Center for Clinical Neurosciences

Neuroimaging And The Quest For Brain Signs Of Consciousness

Neuroimaging and the quest for brain signs of consciousness
Andrew C. Papanicolaou
Address to the Hellenic Neurosciences Society
Friday Nov. 30th 2007

What aspects of conscious experience can be revealed through functional neuroimaging? To address this issue the following propositions must and will be considered:

  1. A person’s discriminant behavioral responses (typically verbal reports and responses to suitable queries) constitute the basis for deciding whether a person is or is not conscious.
  2. Consciousness is not a substance (whether material or mental) nor a unitary cognitive function. Rather, it is a category name for conscious experiences, each of which is the result of the operation of several cognitive functions such as attention, perception, memory, language.
  3. Each cognitive function is mediated by a brain mechanism, that is, by the coordinated pattern of activation of one or several brain structures.
  4. The result of such activation is not always a conscious experience (i.e., sometimes perception can be subliminal).
  5. Yet to each experience (and to each non-conscious event that results from the operation of any functions) corresponds a pattern of activation that accounts exclusively for that and for no other experience (or for that and for no other event).
  6. Each and all experiences that constitute the “stream of conciseness” are unique and unrepeatable; therefore the patterns of brain activation that correspond to them are also unique and unrepeatable. These propositions along with several others of technical nature that will be also described, lead to the following four main conclusions:
  1. No activation pattern corresponding to any unique experience has been thus far obtained. Moreover, no such pattern will ever be obtained, even with ideal neuroimaging devices that may become available in the future. That is, we will never be able to identify any unique experience on the basis of its functional image.
  2. No activation pattern has been thus far obtained that corresponds to particular types or kinds of experiences, that is, to those invariant and repeatable features that allow us to classify each unique experience as a specific case of this or that percept or concept. In other words, there are no functional images that by looking at them the expert can exclaim: “This is the pattern corresponding to the perceptual experience of tables”, say, or “this pattern corresponds to the concept justice”. Nevertheless, it is possible that some such patterns may eventually be derived and such identification may become possible in the future.
  3. It is today possible to record and recognize some parts of the brain mechanisms of some functions with sufficient certainty to use that information in clinical practice. And, it is eminently reasonable to expect rapid progress in this direction in the future.
  4. But no functional image, present or future, may be informative, in the absence of other discriminant responses, as to whether the result of cognitive operations is or is not a conscious experience, though it is possible to infer from such images whether or not several brain mechanisms are or are not operative.

Before starting I should tell you that I am truly honored by your invitation to address you today.
Αλλά επίσης πρέπει να σας προειδοποιήσω ότι μάλλον θα σας απογοητεύσω μιλώντας στα αγγλικά για τον εξής απλούστατο λόγο: Ενώ τα ελληνικά μου παραμένουν, απ ότι μπορώ να ξέρω, ανεκτά, τα αγγλικά τα προφέρω με μια πολύ άσχημη, θα ‘λεγα, προφορά παρ, όλες τις προσπάθειές μου να την συμμαζέψω κάπως τα τελευταία τριάντα επτά τόσα χρόνια. Αλλά τέλος πάντων. This being said, let us get down to business, as we say in Houston but now, apparently, στα Τζουμέρκα, στη Μάνη και στη Θεσσαλονίκη as well.

A big step forward in the discipline that was known for decades as physiological psychology but is now called Systems Neurosciences, was the discovery of the importance of a simple fact, a fact known to humanity for millennia; the fact that people have consciousness.

And no sooner than we realized the importance of that fact the search for its neurophysiological antecedents was on, in many laboratories world-wide, but especially in functional neuroimaging laboratories.

In spite the many disagreements and the prodigious amounts of confusing discourse as to what we really mean by the otherwise obvious fact, considerable progress has been made and I am here to report on it.

In the first place it became obvious, early on, that consciousness is not a thing, it is not a substance, but it is either a process, that is to say, a function, or a set of events – the products of that function, the various conscious experiences. That much, I hope, has been made clear and remains clear to everyone: There might be, we postulate, a function – consciousness, that produces conscious experiences. On the other hand, it is not a hypothesis but a plain fact that conscious experiences which are the products of the hypothesized function of consciousness do exist; they are true, they certainly happen.

Now, in order to find the neurophysiological mechanism of the function of consciousness, but also in order to find the neurophysiological antecedents of each separate conscious experience we had to start with some principles. These I have summarized in five propositions.

I will flash these propositions on the screen, one at the time, because becoming aware of them is of absolute importance if you are to understand the arguments that I am about to present for your consideration.

