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This article was downloaded by: [Adelphi University]On: 20 August 2014, At: 00:01Publisher: RoutledgeInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK
Neuropsychoanalysis: An Interdisciplinary Journal
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The Unconscious Homunculus: Response to the
Commentaries by Francis Crick and Christof KochFrancis Crick & Christof Koch
a
a Division of Biology B9–74, Caltech, Pasadena, CA 91125, e-mail: , Phone: 626-395-6855,
Fax: 626-796-8876, Web: klab.caltech.edu
Published online: 09 Jan 2014.
To cite this article: Francis Crick & Christof Koch (2000) The Unconscious Homunculus: Response to the Commentaries
by Francis Crick and Christof Koch, Neuropsychoanalysis: An Interdisciplinary Journal for Psychoanalysis and the
Neurosciences, 2:1, 48-59, DOI: 10.1080/15294145.2000.10773283
To link to this article: http://dx.doi.org/10.1080/15294145.2000.10773283
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48
Reference
Stevens, R (2000), Phenomenal approaches to the study
of conscious awareness. In:
Investigating Phenomenal
Consciousness: Methodologies
and
Maps
ed.
M
Vel
mans. Advances in Consciousness Research Series. Am
sterdam: John Benjamins.
Richard Stevens
Psychology Department
The Open University
Walton Hall
Milton Keynes
MK
6AA
United Kingdom
e-mail: [email protected]
Phone: 01908-654-545
Fax: 01908-654-488
Crick-Koch
The Unconscious Homunculus: Response to the Commentaries by Francis Crick and Christof Koch
Introduction
We
are grateful to the various commentators for their
remarks, which have helped us rethink our own ideas
and have given us the opportunity to enlarge on them.
Let us first restate the general thrust of our paper. We
think
that qualia are the hard problem, and that, to
begin with, the best tactic is to
try
to find the neural
correlate of particular kinds of qualia, and especially
the activity that correlates with the content of each
kind. To search for this in the brain it might help to
know which psychological processes are likely to be
associated, or not associated, with qUalia For exam
ple, it seems rather unlikely that retinal neural activity,
by itself, is enough to produce any sort of qualia and
we have argued that activity in VI is also not suffi
cient.
We
suggested (following others) that while sen
sory neural activity could produce qualia, thoughts
could not.
In reply to the commentaries, we shall first deal
with several general points, and later consider more
particular ones. The comments by Schall and Stevens
are discussed at the end, after the Addendum,
as
they
were received after the Addendum was written.
What Are Qualia?
To approach the general points, let us first list some
of the psychological activities that might conceivably
have qualia associated with them. For example:
Francis Crick is the co-discoverer, with James Watson, of the double
helical structure of DNA. Since 1976, he has been at the Salk Institute for
Biological Studies in San Diego.
Christof Koch was awarded his Ph.D. in biophysics at the University
of Tiibingen in Germany (with a minor in philosophy). He joined the
California Institute of Technology in 1986, where he is a Professor of
Computation and Neural Systems.
Sensations
Percepts
Images (produced by imagination) not necessarily
visual
Thoughts
Intentions
Actions
Emotions
Mfect (in Freud's sense)
Fringe experiences
Valuations (such as novelty)
This list may well be incomplete. For example,
some might add the self, or meaning as Baars
and McGovern do in their contribution. We suggested
that sensory activities produced qualia, but thoughts
do not. We opted to leave emotions and valuations to
one side, and did not discuss intentions, actions, or
the self. Whether these nonsensory activities can lead
to qualia is an open issue that we did not address. The
primary reason for our reluctance to discuss these is
the current absence
of
a clear experimental program
or model system to study intentions, valuations, fringe
experiences, and the like (introspection by itself being
an unreliable guide).
Marr's Ideas
Instead, we suggested that, in the visual system, we
were directly conscious of something like Marr's
2_1hD sketch. In other terminology, we thought that
what we were conscious of was view-dependent, not
view-independent. The latter included Marr's 3D
model.
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Response to the Commentaries on the Unconscious Homunculus
9
Let us first state what Marr said about the 2 h
sketch in more detail.
Marc (1982, p. 37) wrote:
2
12
Makes explicit the orientation Local surface orientation
Sketch and depth of visible (the needles
surfaces, and contours of primitives)
discontinuities in those Distance from viewer
quantities n a viewer- Discontinuities in depth
centered coordinate frame Discontinuities in surface
Orientation
Marr postponed the consideration of color and motion.
Baars and McGovern
The first general topic is: do thoughts and percepts
have qualia associated with them? Baars and McGov
ern argue that visual consciousness itself is complex
and not simply sensory. They give examples and
ask the reader to assess their examples experientially.
The first example concerns homonyms that preserve
sensory form while changing meaning, such as the
word
s t
in tennis set or chess set. Jackendoffs
proposal was not that the idea of a chess set does not
exist but that the idea itself is unconscious, while what
we are conscious of are sensory images associated
with it, such as unspoken speech, visual images, etc.
We ask Baars and McGovern, and the reader, to
try
to think of a chess set, without at the same time saying
the word (aloud or silently), or visualizing the chess
set, or imagining moving the pieces, etc. We ourselves
find this impossible, which is Jackendoffs point. It is
not just the visual image of the word
s t
which is under
discussion, but all the other possible sensory activities
associated with it.
Baars and McGovern argue
that'
images associ
ated with meanings are not the same
as
the meanings
themselves. No doubt Jackendoff would agree but
would propose that while the meanings are uncon
scious, what we are consciously aware
of
is their sen
sory representations.
Meaning is indeed an important and difficult
topic, though it is obvious that the meaning
of
a word
may depend
on
the context in which it is used.
We
note that Baars and McGovern make no suggestions
as
to what type
of
neural activity is likely to be associ
ated with meaning. For our own tentative proposals,
see the Addendum.
n short, Baars and McGovern would argue, as
opposed to Jackendoff, that we are indeed directly
conscious of
our thoughts and our abstract ideas. They
would, however, agree with us that we are conscious
of both percepts and sensations.
