SIGNATURES OF CONSCIOUSNESS

A Talk by Stanislas Dehaene [11.24.09]

https://www.edge.org/conversation/stanislas_dehaene-signatures-of-consciousness

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.