Because, you see, I do not intend to deliver a panegyric πανηγυρικό θέλω να πω or a eulogy to functional neuroimaging but to develop some rational arguments for you to think about regarding what neuroscience has accomplished, what it will, likely, accomplish, but also what it can not accomplish, in area of consciousness.

Proposition one then:


To every single conscious experience corresponds one activation pattern in the brain, specific to that and to no other experience. The same goes for functions. Any and every function is associated with a specific brain activation pattern indicative of the brain mechanisms of that function. A different pattern of activation is expected for the somatosensory function, the motor, the language, the memory and, also, the hypothesized CONSCIOUSNESS function – such that when we see the activation patterns we will be able to say things like: “This person is now listening to speech; that one is moving his right hand; that other one is conscious”. Because, being able to tell just by looking at a pattern, to what function that pattern corresponds, is the unfailing criterion of whether we have succeeded in obtaining function-specific patterns. And please remember this definition; it will become very useful as we are moving along.

Needless to say, if we do not believe that this proposition is true, we have no reason and no motive to try to find the brain signs of consciousness. The fact that we do proceed with such research testifies to our acceptance of this first proposition.


Let me here mention parenthetically, that these patterns of activation may be electromagnetic – coding neuronal communication, or patterns of local metabolic or blood flow rates, each recorded by a different functional neuroimaging method.

Yet the arguments you will hear are not affected in the least by the peculiarity of each type of activation pattern or method of its construction – I close the parenthesis and move on the propositions 2,3,4 and 5 all of which (unlike the first one which was a hypothesis ) are statements of fact:
it is a fact that

 

 

And here is a pictorial representation of the concrete meaning of these propositions: Assume that this

spatio-temporal pattern of brain activation is the sum total of all patterns of activation corresponding to all functions simultaneously performed by the brain plus any conscious experience that may be transpiring at about the same time.

Buried or submerged in that river of global activity are each of the constituent patterns, this one

and this one

That combined make this one


Now we add the next one that happens to unfold at the same time

and we get this one



And if we add the rest of the simultaneously transpiring patterns, we get this one—that contains all of them for that period of time:


Now if each constituent pattern was in fact made up of qualitatively different materials (represented here by the different colors) we could easily pick them out of the global activation

But, all of them been homogenous (remember proposition two again)

the realistic representation of the global activity is this


where, all the constituent patterns are fused together.

Now, the two facts represented in propositions one and five



make clear why it is absolutely, categorically, and unconditionally true that although they exist, activation patterns corresponding to single. and I repeat single, conscious experiences are impossible, now and for every more, to isolate and recognize. These two propositions make it clear and incontestable why it is and will remain impossible for us to be able to say: “This pattern is the wish of the subject to scratch her nose. That one is a flashback to that occasion that her mother reprimanded her for scratching her nose …. Or that other one the fleeting pleasure that resulted from the smell of jasmine – jasmine, not roses or magnolias” – that sort of thing: Because, remember, being able to tell just by looking at a pattern, to what experience the pattern corresponds, is the only criterion of whether we have succeeded in obtaining experience-specific patterns. If we can not tell just by looking at the patterns, which one belongs to which experience, we really can’t claim that we have discovered patterns corresponding to particular single experiences. But, surprisingly, although this is so obvious, it is exactly what many of our colleagues that go around claiming.

The impossibility is evident from the juxtaposition of propositions 1 and 5. Something that never repeats can not be recognized - κάτι που συμβαίνει μόνο μια φορά στην προσωπική του καθ’ενός μας ιστορία, δεν μπορεί να ανά-γνωριστεί regardless of how fine the recording instruments may be. It happens once for all eternity and, besides, the one and only time it does happen, it is buried into the great river of global brain activity so that we can not see it even that first and only time it does make its appearance.

Maybe some of you that have heard these colleagues I was talking about or have read their statements in βήμα-science or Science, for that matter, and other such places, are already fidgeting, thinking that there is a trick involved here:

Some others among you say to yourselves, “but I do recognize the smell of jasmine – so it must not occur just once in the history of the universe as this guy with the bad southern accent (εννοώ παλιοελλαδίτικο accent ) is trying to tell me!!!!”.

Well, no, I am definitely not playing any games.

What I asserted previously is that the patterns of unique experiences are non-recognizable – the experience of that particular pleasure, resulting from that particular puff of air, on that particular point in time, within the context of that particular set of other concurrent experiences thoughts and tendencies racing through that particular subject’s mind or brain, rather. – That is the pattern that I claim to be un-identifiable and un-recognizable, in principle.