Humphrey
Humphrey
on
the other hand has proposed (Hum
phrey, 1992) that we are conscious only
of
sensations
and not percepts. As an example he suggests that when
we smell a rose we both feel the sweet smell in our
nostrils (sensation) and we perceive the external pres
ence of the rose (perception). Sensation, he says, has
to do with feelings about what's happening to me now;
perception has to do with judgments about the objec
tive facts of the external world.
n
the visual system sensations and percepts are
usually closely associated, but in special cases one
can appear to have the first without the second. This
happens to some blind people who recover their sight
after an operation to remove cataracts (von Senden,
1960). They report seeing colors while still not being
able to see shape. It also occurred to the form agnostic
patient D. F. (Milner et al., 1991), whose brain was
damaged by carbon monoxide poisoning. She could
see color and texture, but not orientation or shape, so
that she could not consciously see objects, though in
some cases she could interact with them uncon
sciously. For a general discussion see Milner and Goo
dale (1995).
Thus Marr's
2 1f2D
sketch is,
on
Humphrey's
definition, a percept. Note that the 2 1f2D sketch is
supposed to make explicit the rough depth of visible
surfaces, distance from the viewer, the orientation of
the visible surfaces and their discontinuities, so it
would include the ambiguous depth and surface orien
tation information compatible with the two views of
the Necker Cube. Marr would probably have described
visual sensations such as orientation, color, and mo
tion
as
primitives. It is not true, as Humphrey
claims in his commentary, that the 21hD sketch cor-
responds to the subjective sensation
of
the image at
the eye.
Are There Many Types o Qualia?
We did not
try
to define qualia. This we regard as
premature. Instead we were exploring the working hy
pothesis that there are percept-qualia, such as cube
qualia and dog-qualia (in other words, for most
of
us,
it feels like something to see or hear a dog). Thus we
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s
agree with what Humphrey reports are the views of
Searle and V. S. Ramachandran, that percepts such as
those of a cube, a duck,
or
rabbit have qualia, and
differ from Humphrey's opinion that they do not. For
this reason we think that the experiments
of
Logothetis
and his colleagues (Logothetis, 1998)
on
neural re
sponses during binocular rivalry will indeed help us
to understand the mystery of consciousness. We are
not enthusiastic about Humphrey's suggestion of
agentic qualia (see our remarks at the end of the Ad
dendum).
We do not consider anyone's opinions about qua
lia, including our own, as compelling, especially as
thoughtful people differ on
the topic. This is not sur
prising
s
the classification
of
qualia depends, at the
moment, on introspection, which is known to be an
unreliable guide to the workings of the brain. What
we are trying to discover experimentally is which con
cepts map best onto the behavior of the brain. The fact
that people's opinions differ means that we shall have
to keep several possibilities in mind.
It may be
that eventually we shall find that it is
best to suppose that there are several kinds of qualia,
perhaps with something in common, so that, for exam
ple, percept-qualia will be, in neural terms, somewhat
different from sensation-qualia. As an analogy, con
sider genes. Some ofour genes code for proteins; some
for structural RNAs (such as transfer RNA). Both
these types of genes are coded
on
DNA, not on pro
tein, so that although they differ in the class
of
product
they produce they still have an important property in
common.
Emotion
The second general topic raised by the commentors is
which other psychological activities, such as emotion
and fringe effects, have special types of qualia associ
ated with them. It has been especially emphasized by
Panksepp, that some type of emotional 'feelings'
may lie at the core
of
human and animal conscious
ness. In our paper we proposed to leave this topic
on one side, if only because it is difficult to study in
a monkey, though we certainly think a monkey can be
angry. Nevertheless we will make a few comments
on it.
Nobody doubts that an emotion, such as anger,
has an effect on people's behavior. It is also very plau
sible that it has an effect on attention. As Panksepp
states,
we
should expect the salience
of
exteroceptive
qualia will be modulated by global emotional
Crick Koch
states. What is less certain is that it alters the charac
ter
of the sensation
or
percept. Anger may make a
person see red but is the redness of the red changed
by this?
It
may make a person pay more, or less, atten
tion to a certain object. In special cases, where the
object is not clearly apparent, it may tip the balance
between seeing an object one way to seeing it another
way, but our impression is that emotion usually has a
rather small effect on the character of a sensation or
a percept. One of us is inclined to remember that (as
a young man) being madly in love made colors seem
brighter, but whether or not this actually affected the
hue of, for instance, roses, remains an open question
that is not easy to test rigorously.
As to the effects of attention, it does not change
the appearance
of
a color. For example, it was shown
recently (Blaser, Sperling, and Lu, 1999) that, in hu
mans, top-down attention directed to a particular
color, in a special type
of
colored moving display, can
radically alter the type
of
motion perceived, without
any significant change in the appearance
of
the color.
It may well be that emotions are more primitive
(that is, occurred earlier in evolution) than percepts,
but general evolutionary arguments, in our view, are
not compelling by themselves, though they may give
hints
s
to how present organisms behave. Thus even
if the basic infrastructure of emotionality was a ma
jor force in the evolution
of
cognitive capabilities
this does not, by itself, say exactly what role emotion
ality plays now.
In
short, we do not see the evidence
that, without emotion, a conscious animal would be
come unconscious. It may be, as Panksepp suggests,
that areas of the brainstem such as the PAG are
more essential for sustaining and building a foundation
for consciousness than the higher brain areas. To us
the more important question is what is specifically
built on such a foundation. We do indeed suspect
(though this may be, as he says, shortsighted) that
the
brainstem components are permissive, in some
relatively unspecific way.
On the other hand, we would not argue with Pank
sepp's view that,
the
internal emotive programs
of
the brain, which are strongly motor need to be ex
pressed before organisms fully experience a variety of
emotional feelings.
Baars and McGovern are concerned with feelings
of emotion, and also with intentions, expectations, and
fringe experiences. These overlap somewhat with
lackendoffs valuations. lackendoff points out, cor
rectly, that we misunderstood his remarks (Jacken
doff, 1987) on what he called affects or (later)
valuations. His point is that these are not, as we wrote,
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Response to the Commentaries on the Unconscious Homunculus
51
a
class of conscious percepts which have a rather
different character from straightforward sensory per
cepts, but rather they are part of how all percepts
can be classified (e.g., any percept can be familiar
or novel, meaningful or senseless and so on). In his
commentary he does not discuss emotions (such as
anger) but such matters as novelty, meaningfulness,
and so on.
Valuation
seems a good term for such mat
ters. Notice, incidentally, that certain persons with
brain damage cannot report the familiarity of faces,
yet will show an unconscious skin response to very
familiar faces (Tranel and Damasio, 1985) so that it
appears one can have unconscious valuations. In any
case we prefer to postpone the study of these until
more vivid forms of consciousness are understood.
