Liminality

It is a common view that consciousness has something to to with the degree of integration of different areas of the brain. The idea is not only a modern scientific notion: it is also a tenet of various esoteric schools (for example that of G.I. Gurdjieff; see here and here) that higher degrees of consciousness are the result of directed effort to integrate the assorted “parts” of the mind/body system, and that diminished consciousness, and sleep, result from their disconnection.

The neuroscientists have been busy with this. Edge.org recently presented a talk by the French researcher Dr. Stanislas Dehaene, in which he explains how his work has enabled him to determine, by examining the activation and coordination of various regions of the brain, whether a visual stimulus has or has not entered conscious awareness. Apparently it is very easy to tell.

Dr. Dehaene stakes out his territory:

Neuroscientists used to wait until they were in their 60’s or 70’s before they dared to raise the topic of consciousness. But I now think that the domain is ripe. Today we can work on real data, rather than talk about the issue in philosophical terms.

In the past, the major problem was that people barely looked at the brain and tried to generate theories of consciousness from the top, based solely on their intuitions. Excellent physicists, for instance tried to tell us that the brain is a quantum computer, and that consciousness will only be understood once we understand quantum computing and quantum gravity. Well, we can discuss that later, but as far as I can see, it’s completely irrelevant to understanding consciousness in the brain. One of the reasons is that the temperature at which the brain operates is incompatible with quantum computing. Another is that my colleagues and I have entered an MRI scanner on a number of occasions, and have probably changed our quantum state in doing so but as I far as I can judge, this experience didn’t change anything related to our consciousness.

Quantum physics is just an example. There has been an enormous wealth of theories about consciousness, but I think that very few are valid or even useful. There is, of course, the dualist notion that we need a special stuff for consciousness and that it cannot be reduced to brain matter. Obviously, this is not going to be my perspective. There is also the idea that every cell contains a little consciousness, and that if you add it all together, you arrive at an increasing amount of consciousness — again this is also not at all my idea, as you will see.

It should be clear already that the results described here will not put to rest any of the fractious philosophical dispute about consciousness. Nevertheless, to study the physical differences between consciousness and unconsciousness seems a necessary prerequisite to any physical account.

Tonight I only want to talk about the simpler and addressable problem of what we call “access to consciousness”. At all times the brain is constantly bombarded with stimulation— and yet, we are only conscious of a very small part of it. In this room, for instance, it’s absolutely obvious. We are conscious of one item here and another item there, say the presence of John behind the camera, or of some of the bottles on this table. You may not have noticed that there is a red label on the bottles. Although this information has been present on your retina since the beginning of my talk, it’s pretty obvious that you are only now actually paying attention to it.

In brief, there is a basic distinction between all the stimuli that enter the nervous system, and the much smaller set of stimuli that actually make it into our conscious awareness. That is the simple distinction that we are trying to capture in our experiments. The first key insight, which is largely due to Francis Crick and Christof Koch, is that we must begin with the much simpler problem of understanding the mechanisms of access to consciousness for simple visual stimuli before we can attack the issue of consciousness and the brain.

Dr. Dehaene’s project involved subliminal presentations of visual signals, taking advantage of an effect called masking:

If you flash words on a screen for a period of roughly 30 milliseconds, you see them perfectly. The short duration, in itself, is not a problem. What matters is that there is enough energy in the stimulus for you to see it. If, however, just after the word, you present another string of letters at the same location, you only see the string, not the word. This surprising invisibility occurs in a range of delays between the word and the consonant string (what we call the mask) that are on the order of 50 milliseconds. If the delay is shorter than 50 milliseconds, you do not see the hidden word. It’s a well-known perceptual phenomenon called masking.

Now, if you lengthen the delay a little, you see the word each time. There is even a specific delay where the subjects can see the stimulus half of the time. So now you are in business, because you have an experimental manipulation that you can reproduce in the lab, and that systematically creates a change in consciousness.

I have had, in the past, some energetic disagreements with those who maintain that consciousness is necessary for intentionality. It has always been my feeling that this is quite obviously not so: that we process meanings and “aboutness” quite unconsciously all the time (this is, of course, the thin edge of a philosophical wedge: I believe that intentionality is an entirely material phenomenon, with a gradualistic, evolutionary origin). Dr. Dehaene’s remarks certainly support that view (emphasis added):

What do we see when we do these experiments? The first thing that we discovered is that, even when you cannot see a word or a picture, because it is presented in a subliminal condition, it does not mean that your cortex is not processing it. Some people initially thought that subliminal processing meant sub-cortical processing — processing that is not done in the cortex. It’s of course completely false and we’ve known this for a while now. We can see a lot of cortical activation created by a subliminal word. It enters the visual parts of the cortex, and travels through the visual areas of the ventral face of the brain. If the conditions are right, a subliminal word can even access higher levels of processing, including semantic levels. This is something that was highly controversial in psychology, but is now very clear from brain imaging: a subliminal message can travel all the way to the level of the meaning of the word. Your brain can take a pattern of shapes on the retina, and successively turn it into a set of letters, recognize it as word, and access a certain meaning — all of that without any form of consciousness.

So what happens as we become conscious of a word on the screen?

Next comes the obvious question: where is there more activity when you are conscious of the word? If we do this experiment with fMRI, what we see is that two major differences occur. You first see an amplification of activation in the early areas: the very same areas begin to activate much more, as much as tenfold, in for instance, this area that we have been studying and which looks at the spelling of words: the visual word form area.