You are right of course: Some aspects, a subset of aspects, of that pattern does repeat. In fact, it repeats innumerable times.

These are the aspects that correspond not to the unique and unrepeatable features that make each experience unique as a whole but those features that allow us to classify each such unique experience as a token of a type, as a specific instance of a kind of thing. Ως δείγμα ενός είδους as Plato would say, not knowing the proper English terms.

So the question becomes: Can we then record, isolate and recognize – if not specific tokens, then types of things?

Can we look at records of functional images showing different patterns and identify each one: “This pattern corresponds to Ερασμίας but also to Ευαγγελίας notion of Table; the other one to the notion Square root of 2 not of 5; the next to the concept Value; the next to the percept Horse” and so on and so forth. Because, once again, being able to tell, just by looking at a pattern, to what concept or percept the pattern corresponds is the only criterion of whether we have succeeded in obtaining concept or percept-specific patterns. If we can not tell just by looking at the patterns, which one belongs to which concept or percept, we really can’t claim that we have discovered patterns corresponding to particular percepts or concepts.

So, can we record and recognize patters specific to different kinds of things, to different concepts or percepts?

The answer here is maybe.
Maybe because it is possible to repeat a stimulus, creating this way, tokens, experiences, each unrepeatable yet all of them sharing a set of common features that repeat reliably from on presentation to the other.

That way the portions of the brain activity that correspond to those features will occur reliably and with statistical manipulations we may extract them from the river of the global gravity in which they are embedded the same way – and here I am jumping the gun – the same way that we extract patterns corresponding to the mechanisms of functions.
- as is this caricature – based on real data-- that the outline of the brain mechanism of the visual perception function is obtained.

Now, I suggest that it is possible to do the same thing with percepts and concepts

I believe that this will be easier for percepts where repetition of the same stimulus leads to similar perceptual experiences but it may be very difficult or impossible in the case of function words

or of abstract notions.


And I am expressing reluctance here because repetition of the same signal – the same word may or may not result in conceptual experiences of the same type every time.
But, here at least, the possibility exists, unlike the case of concrete individual unique experiences we examined before.

I have a feeling that some of you are, once again, uneasy with my talking here about possibilities whereas, in conference like this one and in the printed literature you have already encountered statements --as I said before--, to the effect that we already have obtained activation patterns corresponding to percepts and concepts.

Well, there is a fact here overlooked by whoever claims that, and also a conceptual confusion, which I am about to demonstrate.

The fact first: Nowhere in the world is there a library of images of activation patterns, that one can survey and identify as the pattern corresponding to this or that concept, to this or that percept. Unless of course, someone has these images locked away for his own private edification and enjoyment. And, for a third, -or is it fourth? -and last time:

Being able to tell, just by looking at a pattern, to what concept or percept the pattern corresponds is the only criterion of whether we have succeeded in obtaining concept or percept-specific patterns. If we can not tell just by looking at the patterns, which one belongs to which concept or percept, we really can’t claim that we have discovered patterns corresponding to particular percepts or concepts. And since no one is bringing to the table such sets of images or patterns, claiming the opposite is a false claim.

That’s the fact.

Now the confusion: Although there is no such a “pattern library” anywhere, several laboratories have produced patterns that allegedly are specific to particular percept such as the percept of the rose I showed you before.

These patterns are obtained in the manner I indicated before, or, if obtained with the slower methods, the methods of low temporal resolution, i.e. fMRI or PET, they are obtained in roughly the following manner.

In a control condition the subject does nothing and a pattern of his resting brain activity is recorded, whereas in an experimental condition the subject is shown the rose while the brain activity that includes the pattern allegedly specific to the rose-percept is obtained. Then subtracting the second from the first image one gets the pattern corresponding to the rose.

The trouble is though, that the pattern could well be that of round shapes.

And were you to take a picture of brain activity while the subject was exposed to round shapes and subtract it from the previous one, there is no real reason to believe that the new pattern is now of the percept rose for the very elementary reason that the new pattern could very well be that of the color red – because we did expose the subject to a red stimulus didn’t we? Of course we did. So a new experiment is necessary to exclude the possibility of the pattern being specific to color red, and so on to virtual infinity.

Now, it is true that if we go on experimentally excluding alternative aspects of the stimulus we may reach the point that any reasonable person will be willing to accept the explanation that we have in fact arrived at the pattern of the concept rose, especially if we have demonstrated, on the way, that the patterns corresponding to round things or to red things are reliably different than the pattern for rose. This is why I said before that it is possible to isolate patterns specific to percepts even if we have not done it yet.