This is not to say that emotions and valuations are not
important, but they are not our immediate concern.
A third general topic is the matter of global activ
ity versus local activity. To discuss this we need to
describe our more recent views on this topic, so we
have relegated it to the Addendum.
Particular Comments
Schwartz has provided a brief sketch, which we found
quite interesting, of the history of the homunculus con
cept. We agree with him that the question, must de
sires and feelings first be converted into sensory
perceptions to become conscious? is a good one,
even though it is not our immediate concern.
Smith has written a lengthy description of
Freud's theories of consciousness, how they devel
oped over time, and how some
of
them appear to pre
figure recent ideas on the subject. While we found this
of
some interest, especially as it filled out our own
very brief references to Freud's ideas, we think it
im-
portant to stress that from the scientific point of view
it is irrelevant what Freud wrote, especially as there
are numerous features of Freud's theories that reflect
nineteenth-century misconceptions
of
the nervous sys
tem. n fact Freud's ideas map rather poorly onto
what we know today of the primate brain, and where
they do they lack precision. For example, if there is
indeed a temporal code, as Freud suggested (and
as
we also have outlined in the Addendum) it is important
to know whether it merely involves the correlated fir-
ing of neurons or, if it is really periodic, what frequen
cies are involved. The few sentences of Smith on the
present body of neuroscientific evidence seems to us
rather inadequate and somewhat uncritical.
Some of Freud's theories may suggest possible
useful ideas but if they do they should be formulated
in modern terms, and some thought should be given
as to the best way to test them experimentally.
Freud's Terminology
To avoid misunderstanding it might be useful to record
our view
of
the various conscious terms used by
Freud (see Schwartz's use of
preconscious .
As we
understand it, the Freudian term
preconscious
corres
ponds to information coded in such places as synapses
(as in the weights of
a neural network) and does not
require neural activity. A few years ago one
of
us,
during an operation, had his brain cooled so much that
his EEG was flat. Nevertheless he was able to recall,
after the operation, many past incidents. These were
coded in his Freudian preconscious. This rather obvi
ous idea is not always appreciated by philosophers.
When we C and K) say unconscious this usu
ally refers to the Freudian subconscious. Roughly
speaking, this is, in our terms, the detailed neural ac
tivity that leads up to consciousness (as in the retina)
or bypasses it (as in on-line systems).
n
cognitive
terms, it includes the unconscious computations lead
ing up to the conscious results of the computations,
though these results may be fairly transient, as well
as at least some of the neural activity that is triggered
by the NCC neurons.
The Freudian unconscious is usually taken to
mean (neural) activity that could in principle become
conscious but is being actively repressed at that time.
We
have not dealt with this topic.
Libet
Libet has given a crisp summary of some of his im
portant work on human consciousness. He is, of
course, correct that we cannot at this stage be certain
that a macaque monkey is conscious, but we do not
agree with im that for this reason one should not try
to study consciousness on monkeys. There would be
little point in working on monkeys if one could do
exactly the same experiments on humans but, for ethi
cal reasons, this is often impossible. So we adopt the
working hypothesis that monkeys can be conscious,
based on the similarity in neuroanatomy and behavior
between humans and monkeys and their evolutionary
relatedness. When comparing the behavior of these
two species, we must take care that the character of
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S
the monkey's (nonverbal) responses are similar in
character to those
of
a conscious human doing the
same task.
f
these experiments, as we hope, give us
some important clues as the nature of consciousness
this will make
it
easier to perform the relevant experi
ments
on
humans, which, without such clues, we
might not have thought of. Note that, so far, there has
been no repetition
of
Libet's important work, while
in the same period, several informative experiments
have been done
on
alert monkeys.
As to Pollen' s remarks (Pollen, 1999), quoted by
Libet, we do not doubt that
VI
plays an important role
in conscious vision. Our hypothesis (Crick and Koch,
1995) was that the activity of the neurons there does
not correlate with the relevant visual percepts. This
hypothesis is being tested experimentally (e.g., He,
Cavanagh, and Intriligator, 1996; Logothetis, 1998).
Libet has stated in his commentary that patients
with bilateral lesions in the temporal lobes can lose
all ability to form short-term memories. This state
ment is correct
if short-term
means times
of
the
order
of
10 seconds, but it is not true for very short
term memories,
of
the order
of
less than a se
cond--often
called for vision, iconic memory. We
consider this form
of
very short-term memory essen
tial for consciousness.
As to Libet's proposal for a conscious mental
field (CMF) or the one alluded to by Smith in his
commentary, we ourselves regard this hypothesis as
very unlikely. In the absence
of
dendrites and axons,
we know
of
no physical mechanism operating in brains
that can transmit the relevant information between
groups
of
neurons within a fraction
of
a second with
the required point-to-point specificity. In particular, it
remains utterly unclear how quantum mechanical ef
fects would solve any
of
these problems. However,
this does not mean that Libet's proposal should not be
tested, though as
he
said, a test would not be easy.
We have already dealt with
emotions-the m in
topic
of
Panksepp's commentary and will deal with
the local-global topic in the Addendum. We agree with
Panksepp
on
the usefulness of anesthetics
in
the study
of
consciousness. Studies on the detailed effects
of
anesthetics
on
neural activity
in
macaque monkeys are
in
progress (N. Logothetis, personal communication).
t
should be noted that restricted brain areas can also
be reversibly inactivated by local cooling, or by in
jecting GABA analogues.
Jackendoff
Jackendoff rightly surmises that even though we are
reductionists we try to study the activity
of
the brain
Crick Koch
(or
at
least certain aspects
of
that activity)
at
all levels,
from the philosophical to the molecular. It is a good
general rule, however, that the best way to establish a
theory
at
one level is to use evidence from a lower
level. For example the trichomatic theory
of
photopic
vision has been firmly supported by molecular biologi
cal studies
on
the different rhodopsins. So we think
that, in the long run, the neural and molecular levels
may be decisive in establishing theories
at
the psycho
logical levels.
We do not think, as Jackendoff rather implies,
the unconscious homunculus will be any single brain
area. We believe that various aspects
of it
will be rep
resented in various special (mainly prefrontal) areas
and that the activities
of
the homunculus will require
special interactions between these areas. We have not,
as yet, developed any outline theory (on the lines
of
Marr's general ideas about vision)
of
how this might
happen. We do not like Dennett's analogy, quoted
by
Jackendoff, that the experienced ego is like a center
of
gravity. This seems altogether too simple a concept
for such a complex thing as the ego. Whether our tenta
tive ideas about the (unconscious) homunculus corre
spond to a real entity or a virtual entity we leave
Jackendoff to decide.
