way the brain
works.If it turns out that the neural correlate of
blue is exactly the same in your brain as in mine, it would be scientifically
plausible to infer that you see blue as I do.How precise one has to be will depend on a detailed
knowledge of the processes involved.If the neural correlate of blue depends,
in an important way, on my past experience, and if my past experience is
significantly different from yours, then it may not be possible to deduce that
we both see blue in exactly the same way (Crick, 1994).Could this problem be solved by connecting two brains together in some
elaborate way?It is impossible to do this at the moment, or in the easily
foreseeable future.Unfortunately, this enterprise is fraught with
hazards, since it inevitably makes assumptions about how brains behave, and
most of these assumptions have so little experimental support that conclusions
based on them are valueless.For example, how much is a person's percept of the
blue of the sky due to early visual experiences?They assume that because they, as outside observers, are conscious of the
correlation, the firing must be part of the NCC.This by no means follows, as
we have argued for neurons in V1.But this is not the major problem, which is: How do other parts of the brain
know that the firing of a neuron (or of a set of similar neurons) produces the
conscious percept of, say, a face?Put in other words, how is meaning generated by the
brain?How is meaning expressed in neural terms?And
how does this expression of meaning arise?We suspect (Crick and Koch, 1995c)
that meaning derives both from the correlated firing described above and from
the linkages to related representations.For example, neurons related to a
certain face might be connected to ones expressing the name of the person whose
face it is, and to others for her voice, memories involving her and so on, in a
vast associational network, similar to a dictionary or a relational database.The obvious idea is that they
depend very largely on the consistency of the interactions with the
environment, especially during early development.He comes to feel
what the stick is touching, not merely the stick itself.NCC
easier, at the moment the most promising experiments are those on bistable
percepts.These experiments should be continued in numerous cortical and
thalamic areas and need extending to cover other such percepts.GABA agonists, perhaps using the relatively smooth cortex of
an owl monkey.Inevitably, it will be necessary to compare the studies on monkeys with
similar studies on humans, using both psychophysical experiments as well as
functional imaging methods such as PET or fMRI.Present methods are not specific enough to do this, but new methods in
molecular biology should, in time, make this possible.The time course of this illusion parallels the time course
of activity as assayed using fMRI.We have assumed that the visual NCC in humans is very similar to
the NCC in the macaque, mainly because of the similarity of their visual
systems.Ultimately, the link between neurons and perception will need to be
made in humans.We hope that some of the younger neuroscientists will
seriously consider working on this fascinating problem.For
helpful comments we thank David Chalmers, Leslie Orgel, John Searle and Larry
Weiskrantz.References
Adolphs R, Tranel D, Damasio H, Damasio A (1994) Impaired recognition of
emotion in facial expressions following bilateral damage to the human amygdala.Blackwell KT, Buchsbaum G (1988) Quantitative studies of color constancy.Blake R, Fox R (1974) Adaptation to invisible gratings and the site of
binocular rivalry suppression.MT reflect the perception of depth.Braun J, Julesz B (1997) Dividing attention at little cost.Britten KH, Shadlen MN, Newsome WT, Movshon JA (1992) The analysis of visual
motion: a comparison of neuronal and psychophysical performance.Chalmers D (1995) The Conscious Mind: In Search of a Fundamental Theory.Cowey A, Stoerig P (1995) Blindsight in monkeys.Crick, F (1996) Visual perception: rivalry and consciousness.Crick F, Koch C (1990) Towards a neurobiological theory of consciousness.Crick F, Koch C (1995a) Are we aware of neural activity in primary visual
cortex?Cumming BG, Parker AJ (1997) Responses of primary visual cortical neurons to
binocular disparity without depth perception.Damasio AR, Anderson SW (1993) The frontal lobes.Dennett D (1996) Kinds of minds: Toward an understanding of consciousness.Distler C, Boussaoud D, Desimone R, Ungerleider LG (1993) Cortical
connections of inferior temporal area IEO in macaque monkeys.Eccles JC (1994) How the self controls its brain.Engel S, Zhang X, Wandell B (1997) Colour tuning in human visual cortex
measured with functional magnetic resonance imaging.Gur M, Snodderly DM (1997) A dissociation between brain activity and
perception: chromatically opponent cortical neurons signal chromatic flicker
that is not perceived.Koch C, Braun J (1996) On the functional anatomy of visual awareness.Kosslyn SM, Thompson WL, Kim IJ, Alpert NM (1995) Topographical
representations of mental images in primary visual cortex.Libet B (1993) Neurophysiology of consciousness: selected papers and new
essays by Benjamin Libet.Logothetis N, Schall J (1989) Neuronal correlates of subjective visual
perception.Morgan MJ, Mason AJS, Solomon JA (1997) Blindsight in normal subjects?Nakamura RK, Mishkin M (1986) Chronic blindness following lesions of
nonvisual cortex in the monkey.Nagle AHM (1997) Are plants conscious?Penfield W (1958) The excitable cortex in conscious man.Pollen, DA (1995) Cortical areas in visual awareness.Rock I, Linnett CM, Grant P, Mack A (1992) Perception without attention:
results of a new method.Cyr JA, Ungerleider LG, Desimone R (1990) Organization of visual
cortex inputs to the striatum and subsequent outputs to the pallidonigral
complex in the monkey.A, Bullier J (1995) Corticocortical connections in the visual
system: structure and function.Salzman CD, Britten KH, Newsome WT (1990) Cortical microstimulation
influences perceptual judgements of motion direction.Sheinberg DL, Logothetis NK (1997) The role of temporal cortical areas in
perceptual organization.Sherk H (1986) The claustrum and the cerebral cortex.Tootell RBH, Dale AM, Sereno MI, Malach R (1996) New images from human
visual cortex.Ungerleider LG, Mishkin M (1982) Two cortical visual systems.Malsburg C (1995) Binding in models of perception and brain
function.Morgan S, Squire LR (1993) Neuroanatomy of memory.We are looking for paid volunteers for fMRI and behavioral experiments......So this is what we came up with; do join us!May, 2008 at The Annexe, Central Market.Each table can be taken up by one publisher, or it can be shared between various people who each have books to sell.There are many happenin' Malaysian books now being published; the trouble is, not many of them are readily available in the main bookshops.Gerakbudaya, SIRD, Silverfish, and Matahari Books.The idea is to make interesting books available even to those who can't afford to buy 'em.Deadline to receive entries (including payment) is April 26Payment goes directly to The AnnexeInterested?Freedom of Information thing, but I didn't hear that the copyright was passed.I'll have to ask him about that sometime.I'm craving for something from late Pak Pramoedya.But don't know where to get it.And yes, I'll be there too...Must be an interesting event.April
(13)
The KLAB is this weekend!
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