Neural Correlates of Consciousness - T. Metzinger (1999) WW.pdf - Neurobiology - koloidalnyy

Preface

This volume tries to make a bold step forward.

For the ®rst time ever, it unites a number of

highly renowned neuroscientists and philoso-

phers, from all over the world, who are together

investigating the way in which the content of

subjective experience is correlated with events in

the brain. The current collection grew out of the

conference ``Neural Correlates of Consciousness:

Empirical and Conceptual Questions,'' which

took place June 19±22, 1998, in Bremen, Ger-

many. I had the honor to act as the principal

organizer for this conference, and I am now very

pleased to be able to present a small selection of

the many excellent contributions that were made

there.

The 1998 Bremen meeting was a special event

for the consciousness research community for

several reasons. One was that it was organized

and supported by two newly founded and partic-

ularly dynamic academic institutions: the Asso-

ciation for the Scienti®c Study of Consciousness

(ASSC) and the Hanse-Wissenschaftskolleg in

Delmenhorst. I am deeply indebted to both

organizations, and want to take the opportunity

to thank some of the people who have helped

me. First, the members of the ASSC conference

committee: Bill Banks, David Chalmers, Christof

Koch, Antti Revonsuo, and Patrick Wilken. They

assured the scienti®c quality of the meeting, and

in doing so indirectly made it possible for this

volume to come into existence. Second, I wish to

thank all those who have supported me on the

level of local organization. Professor Dr. Dr.

Gerhard Roth, as director of the hosting institu-

tion, ®nancially secured the enterprise and, with

continuous enthusiasm, backed it up on many

levels. Ingeborg Mehser, Diana Garde, Martin

Meier, Beatrice Riewe, and Marion Wachholz-

Logemann mastered a number of unforeseeable

di½culties and took countless burdens o¨ my

back. I could not have done without the contin-

uous support from the superb team in the

Hanse-Wissenschaftskolleg. The same is true, in

particular, of my wife, Anja. She is my contin-

uous source of strength and inspiration. In a way,

however, I am most proud of the student team

led by John-Dylan Haynes, Harald Schmidt, and

Sven SchÈ tt: Jakob BÈ senberg, Christina Egger,

Christine HÈûler, Martin Haindor¨, JÈ rgen

Hanken, Alexander Hillmann, Stefan Huth,

Tobias Gronau, Ulrich KÈ hne, Heide MÈ ller-

Leiendecker, Marcus Naumer, Anett Noster,

and Andreas Wawrzinek. Some of them may not

yet know it, but they are the consciousness

researchers of the future.

San Diego, July 1999

Introduction: Consciousness Research at the End of the Twentieth Century

From False Intuitions to Psychophysical

Correlations

scious experience. This content can be a simple

qualitative feature like ``grayness'' or ``soggi-

ness.'' There are also complex, nested forms of

conscious content like ``the self in the act of

knowing'' (see, e.g., chapters 7 and 20 in this

volume) or high-level phenomenal properties like

``coherence'' or ``holism'' (e.g., chapters 8 and 9

in this volume). But what, precisely, does it mean

that conscious experience has a ``content''? Is

this an entity open to empirical research pro-

grams and interdisciplinary cooperation? And

what would it mean to map this content onto

physical states ``under a certain description''? In

other words: What kinds of relations are psy-

chophysical correlations? Do we have a work-

able conception of the isomorphism we are

obviously assuming? If one is seriously interested

in getting away from the naÈvetÂ of popular dis-

cussions concerning consciousness, the ®rst thing

one has to understand is that we know the world

only under representations. For philosophers this

is a point of great triviality, but since the large

majority of contributors in this volume address

empirical issues, a few short remarks may be in

order. Let me explain.

In 1989 the philosopher Colin McGinn asked the

following question: ``How can technicolor phe-

nomenology arise from soggy gray matter?''

(1989: 349). Since then many authors in the ®eld

of consciousness research have quoted this ques-

tion over and over, like a slogan that in a nut-

shell conveys a deep and important theoretical

problem. It seems that almost none of them dis-

covered the subtle trap inherent in this question.

The brain is not gray. The brain is colorless.