We are not enthusiastic about Jackendoff's ideas
on the usefulness
of
consciousness, preferring our own
suggestion. He suggests that
the
conscious
field is the domain over which attentional pro
cesses can operate. We will not enlarge here
on
the
difficult topic of attention, except to say that Jacken
doff is mainly concerned with what is sometimes
called top-down attention, and that there is a recent
claim (Kentridge, Heywood, and Weiskrantz, 1999)
that responses in blindsight can
be
influenced
by
con
scious top-down attention. There is also experimental
data that top-down attention can have effects
at
least
as early as
VI
in both the macaque (Vanduffel, Too
tell, and Orban, 2(00) as well as
in
the human visual
system (Brefczynski and DeYoe, 1999). We also note
that we suspect that the idea, which J ackendoff does
not like, that something
can be
briefly conscious may
turn out to be a useful way to describe certain brain ac
tivities.
ddendum
ntroduction
To deal with the topic
of
local versus global activity
the Editors have kindly allowed us to develop briefly
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Response to the Commentaries on the Unconscious Homunculus
53
our ideas beyond those presented in our original paper.
Much of this was written down before we saw the
commentaries.
First let us state the apparent paradox. The local
effects
of
brain damage can remove from conscious
ness the content
of
certain aspects of (visual) con
sciousness, as in cases of
achromatopsia-the
inability
to consciously perceive colors (Zeki, 1993), prosopag
nosia or the inability to identify specific faces
or
to
recognize faces
as
a class (Bauer, 1993), or akinetop
sia, the inability to experience motion (Zihl, Von Cra
mon, and Mai, 1983). Notice that two
of
these (color,
motion) might be considered sensations (or primitives
of
vision) whereas a face would be considered a per
cept. On the other hand functional brain imaging tech
niques such as PET or fMRI in human subjects reveal
that seeing a simple stimulus, such as fringes, moving
dots, faces, and so on leads to widespread and distrib
uted activity in many parts
of
the brain (as assayed
via changes in brain hemodynamics). In short, brain
damage makes consciousness appear somewhat local,
whereas scans suggest it can be more global.)
Let us now sketch our very tentative ideas about
the neural correlates
of
the content
of
some aspects
of
viSUal) consciousness. As a background we outline
briefly our present overall view
of
the activities
of
the
cerebral cortex. Any particular cortical area receives
a variety of fairly specific inputs. It learns to detect
some
of
the correlations in and between these inputs
and to embody them in the strengths of its synapses,
or in other neural parameters, so that it can in the
future respond strongly and rapidly to such correlated
inputs. That is, its neurons learn to create receptive
fields whose
features
reflect the correlations that it
has learned, such as orientation and disparity in VI,
or faces and heads in IT.
The visual cortical areas are arranged in a crude
hierarchy (Felleman and van Essen, 1991). This means
that higher up in the hierarchy the features are more
complex (in the sense that a face is more complex than
an oriented line) since the higher area has extracted
further correlations between the correlations in the
lower areas that feed into it.
A further point should be made about the known
behavior
of
cortical neurons. Many. such neurons are
sensitive to context. The context (the so-called non
classical receptive field) does not, by itself, fire the
lit is only when the differential response
of
the brain is computed
(that is, which areas respond stronger say. to drawings of faces than to
images of houses). that one finds highly localized brain regions. When
contemplating these images one should always keep this fact in mind
(Frostig. 1994).
neuron but it can modulate the firing produced by the
classical receptive field
of
that neuron (Allman, Mie
zin, and McGuinness, 1985). Some
of
these effects of
context are slightly delayed and (at least in VI) may
depend in part on feedback from higher levels in the
visual hierarchy (Lamme, Super; and Spekreijse,
1998).
In considering the NCC we have to make a rough
distinction between enabling factors, on the one hand,
and specific factors on the other. What we are looking
for are the specific factors. That is, the neural activity
that is specific for the particular percept under consid
eration. There is experimental evidence from single
electrode electrophysiology and dye imaging on mon
keys, and from the results
of brain damage on humans,
that any particular part of the visual cortex responds
strongly to only certain aspects
of
the visual scene. In
other words, there is a high degree
of
localization in
the brain. Exactly what is being localized is not always
completely clear and, moreover, it can change some
what with experience.
ssential Nodes
The evidence from brain damage, especially in hu
mans, suggests that certain parts
of
the cortex are es
sential for a person to be conscious of certain aspects
of
the visual sensation or percept, such as color, mo
tion, faces, etc. For example,
as
already mentioned, in
the condition known as achromatopsia the patient is
quite unable to see color in part or all
of
his visual
field, depending on the exact nature of the brain dam
age. Zeki has, very reasonably, described such a piece
of the cortex as an essential node for that aspect of
the percept (Zeki and Bartels, 1999).
The term should not be taken to imply that a
person who possessed only the relevant essential node
would be conscious
of
that aspect of the percept. It is
highly probable that to produce that aspect of con
sciousness the node would have to interact with other
parts
of
the brain. The point is that damage to that
essential node would specifically remove that particu
lar aspect of the sensation or percept, while leaving
other aspects relatively intact. Our basic assumption
is that the NCC at any moment involves a certain
activity in a group
of
neurons in the brain. Let us
consider one
of
these groups. One set
of
questions is:
Exactly what is the nature of this
activity ?
What
leads up to the production
of
it? How long does it last?
What effect does it have on other parts
of
the brain?
Another set of questions is: What neurons (at that
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S
particular time) form this local group? Are they only
of certain neuronal types? and,
can
the group usefully
be considered to be made of subgroups?
f
so, about
how many subgroups are there likely to be? And typi
cally how many neurons are there in any particular
subgroup? Moreover we need to ask: What do mem
bers
of
one subgroup have in common? And, how are
they and different subgroups connected?
We can also ask how this active group
of neurons
changes for different percepts.
n
particular, are there
types of neurons that never form partof such a group?
Or, alternatively, can every type
of
neuron in the brain
(or, more plausibly, every type
of
neuron in the cere
bral cortex) form part of the
Nee at
one time or an
other?