The second aspect is that several other distant areas of the brain activate. These include areas in the so-called prefrontal cortex, which is in the front of the brain here. In particular, we see activation in the inferior frontal region, as well as in the inferior parietal sectors of the brain. What we find also is that these areas begin to correlate with each other — they co-activate in a coordinated manner. I am for the moment just giving you the facts: amplification and access to distant areas are some of the signatures of consciousness.

What, then, is the practical effect of all this? Some activity in the brain has passed some threshold at which it begins to propagate itself throughout the system, almost like a “viral” meme. What happens then?

This idea is relatively simple, and it is not far from the one that Daniel Dennett proposed when he said that consciousness is “fame in the brain”. What I propose is that “consciousness is global information in the brain” — information which is shared across different brain areas. I am putting it very strongly, as “consciousness is”, because I literally think that’s all there is. What we mean by being conscious of a certain piece of information is that it has reached a level of processing in the brain where it can be shared.

Because it is sharable, your Broca’s area (or the part of it involved in selecting the words that you are going to speak) is being informed about the identity of what you are seeing, and you become able to name what you are seeing. At the same time, your hippocampus is perhaps informed about what you have just seen, so you can store this representation in memory. Your parietal areas also become informed of what you have seen, so they can orient attention, or decide that this is not something you want to attend to… and so on and so forth. The criterion of information sharing relates to the feeling that we have that, whenever a piece of information is conscious, we can do a very broad array of things with it. It is available.

But what’s the point? Dr. Dehaene discusses this at some length at the end of his talk, beginning with this:

This type of model may help answer a question that was difficult to address before, namely, what is consciousness useful for? It is a very important question, because it relates to the evolution of consciousness. If this theory is right, then we have a number of answers to what consciousness actually does. It is unnecessary for computations of a Bayesian statistical nature, such as the extraction of the best interpretation of an image. It seems that the visual brain does that in a massively parallel manner, and is able to compute an optimal Bayesian interpretation of the incoming image, thus coming up with what is essentially the posterior distribution of all the possible interpretations of the incoming image. This operation seems to occur completely non-consciously in the brain. What the conscious brain seems to be doing is amplify and select one of the interpretations, which is relevant to the current goals of the organism.

In several experiments, we have contrasted directly what you can do subliminally and what you can only do consciously. Our results suggest that one very important difference is the time duration over which you can hold on to information. If information is subliminal, it enters the system, creates a temporary activation, but quickly dies out. It does so in the space of about one second, a little bit more perhaps depending on the experiments, but it dies out very fast anyway. This finding also provides an answer for people who think that subliminal images can be used in advertising, which is of course a gigantic myth. It’s not that subliminal images don’t have any impact, but their effect, in the very vast majority of experiments, is very short-lived. When you are conscious of information, however, you can hold on to it essentially for as long as you wish,. It is now in your working memory, and is now meta-stable. The claim is that conscious information is reverberating in your brain, and this reverberating state includes a self-stabilizing loop that keeps the information stable over a long duration. Think of repeating a telephone number. If you stop attending to it, you lose it. But as long as you attend to it, you can keep it in mind.

There’s a good deal more, and you can read it all here.

Now I want to make it clear once again that none of this addresses the “hard question” of consciousness, to wit: in virtue of what can matter, suitably arranged, give rise to the subjective experiencing — never mind the information-processing and neural razzle-dazzle examined here — that we consider to be the hallmark of consciousness? But if you expect, as I do, that the patient work of scientific inquiry will one day crack this most challenging of puzzles, this is the sort of work we need to be doing.

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2 Comments

  1. Kevin Kim says

    Charles? You there?

    Posted December 8, 2009 at 11:25 am | Permalink
  2. Charles says

    Yes, I am indeed here! I was quite surprised to see this post title pop up in my feed reader, and eager to see what the post was about.

    I honestly don’t think I am qualified to say anything here, but for some odd reason I feel compelled to comment…

    Even if I don’t quite grasp everything above, I still find it fascinating. When liminality is used in the field of psychology (if I’m not mistaken, this was the field that originally gave birth to the word), it is often used to describe things that are below the threshold of consciousness, or subliminal. Rarely do you hear the word “superliminal” being used, since things of that nature are generally discussed in terms of “consciousness.”

    What fascinates me about the above article, though (or at least the excerpts I have read above–I admit that I haven’t had the chance yet to read the whole thing), is the idea of the transition from subliminal to superliminal. Perhaps the full article discusses it in more detail, but I have always been curious about what exactly happens in this psychological liminal state.

    Unlike sociological liminality, psychological liminality must be measured in milliseconds, I imagine. Either something is in your conscious mind or it is not, right? As the article says, if you do not make subliminal information superliminal, it fades away in the space of one second, or at least very quickly. So the liminal state can’t be very long. Still, I am very curious about what happens during that brief time. I guess it boils down to how the brain is wired and how information passes from one isolated area to be shared with the rest of the brain–and I probably wouldn’t understand it even if someone explained it to me.

    Well, there you have it. That’s most likely a pile of gibberish, but I couldn’t resist commenting. With a post title like that, I imagine you might have been expecting this.

    Posted December 9, 2009 at 1:23 am | Permalink