SIGNATURES OF CONSCIOUSNESS
A Talk by Stanislas Dehaene [11.24.09]
On October
17, Edge organized a meeting at The Hotel Ritz in Paris to allow
neuroscientist Stanislas Dehaene to present
his new theory on how consciousness arises in the brain to a group of Parisian
scientists and thinkers. The theory, based on Dehaene's past twelve years of
brain-imaging research is called the global
neuronal workspace.
Dehaene is a Professor at the Collège de
France and Chair of Experimental Cognitive Psychology. His research focuses on
the cerebral bases of specifically human cognitive functions such as language,
calculation, and reasoning. His work centers on the cognitive neuropsychology
of language and reading, and his main scientific contributions include the
study of the organization of the cerebral system for number processing.
Consciousness is a difficult subject
which is highly philosophical. We assume
consciousness is biological, a result of the brain's activity. But how can the activity of a network of
neurons in the brain give rise to a conscious experience? We are far from answering this question. Cognitive studies like Dehaene's give us
modest insights that can inform our thinking about this subject.
At all times the brain is constantly
bombarded with stimulation— and yet, we are only conscious of a very small part
of it. When you read this post you are
probably in a room at home, at work or maybe in a coffee shop. You probably
haven't noticed the color of the doorpost. This information has been present on
your retina, and you may have processed it to some level, but it had not become
conscious. Dehaene talks about the differences between what
happens in the brain before and after a stimulus becomes conscious.
If you flash words on a screen for a period of roughly
30 milliseconds, you see them perfectly. 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 (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.
In the masking procedure, even when
subjects claim to be unaware of the word and report that they cannot see any
word at all, they still do better than chance on classification tasks like guessing
if the word is the name of an animal or not, or if it's a word in the lexicon
or not. The 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.
In a similar way, it's possible to flash
a digit on a screen, and this
digit is subliminal because it was masked. Now suppose you are asked, "Is
it larger or smaller than five?" You get two buttons, one for larger and one
for smaller, and you are forced to respond. Although you claim that you have not seen
anything, and you have to force yourself to respond, you do much better than
chance. You are typically around 60 percent correct, while pure chance would be
50 percent. So this is subliminal processing. Some information gets through,
but not enough to trigger a global state of consciousness.
However, if you are given a task that
involves two serial processing steps, you cannot do it. If you
are asked to give the number plus two, you can do it — but if you are asked to
compute the initial number plus two, and then decide if the result of that +2
operation is larger or smaller than five, you cannot do it. It's a strange
result, because the initial experiments show that you possess a lot of
information about this subliminal digit. If you just named it, you would have
enough information to do so correctly, much better than chance alone would
predict. However when you are engaged in a chain of processing, where you have
to compute x+2, and then decide if the outcome is larger or smaller than five,
there are two successive steps that make performance fall back down to chance.
Presumably, this is because you haven't had access to the workspace system that allows you to execute this kind of serial
mental rule. Access to the
global neuronal workspace requires consciousness.
The way Dehaene
describes the global neuronal workspace reminds me
of working memory, to which Dehaene explicitly refers:
"When you are conscious of information… 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…Our model proposes that this
is really one of the main functions of consciousness: to provide an internal
space where you can perform thought experiments, as it were, in an isolated
way, detached from the external world. You can select a stimulus that comes from the outside
world, and then lock it into this internal global workspace. You may stop other
inputs from getting in, and play with this mental representation in your mind
for as long as you wish".
Dehaene used masking
procedures like those described with subjects in an fMRI machine. This machine is able to identify changes in
blood flow in different areas in the brain. These changes inform us about the
activity in these areas at a specific point in time.
fMRI
Dehaene learned
from such experiments that subliminal stimuli can create a small and quickly
decaying P3 wave in the brain, whereas a very big and nonlinear increase in
activation, leading to a large event-related potential, can be seen when the
same stimuli cross the threshold and become conscious. Prior to conscious ignition,
processing is essentially modular, with several simultaneous activations
occurring independently and in parallel. However, at the point where scientists
begin to see conscious access, their records show a synchronization of many
areas that begin to work together. Dehaene thinks that we become aware of a stimulus
when different areas in the brain share the products of the stimulus'
processing with each other. Synchronization
is probably a signal for agreement between different brain areas. The areas
begin to agree with each other. They converge onto a single mental object. The global neuronal workspace is needed for this sharing process to happen.