Obviously, to make any progress we have to
make a few basic guesses. We shall assume that the
smallest group
of
neurons we can usefully consider
consists
of
neurons (probably pyramidal cells)
of
a
single type-so that they all project in a similar man
ner to roughly the same
area-located
fairly close to
gether in one place and in the same cortical sublayer.
By place
we mean, crudely speaking, in one or a
few patches or columns of the cortex, and any
corresponding patches in subcortical structures.
We suggest this because, in this context, we do
not believe in so-called grandmother cells, but in a
distributed representation. That is, the information is
expressed in the way the activity is distributed across
the group, in order to have the advantages
of
popula
tion-coding. We leave open the question as to whether
the activity
of each neuron is binary (yes or no) or
graded.
This is not to suggest that in
anyone
patch
of
the cortex theNee is expressed by only a single type
of cortical neuron. On the contrary we believe it is
more likely that in
anyone
cortical patch there are
likely to be several types of neurons expressing the
Nee at the moment, stacked crudely one above the
other. ur reason is that (as emphasized
by
Bernard
Baars [1988]) we expect the information in the
Nee
will be widely distributed to many parts
of
the brain.
A single type
of
pyramidal cell rarely projects to many
very separate places. Most of them project to one, or
occasionally to two or three distinct places. Therefore
we guess that the
Nee
will inVOlve in one place,
more than one type
of
neuron.
ctivity
Let us now turn to
activity.
We shall assume that
the
Nee
is associated with the rise
of
the activity
Crick Koch
above a threshold that is enough for the activity tobe
maintained for a sufficient duration. That is, a time
such that this activity can have significant effects at
multiple places in the brain.
What does one mean
by activity ?
The first
guess would be a very high average rateof firing such
as 300 to 400 Hz. However, it may be that since this
activity has to be maintained, probably for at least one
or two hundred milliseconds, so many action poten
tials arriving at anyone synapse would rapidly exhaust
the supply of synaptic vesicles available for release at
that synapse. Moreover so far there is no sign of any
neurons in the cortex firing at such a high rate for any
considerable time.
Another possibility is that the activity is really
the onset of correlated firing in the set of neurons that
make up theNee certainly
in
the neurons
of
the same
and of different types
at
one place, and possibly (to
help with the binding problem)at other relevant places
(Singer, 1999). This correlated firing would produce
a stronger effect on recipient neurons than the corres
ponding uncorrelated firing. It is possible that by this
means one can provide
an
effect appreciably bigger
than any possible high average rate of firing, such as
400 Hz. We return to this topic later.
Another possible way to produce a bigger punch
from a train
of
axonal spikes is to band them together
in small groups, often described as bursts. This is be
cause in some circumstances
it
is known that a spike
arriving
at
a synapse will not release a synaptic vesicle
with certainty but only with a certain rather lower
probability. However, two spikes close together in
time will always release at least one vesicle, since if
the first one fails the second always succeeds (Koch,
1999).
How long should this activity stay above thresh
old? Presumably long enough to produce significant
activity at least at the next stage. Thus some minimum
time seems likely, possibly in the range
of
100 to 300
msecs. It is not obvious how one might get a better
estimate of this time.
Reentrant Pathways
What
is required to maintain the activity above a
threshold? This could be something special about the
internal dynamics
of
the neuron but, more likely,
it
is
due to the existence
of
reentrant circuits (Edelman,
1989). Such excitatory feedback loops could, by reiter
atively exciting the neuron, push its activity increas
ingly upwards so that
it
not only reached above some
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Response to the ommentaries on the Unconscious Homunculus
ss
critical level but was maintained there for a time. t
is well known that, speaking loosely, there are many
reentrant pathways in the brain, so the problem is to
decide which ones might be relevant, and exactly what
properties they might have. Some
of
these loops may
be within a given cortical area but there are many
loops between regions which are further apart.
There are several parameters that need to be con
sidered.
n
rather general terms these are the number
of synaptic steps in the pathway, the overall strength
of the elements in the reentrant pathway (such matters
as how many synapses are involved at each step, and
the strength of the synapses), and the specificity. That
is, are the return connections rather widespread and
diffuse, or do they concentrate on the neurons closely
active in the patch of cortex being considered and, if
so, exactly how are they concentrated?
Having enumerated these parameters (if one
can ) for all the possible reentrant pathways originat
ing from the patch of cortex being considered, one
then has to decide on their relative importance. Is there
one pathway which is dominant, with all the other
pathways having a negligible effect or, at the other
extreme, do all the major reentrant pathways contrib
ute, so that the threshold for maintained activity is
only reached
if
all of these pathways are fairly active?
Obviously there are many possibilities between these
two extremes.
All this assumes that the reentrant pathways are
solely excitatory. This is not unreasonable, since
ll
connections between different cortical areas are excit
atory. However, inhibition may play important roles
in keeping the firing rate within reasonable bounds, in
controlling competition between groups of neurons,
and possibly helping to produce some type of corre
lated firing.
We
are so ignorant of most of the parameters
mentioned above that it is not easy to guess which
reentrant pathways are likely to be important for pro
ducing the NCe However, two kinds of pathways
probably deserve special consideration. The first are
the cortical-thalamic loops: one might guess that these
help to provide some of the biases that produce what
is commonly called attention, which is known to help
produce consciousness of the object or event attended
to. The other is the reentrant pathways between the
higher levels of the perceptual system (such as IT in
the case
of
vision) and various parts
of
prefrontal cor
tex. We have already hinted (Crick and Koch, 1998)
that the pathways from the cortical area near the ma
caque principal sulcus back to the higher ventral visual
area (such as IT) may have unusual properties (Web-
ster, Bachevalier, and Ungerleider, 1994). More re
cently Thompson and Schall (1999) have suggested
from their studies
of
the effects of masking on neurons
in the frontal eye fields (PEp) in prefrontal cortex that
the pathway from the PEF back to extrastriate visual
cortex is such that all "selective postmask activation
of
the PEF is correlated with a perceptual experi
ence of the target" p. 286). This is certainly along
the lines we have been thinking. Pathways from other
parts of prefrontal cortex back to extrastriate visual
areas may also have to be taken into account.
orrelated Firing
Unfortunately these ideas are not easy to develop fur
ther and at the moment seem very difficult to test in
any decisive manner. On the other hand our postulate
of correlated firing between the neurons expressing the
NCC in the relevant cortical patch should be testable,
though the exact composition of the active neurons in
the patch is only likely to last for a relatively short
time and will usually change when the percept
changes. One could also study the effects of training
on correlated firing. Correlated firing could take many
forms. The set of neurons might fire not only in a
correlated manner but also rhythmically, as in the so
called 40 Hz oscillations. In this case the firing of the
whole set of neurons (as seen in the field potentials)
might define a phase, with some neurons producing a
lesser punch by firing somewhat out of phase. This is
believed to happen in the locust's olfactory system
(Laurent, 1997), albeit at 20 Hz.