Obviously, the fundamental methodological

problem faced by any rigorous research program

on consciousness is the subjectivity of the target

phenomenon. It consists in the simple fact that

conscious experience, under standard conditions,

is always tied to an individual, ®rst-person per-

spective. The subjective qualities inherent in a

phenomenal color experience are a paradigm ex-

ample of something that is accessible from a ®rst-

person perspective only. Color consciousnessÐ

regardless whether in gray or in TechnicolorÐis

a subjective phenomenon. However, the precise

nature of the relationship of such ®rst-person

phenomena to elements within the domain of

objectively describable events is unclear. From

an objective, third-person perspective all we ®nd

in the world are electromagnetic radiation and

the re¯ectance properties of middle-sized objects,

wavelength mixtures and metamers, retinal input

vectors and activation patterns in the visual

system. None of these, so far, map nicely and

systematically onto the chromatic primitives of

subjective, visual experience. It is just as our

physics teacher in high school always told us:

From a strictly objective perspective, no such

things as colors exist in the world. Therefore, the

pivotal question is not How do we get from gray

to Technicolor?

The core question is if at allÐand if so, in

what senseÐphysical states of the human ner-

vous system, under a certain description, can be

successfully mapped onto the content of con-

Theoretical and Phenomenal Models of Reality

One way to know the world (and ourselves) is

under theoretical representations. For instance,

we can use descriptions of the brain generated by

empirical research in the cognitive neurosciences.

Neurophysiological descriptions of certain brain

areas or neural algorithms describing their com-

putational properties are typical and well-known

examples. We can also gain further knowledge

under conceptual interpretations of such descrip-

tions generated by analytical philosophers of

mind. For instance, philosophers might speak

about the way in which a certain abstract prop-

erty, such as a causal role, is ``realized'' by a

certain concrete state in the brain. Both types of

descriptions are linguistic representations, and

their content is propositional.

Introduction

Another way to know the world (and our-

selves) is under a phenomenal representation. For

instance, to come back to our initial example, we

can use the content of conscious experience gen-

erated by our own brain in the act of visually

perceiving another brain in order to gain knowl-

edge about the world. ``Grayness,'' for instance,

is one important aspect of the content of a phe-

nomenal representation. The subjectively experi-

enced colors of a rainbow or those of a movie in

Technicolor are further examples. The format of

phenomenal representations is something for

which we currently possess no precise terminol-

ogy, but it is obviously not of a syntactically

structured, linguistic kind, and their content is

only very rarely of a conceptual or propositional

nature. You don't need language to be con-

sciousÐa nonlinguistic creature could certainly

have the subjective experience of ``grayness.''1

Again, there are also conceptual interpretations

of the content of conscious representations itself

(for instance, generated by phenomenologically

oriented philosophers of mind), and in some

cases such descriptions constitute a valuable

source of information.

At the end of the twentieth century we have

some good ideas about what it could mean for

an empirical theory (the ®rst type of representa-

tion) to possess ``content.'' However, it is unclear

what it means, precisely, to claim that states of

consciousness (the second type of representation)

have ``content.'' I am not going to answer this

question here. But let me frame it in a simpli®ed

way that may serve to illustrate an important

aspect of the underlying issue. The problem may

consist in the fact that phenomenal representa-

tions are special in having two kinds of content.

Philosophers sometimes speak of the intentional

content and of the phenomenal content of mental

representations. Consider the following example:

While visiting one of the new underground lab-

oratories for experimental philosophy of mind,

which are mushrooming all over the world, you

suddenly ®nd yourself holding a freshly excised

human brain in your hand and, looking at it, you

have the phenomenal experience of ``grayness''

and ``sogginess.'' The next night, after awaking

from a nightmare in which you subjectively

relived exactly the same scene, including pre-

cisely the same visual and tactile qualities, you

realize that you have just had a complex hallu-

cination. This time, fortunately, it was all a

dream.

What was the di¨erence between the two epi-

sodes? In a ®rst and very rough approximation

one might say the following: In the initial case

your relevant mental state had intentional and

phenomenal content. The intentional content

consisted in the fact that this mental state actu-

ally referred to something in the external world;

there really was a brain in your hand. The phe-

nomenal content consisted, for example, in the

subjectively experienced qualities of ``grayness''

and ``sogginess.'' In the second case, however,

there was only phenomenal content, because no

such thing as a brain existed in your present

environmentÐyour hand was paralyzed and your

visual system was decoupled from external input

(regarding dreams as a model system for phe-

nomenal experience, see chapter 4 in this vol-

ume). If you remove the external component, you

seem to get very close to the pure experiential

content (on the neural correlates of spontaneous

visual hallucinations and on bistable phenom-

ena, see chapters 14 and 15 in this volume).