As for the confusion, it consists in equating the ability to identify a pattern among many as being specific to a concept with the ability to say, “since I showed the subject a rose, the differences in the pattern of his brain activity during that condition and the resting control condition must be the pattern specific to the percept rose”. This is the sort of confusion that goes around more frequently than can fairly be excused

And, anyone not rendered blind by the expectation of miracles or by an overgrown admiration of the potential of sophisticated technologies and technologists, anyone with simple common sense can, I expect, easily see these points.

So what aspects of consciousness can we image and what have we imaged.

Well, plenty of aspects. We have in fact imaged the patterns corresponding to the cerebral mechanisms of many functions that produce conscious experience like the Somatosensory function producing somatic sensations,

the speech production

and speech perception function


click here to play movie clip

And we are slowly isolating others, like this one that possibly corresponds to the function of affect production



click here to play movie clip

And many more

And the question arises: what about the brain activity pattern corresponding to consciousness, if consciousness were indeed a brain function.

But is there a single function, distinct from all others with its own cerebral circuitry that produces conscious experiences?

Nothing in our experience appears to support such a notion. Nothing indicates that the function that produces awareness is separate from the other functions; the different perceptual, mnemonic, linguistic and executive functions. When we are aware, when we are conscious, we are aware of sensations, percepts, and concepts. And, all these are products of the functions I just mentioned. There is never the case that we are aware, but aware of none of the products of particular cognitive or perceptual functions

To the very same conclusion leads the consideration of the clinical data: Consciousness is not an independent function separate from all others but is the extension or the culmination of each of the several perceptual and cognitive functions:

No brain pathology whatsoever, whether focal multifocal or diffuse has ever interfered with consciousness while leaving all other cognitive functions intact. On the contrary, brain lesions that result in loss of consciousness result also in the loss of one or more or all functions. Thus the amnesias, the agnosias, the aphasias. Thus, also, coma and the vegetative state.

To the very same conclusion lead whatever functional neuroimaging data we now have. Never has there been the case that an activation pattern specific to consciousness at large is identified, that it is not also a pattern specific to some perceptual or cognitive function.

And this brings me to the last point: Is it possible to tell, by consulting such brain activation patterns, whether or not a person is conscious?

Let us think this one through together:

Is it possible to tell that a person is or is not conscious on the basis of brain activity patterns?

If I have concluded correctly that there is no separate cerebral mechanism of consciousness, then to be correctly claiming that a person is unconscious on the basis of brain activity patterns presupposes the following:
It presupposes that we know what features of brain activation patterns correspond to the carrying out of a function that results in unconscious experience, and what additional or different ones are present when the outcome of carrying out of the same function results in a conscious experience.

It presupposes, for example, that we know what an activation pattern looks like when a person perceives subliminally and how it is different when the same person perceives consciously.

Now, as far as I can tell, such subtle differences in patterns have not been specified as yet, since at present, we can only boast of having some idea about the general features of some brain patterns of some functions -and even those are incompletely specified.

Ergo, there is no way now, of looking at a pattern resulting from auditory language stimulation and on the strength of that alone pronounce a person conscious or unconscious.

This is why as long as one records any brain activation pattern one has to verify that the corresponding function does or does not result in conscious experiences by checking other discriminate responses - verbal, first of all.

Needles to say, the situation is different when in the presence of the appropriate conditions, no activation pattern whatsoever appears. When, for example, there is no response of the occipital cortex to the presentation of any visual stimuli and of the auditory and somatosensory cortex in response to the appropriate stimuli, while the sensory pathways are demonstrably intact.

There, one may safely assume absence of consciousness but even there, as long as there is baseline or background brain activity present, no one will act on this assumption without checking other discriminate responses.

It is simply against the standards of medical practice and against plain common sense to do so because, not knowing the neurological signature of conscious experience on the brain background activity, any more than we can recognize them on brain activation patterns, we need the reassurance that the other, the overt discriminate responses, provide.

An exception to all that is, of course, when there is absence of all brain activity. There, we can pronounce absence of consciousness, but not only absence of consciousness but also cessation of all perceptual and cognitive functions and cessation of basic biological ones as well. Moreover in such cases we do not need neuroimaging since isoelectric lines on conventional EEG tell us the same story.

Is there then no role for neuroimaging here? Of course there is:
1st, to establish the difference in activation patterns in those cases that the various perceptual and cognitive functions result in conscious experiences and in cases they do not.

And, 2nd, to establish the level of global brain activity and activation below which no function whether conscious or unconscious is possible.

And I am looking forward to hearing about such developments from my colleague from Berlgium.

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