Unfortunately, it would be surprising if the firing
was not contaminated somewhat by noise, for example
due to the weak effects of other, irregular inputs to
the neurons.
If the correlated firing of the relevant neurons in
roughly the same place does exist, another important
topic is how this correlation comes about. A number
of different mechanisms can be imagined but we will
touch on them only very briefly. For example, do in
hibitory neurons place a crucial role (as in the locust),
especially those that synapse mainly on the soma of
cortical pyramidal neurons, as basket cells or clutch
cells do, or on the axon initial segment, as chandelier
cells do?
This type of correlated firing is worth looking for
by using multiple electrodes very close together, and
at a variety of cortical depths, in the hope that at least
some of them could sample the neurons in the relevant
patch. The general location of the relevant patches
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56
might be discovered by optical methods (see Wang,
Tanaka, and Tanifugi, 1996).
he
Effects
o
the
ee
Having discussed the possible nature of the NCC and
the events leading up to it (which includes competitive
processes) let us now turn to the effects that the NCC
might be expected to produce. These are probably of
two major kinds: the NCC activity is likely to influ
ence thoughts and the planning of actions. We have
argued (Crick and Koch, 1995) that these take place
mainly in prefrontal cortex. The NCC activity will
probably activate, to some extent, many related repre
sentations in the brain. These can be located in many
places in the brain, especially in the sensory systems.
It is probably these two kinds of activities that allow
the brain to grasp the significance (or meaning ) of
the particular neural representation being expressed at
that moment by the NCC. Notice that the significance
will be strongly dependent on the past experience of
the animal.
We shall not explore here all the interesting rami
fications of these two kinds of activity, except for one
parti Ular
aspect of them. We have just spoken as
if
the neural activity went only from the region of the
NCC to the regions for planning and the regions for
associations. However, it is more than likely that, in
all these cases, there are reciprocal neural pathways.
So we must now speculate what effect these return
pathways might have on the NCC. In particular, is the
neural activity in some or all of these return pathways
essential for the neural activity in the essential nodes
(that can express that particular percept) to reach
above the threshold required for consciousness (as dis
cussed above)? This might be the neural correlate of
significance (or
meaning )
.
f
this were the case,
then some might prefer to use the term ee for the
many activities in these reciprocal pathways. Person
ally we would prefer to keep the term ee for the
critical activity at the essential nodes, while accepting
that this activity could not reach above the threshold
necessary for consciousness unless the return path
ways were active.
Notice that the act ivi ty in the regions to which
a critical node projects need not itself reach above the
critical level for consciousness though it may be
enough to produce unconscious priming elsewhere, re
sulting in implicit memory (Schacter, 1987). Most of
the associations we have to a conscious percept are
latent and do not reach consciousness, though some
Crick Koch
can, in some cases, become a later item of conscious
ness as the result of the association. Do we also have
latent thoughts and plans, that is, ones that are primed
but not strongly expressed? In terms of average rate
of
firing all these latent activities may be quite strong,
but if, as we have argued, consciousness requires cor
related firing, then they would not reach consciousness
in these related places
if
the firing there was moder
ately rapid but uncorrelated. This lack of correlated
activity might also explain why, in recordings
of
indi
vidual neurons throughout the medial temporal lobe
in epileptic patients, neural activity following direct
perception
of
particular classes of images (say faces
versus animals) is only somewhat stronger than the
firing rate of the same neuron when the subject is
asked to imagine the previously viewed pictures, de
spite the fact that for most subjects imagery is much
less vivid and immediate than vision (Kreiman, Koch,
and Fried, 2000).
Summary
In summary, we postulate that the proper neural activ
ity at the appropriate essential (local) nodes has to be
maintained above some critical value for some appre
ciable time; that widespread (global) neural activity
may normally be required to produce this essential
activity, and that the effects of this activity on the rest
of
the brain may also be widespread (global). We do
not claim originality for most of these ideas. They
are obviously related to those of Tononi and Edelman
(1998) and to those of Singer (1999) and many others.
These ideas are at the moment not easy to test.
However,
if
the essential activity involves some form
of correlated firing, then at least this could be looked
for, though care must be taken to search for it in care
fully considered sets of neurons, rather than in neurons
sampled at random. Note that we are not suggesting
that correlated firing in the brain only occurs for con
scious neural activity. It is likely to be used also for
other purposes.
We hope this brief sketch will show Panksepp
how, at the moment, we view the local-versus-global
issue.
gentic Qualia
Humphrey (as well as Patrick Wall) has suggested the
idea of agentic qualia.' , We do not think that agentic
qualia are the solution to his Necker Cube problem.
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Response
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the Commentaries on the Unconscious Homunculus
7
He states whether we experience a sight, sound
etc., the conscious experience can be expected to
consist not only of the sensory qualia appropriate to
the particular sensation but also what agentic qualia
are being called into being. These agentic qualia,
he suggests, have small but noticeable feels to them,
produced by an implicitly formulated plan
of
action.
At the moment we prefer to think, as we have de
scribed above, that these implicit plans of action may
indeed contribute to the production
of
the qualia of
objects but may not themselves have qualia at that
moment.
Schall
The commentary by Schall was received after the Ad
dendum was written. We are very enthusiastic about
both Schall's experiments and his attitude to the prob
lem. We are thinking about visual awareness on some
what similar lines to him,
as
can be seen by comparing
his suggestions with ones in our Addendum.
We
certainly agree with him that the intelligent
interpretation
of
the neurophysiological data will re
quire more sophisticated and self-consistent con
cepts,
but
we
rather doubt whether philosophers will
provide much help. There is, after all, no reason why
neuroscientists should not refine the concepts them
selves, and they have the advantage that they are pre
pared to take the trouble to master the experimental
details (which is especially important in biology) and
which may contain vital clues to the neurophysiologi
cal processes underlying visual awareness.