It is probably safe to say that a majority of

experts in the relevant areas of philosophy

would, while wholeheartedly disagreeing about

the nature of intentional content, at least sub-

scribe to the thesis that phenomenal content, in a

strong sense, supervenes on properties of the

brain.2 That is, as soon as all internal and con-

temporaneous properties of your brain are ®xed,

all properties of your conscious experience are

fully determined as well. What is determined is

how being in these states feels to you, not if

these states are what philosophers would call

``epistemic states''Ðstates that actually carry

knowledge by relating you to the world in a

meaningful way. In the short introductions

Introduction

written for the parts of this volume, I will use the

concept of ``phenomenal content'' in accordance

with this loose, nontechnical de®nition: The

phenomenal content of your mental representa-

tions is that aspect which, being independent of

their veridicality, is available for conscious ex-

perience from the ®rst-person perspective while

simultaneously being determined by inclusively

internal properties of your brain.

What is the upshot of this ®rst conceptual

clari®cation? Consciously experienced colors or

the tactile experience of ``sogginess'' are parts of

a phenomenal model of reality. The content of

global conscious states like waking or dreaming

is the content of phenomenal models of reality,

episodically activated by the brain of an individ-

ual human being. Wavelength mixtures and the

like are theoretical entities in scienti®c models of

reality. Scienti®c models of reality are generated

by socially interacting groups of human beings.

This point is important in order to prevent a

second possible form of popular naÈvetÂ lurking

in the background. The reality of the brain as

well as the reality of consciousness as described

by science are, strictly speaking, not ``the'' objec-

tive domain. They are the result of intersubjective

cooperation within scienti®c communities. If

readers will permit the use of a connectionist

metaphor: A theoretical model is more like a

distributed and coherent pattern in a social net-

work, dynamically unfolding its informational

content while subtly changing the internal land-

scape of the overall system. It is also interesting

to note that, in parallel with the renaissance of

systematic research programs on conscious ex-

perience, we are starting to discover the neural

correlates of social cognition as well (see chapter

22 in this volume).

If individual human beings, maybe as ob-

servers of a neurosurgical operation or, while in

the basement of a pathology institute as witnesses

of the dissection of a corpse, consciously look at

the exposed brain of a fellow human being, then

they will, under standard conditions, experience

this brain as having the color gray. Their brains

activate individual phenomenal models of real-

ity, including the visually perceived brain. From

an objective point of view, however, both brains

involved in this perceptual relation are abso-

lutely colorless. There are no colors in the exter-

nal world. Matter never was gray. So what is it

that generates those false intuitions often lead-

ing us astray? It is the fact that theoretical

reality-modeling is anchored in phenomenal

reality-modeling, and that phenomenal reality-

modeling is characterized by an all-pervading

naive realism.

From a strictly subjective point of view there

is only one brain, and in all its concrete sogginess

and grayness it is certainly not perceived by

another brain, but by a self-conscious person.

This person enjoys what Revonsuo (see chapter 4

in this volume) has called an ``out-of-the-brain-

experience'': a very robust sense of presence in

and the immersion into a seemingly real world

outside the brain. Yet many theoretical consid-

erations and a ¯ood of empirical data now

strongly point to the conclusion that in all its

ultimate realism, this form of experiential con-

tent is itself entirely dependent on the internal

workings of an individual brain. And trying to

understand this nexus between the virtuality of

an external existence and the internal dynamics

of biological information-processing certainly is

more exciting than any popular debate could

ever be. While the Mysterian's trap is just a rhe-

torical bogeyman, we are actually faced with

much deeper theoretical issues and an extremely

interesting set of empirical challenges. This book

is about these challenges. How could genuine

®rst-person phenomenal experience emerge in a

self-organizing physical universe?

The NCC: Correlating Phenomenal Content with

Properties of the Brain

Given this context, what does it mean to look for

the ``neural correlates of consciousness'' (NCC)?

The idea of an NCC has been around in dis-

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