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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

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Neurosciences, 2:1, 48-59, DOI: 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.



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



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,




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



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




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



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



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



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.



Response

t

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



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.

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Christof Koch

Division

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B9 74

Caltech

Pasadena, CA 9//25

e-mail: [email protected]

Phone: 626-395-6855

Fax: 626-796-8876

Web:

klab.caltech.edu

Documents

11 the Unconscious Homunculus: Response to the Commentaries by Francis Crick and Christof Koch