Two short footnotes about Schall's comments.
The words he quotes from
us
(in italics) were preceded
by I t is plausible that. We agree with Schall
that neurons with unique neuronal properties may not
be necessary. However, given the great specificity
of
biological mechanisms in molecular, cell, and neurobi
ology, it is not unreasonable to suppose that this speci
ficity will extend to the neuronal correlates of
consciousness. Furthermore, it would be an enormous
advantage experimentally
if
such neurons existed, so
the possibility should not be overlooked.
Schall's important experiments with Logothetis
(Logothetis and Schall, 1989) on cortical area MT
were indeed the first explicit demonstration
of
the neu
ral activity related to visual awareness as opposed to
stimulus properties. It should be noted, however, that
more recently Leopold and Logothetis (1999) have
written that in earlier visual areas such as MT most
modulating neurons [in binocular rivalry] exhibited
changes in their spiking rates that were short-lived,
and more closely associated with transitions than with
lasting perceptual states. Only in the inferotemporal
cortex (IT) was elevation or suppression [of cortical]
activity commonly sustained throughout a period
of
perceptual dominance
(p.
258). This makes it some
what unlikely that these activities in MT and other
early visual areas were the real correlates of visual
awareness in these experiments.
Stevens
Stevens, in his commentary, has made a useful point
about introspection.
As
he states, it is usually carried
out in retrospect, rather than at the time or shortly
after. We look forward to his discussion in press
of
this and phenomenological methods in general. In the
long run
we
shall need to know what is happening
in the brain when an observer reports and also what
neuronal events led up to the report. This may indeed
involve a sequence
of
operations.
We
have already discussed in the Addendum
Freud's usage of conscious. What
we
call uncon
scious Freud would have called subconscious.
Whether Freud's views (omitting his ideas about re
pressed ideas) really are the same
as
Stevens's and
Jackendoffs we leave to Freudian scholars. .
We
agree with Stevens that there
is
more to phe
nomenal consciousness than pain, color, taste, etc.
That is why we extended the use of the word qualia
to include dog-qualia face-qualia etc. (see our com
ments on Humphrey's ideas).
We
have already made
some tentative suggestions in the Addendum on the
difficult problem of meaning. As to the unconscious
homunculus this is, at the moment, just a catch phrase.
We
certainly think
of
it
as
involving multiple pro
cesses.
We
agree with Stevens that the conceptual is
sues surrounding
will and its neuronal basis
constitute a somewhat neglected topic, but see Wegner
and Wheatley (1999).
Conclusion
Our overall impression from the commentaries is that
there
is
no general agreement
as
to what mental activi
ties have qualia associated with them, except that ev
eryone thinks that sensations do. We rather doubt
that further discussion
of
the topic will be fruitful,
though it might be interesting to hear Baars, McGov
ern, Jackendoff, Stevens, and Humphrey debate the
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8
topic together,
if
only to hear how they disagree. It
seems very difficult to decide solely by introspection
which psychological processes (such as those associ
ated with sensations, percepts, thoughts, emotions,
valuati01.1s
etc.) are associated with qualia
of
their
own. As Baars and McGovern rightly point out, the
discussions in the latter parts
of
the nineteenth century
on imageless thought reached no conclusion.
It is very important to realize that these matters
will not be finally settled by learned discussion, nor
solely by psychological experiments, but that all theo
ries, some produced by these kinds
of
approach, must
be tested to see how they map onto the detailed behav
ior
of
the brain. Only this will allow us to see which
concepts, and which theories about them, have real
scientific validity.
Let us consider one example. The percept
of a
face could
be
considered merely as a conjunction
of
the outline, surface orientation, color, movement, etc.,
of the parts of the face. On this view there would be
no face-qualia as such. The main reason we prefer the
proposal that face-qualia exist is that patches of neu
rons are found in inferotemporal cortex (and else
where) that respond strongly when a monkey appears
to report that it sees a face, and also that localized
brain damage in humans can prevent the patient from
recognizing his own face or, in other cases, of not
recognizing a face as a face. In short, there appear to
be
essential nodes for faces in patches in the primate
brain.
By analogy we can ask: Is there an essential node
for (some aspects of) the thought
of
a face? In other
terminology, are there essential nodes for a viewer
independent model
of
a head? This might be con
structed by the brain by combining the inputs
of
a
patch of neurons coding for all the different views
of
the head, and indeed Perrett (Perrett, Hietanen, Oram,
and Benson, 1992), Logothetis (Logothetis and Pauls,
1995), and Booth and Rolls (1998) have reported a
small number of neurons in the macaque that have
such properties. Whether, as we have speculated, there
is a higher percentage
of
them in prefrontal face
patches 0 Scalaidhe, Wilson, and Goldman-Rakic,
1997) remains to be discovered.
We can then ask of such neurons, do they have
other properties that might possibly lead to qualia
of
some sort? For example, does the set of them project
widely? Do they show correlated firing? And so on.
And it may be that they have some of the properties
of view-dependent face neurons, but lack others. For
example, they may project less widely, and to some
what different places.
Crick Koch
Our own strategy has been to concentrate in the
first instance on cases which we think are very likely
to have qualia associated with them (such as visual
sensations and percepts) and which can be studied in
the macaque monkey, and for the moment to leave the
more questionable qualia on one side. Naturally others
may prefer a different approach.
This leads to our final comment, which may not
be
a welcome one. We are disappointed that (with the
exception of Libet and Schall) no one else made any
suggestions
of
any experimental test
of
a neurobiolog
ical nature that might advance the subject.
f
psycho
analysis and neuroscience are to interact effectively
there must be more emphasis
on
possible experiments,
especially neuroscientific ones, and less time devoted
to describing, ad nauseam, what people thought in the
past. One
of
us proposes that, in future, any reference
to Freud should be barred from this journal, at least
for the next 1
0
years, though,
of
course, one could
hardly object if authors read his work in secret.
References
Allman, J., Miezin, F., McGuinness, E. (1985), Stimulus
specific responses from beyond the classical receptive
field: Neurophysiological mechanisms for local-global
comparisons in visual neurons.
Ann. Rev. Neuros ci.
8:407-430.
Baars, B. J. (1988), A Cognitive Theory
o
Consciousness.
Cambridge, U.K.: Cambridge University Press.
Bauer, R. M. (1993), Agnosia.
In:
Clinical Neuropsychol
ogy 3rd ed., ed. E. Valenstein & K. M. Heilman. New
York: Oxford University Press, pp. 215-277.
Blaser, E., Sperling, G., Lu, Z. L. (1999), Measuring the
amplification of attention. Proc. Nat. Acad. Sci.
96(20):11681-11686.
Booth, M. C.
A &
Rolls, E. T. (1998), View-invariant
representations of familiar objects in the inferior tempo
ral visual cortex. Cereb. Cortex 2:510-523.
Brefczynski, J. A DeYoe, A E. (1999), A physiological
correlate of the spotlight of visual attention. Nature
Neurosci. 2:370-374.
Crick, F., & Koch, C. (1995), Are we aware of neural activ
ity in primary visual cortex? Nature 375:121-124.
(1998), Constraints
on
cortical
and
thala
mic projections: The no-strong-loops hypothesis. Na
ture 391:245-250.
Edelman, G. M. (1989), The Remembered Present: A Bio
logical Theory
o
Consciousness. New York: Basic
Books.
Felleman, D. J., Van Essen, D. C. (1991), Distributed
hierarchical processing in the primate cerebral cortex.
Cereb. Cortex 1:1-47.
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Response
to
the Commentaries on the Unconscious Homunculus
S
Frostig, R D. (1994), What does in vivo optical imaging
tell us about the primary visual cortex in primates? In:
Cerebral Cortex, Vol. 10, ed. A. Peters K S. Rock
land. New York: Plenum Press, pp. 331-357.
He, S., Cavanagh, P., Intriligator, J. (1996), Attentional
resolution and the locus
of
visual awareness.
Nature,
383:334-337.
Humphrey, N. (1992),
A History
of
the Mind.
London: Si
mon Schuster.
Jackendoff, R (1987), Consciousness and the Computa
tional Mind. Cambridge, MA: MIT Press.
Kentridge, R. W., Heywood, C.
A
Weiskrantz, L.
(1999), Attention without awareness in blindsight. Proc.
Roy. Soc. Lond. B 266:1805-1811.
Koch, C. (1999),
Biophysics of Computation.
New York:
Oxford University Press.
Kreiman, G., Koch, c. Fried,
I.
(2000), Imagery neurons
in the human brain. (Submitted.)
Lamme, V. A F., Super, H., Spekreijse, H. (1998), Feed
forward, horizontal, and feedback processing
in
the vi
sual cortext. Curro Op.
Neurobiology, 8:529-535.
Laurent, G. (1997), Olfactory processing: Maps, time and
codes. Curro Op. Neurobiol., 7:547-553.
Leopold, D. A Logothetis, N. (1999), Multistable phe
nomena: Changing views in perception. Trends Cog.
Sci., 3:254-264.
Logothetis, N. (1998), Single units and conscious vision.
Philosoph. Trans. Roy. Soc. Lond. B 353:1801-1818.
Schall, J (1989), Neuronal correlates
of
subjective
visual perception.
Science, 245:761-763.
Pauls,
1
(1995), Psychophysical and physiological
evidence for viewer-centered object representations in
the primate.
Cereb. Cortex, 3:270-288.
Marr, D. (1982),
Vision: A Computational Investigation into
the Human Representation and Processing of Visual In
formation.
San Francisco: W. H. Freeeman.
Milner, A D., Goodale, M. (1995), The
Visual Brain in
Action.
Oxford: Oxford University Press.
Perrett, D.
I.,
Johnston, R S., Benson, P. J., Jordan,
T.
R
Heeley, D. W. (1991), Perception and action in
visual
form
agnosia.
Brain, 114:405-428.
o Scalaidhe, S. P., Wilson,
F. A
W., Goldman-Rakic,
P. S. (1997), Areal segregation
of
face-processing neu
rons
in
prefrontal cortex.
Science, 278:1135-1138.
Perrett, D. I., Hietanen, 1 K., Oram, M. W., Benson, P.
J. (1992), Organization and functions of cells responsive
to faces in the temporal cortex. Phil. Trans. Roy. Soc.
Lond.
B
335:23-30.
Pollen, D. (1999), On the neural correlates of visual percep
tion. Cereb. Cortex, 9:4-19.
Schacter, D. L. (1987), Implicit memory: History and cur
rent status. J
Exp. Psychology, 13 3):501-518.
Singer, W. (1999), Neuronal synchrony: A versatile code
for the definitions
of
relations?
Neuron, 24:49-65.
Thompson,
KG.
Schall, J. D. (1999), The detection
of
visual signals by macaque frontal eye field during mask
ing. Nature Neurosci., 2 3):283-288.
Tononi, G., Edelman, G. M. (1998), Consciousness and
complexity.
Science, 282:1846-1851.
Tranel, D., Damasio, A (1985), Knowledge without
awareness:
An
autonomic index of facial recognition by
prosopagnosics.
Science, 228:1453-1454.
Vanduffel, W., Tootell, R. B. H., Orban, G. A (2000),
Attention-dependent suppression
of
metabolic activity in
the early stages
of
the macaque visual system.
Cereb.
Cortex, 10:109-126.
von
Senden, M. (1960),
Space
and
Sight: The Perception
of
Space and Shape in the Congenitally Blind Before
and
Aft er Operation.
Glencoe, IL: Free Press.
Wang, G., Tanaka, K Tanifuji, M. (1996), Optical im
aging
of
functional organization in the monkey infero
temporal cortex.
Science, 272:1665-1668.
Webster, M. J., Bachevalier , J., Ungerleider, L. G. (1994),
Connections of inferior temporal areas TEO and TE with
parietal and frontal cortex in macaque monkeys. Cereb.
Cortex, 5:470-483.
Wegner, D. M., Wheatley, T. (1999), Apparent mental
causation. Sources of the experience of will. Amer. Psy
chologist, 54:480-491.
Zeki, S. (1993),
A Vision
of
he Brain.
Oxford: Oxford Uni
versity Press.
Bartels,
A
(1999), A theory
of
visual conscious
ness.
Consciousness Cognit., 8:225-259.
Zihl, J., Von Cramon, D., Mai, N. (1983), Selective dis
turbance
of
movement vision after bilateral brain-dam
age. Brain, 106:313-340.
Christof Koch
Division
of
Biology
B9 74
Caltech
Pasadena, CA 9//25
e-mail: [email protected]
Phone: 626-395-6855
Fax: 626-796-8876
Web:
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