Article: The conscious access hypothesis: origins and recent evidence
Author: Bernard J Baars
Citations: 581 (note: as of 03/2014)
Link: Here (note: not a permalink).
In 1988, Bernard Baars authored A Cognitive Theory of Consciousness, which presented his Global Workspace Theory (GWT) of consciousness. In short, he argues that consciousness is caused by global inter-brain sharing of information. This theory does not concern itself much with the construction of phenomenology, and thus does not qualify as a solution to the Hard Problem of Consciousness (which is well explained here).
Scientific efforts to understand consciousness evoked vigorous philosophical objections. These were essentially the classic mind-body problems: how does private experience relate to the physical world? … Difficult conceptual questions are routine when the sciences turn to new topics. The traditional scientific response is simply to gather relevant evidence and develop careful theory. Ultimately, philosophical controversies either fade, or they compel changes in science if they have empirical consequences.
I like this quote. While it doesn’t encapsulate my sentiments on the role of philosophy, its call for empirical analysis was long overdue.
You may find yourself asking: how can neuroscience examine consciousness, if consciousness is private to the individual? Baars advocates using an operational definition of conscious awareness: consciousness is the ability to produce a reliable report. An example: suppose I flash a number (0-9) on your monitor, and then ask its value. Say I present the number three for 200 milliseconds. If I ask you what you saw, you would be able to report your conscious experience. But, say I present the same number for 2 milliseconds. If I then ask you what you saw, you would not be able to report the correct value better than a ten-sided die. By this means, I have acquired a variable that represents whether a task is associated with consciousness.
How can we causally distinguish between the effect of consciousness and, say, the effect of low IQ on a given task? Well, most neuroscientific inquiries into consciousness employ a technique Baars refers to as contrastive analysis. This technique involves comparing processes that induce conscious awareness only occasionally. Let’s suppose that, in the above example, 200ms corresponded to 98% correct reports, whereas 2ms corresponded to 3% of subjects being aware of the change consciously. I would then be tempted to “turn the display-time knob” so any one person has a 50% chance of perceiving the number, and then analyzing the differences between the two groups. To see an example of contrastive analysis beyond the above toy model, Baars cites Dehaene et al  as an exemplar.
A Philosophical Aside
It is, first, important to distinguish between operational definitions such as the above, and operationalism, which is a more extreme call to operationalize all scientific concepts. While the latter movement is today widely regarded as unhelpful, that doesn’t seem to problematize the desire to operationalize some definitions, such as consciousness or volition.
Let me sketch a problem that will be familiar to any philosophers. The question of philosophical zombie was memorably treated by Descartes: is it possible for a human being behave exactly as one who is conscious, reporting conscious experiences to anyone who may ask, but entirely devoid of an inner life? This metaphysical question has not been satisfactorily resolved. However, let us reframe this question in nomological terms: is consciousness causally linked to the human nervous system? If we provisionally accept the operational definition of consciousness above, we are in position to answer this question with data.
The data seems to say yes. Consciousness hugely contributes to the functioning of our nervous system. In this paper, Baars sketches seven lines of evidence that have accumulated since his theory’s inception (1988).
- Conscious perception involves more than sensory analysis; it enables access to widespread brain sources, whereas unconscious input processing is limited to sensory regions.
- Consciousness enables comprehension of novel information, such as new combinations of words.
- Working memory depends on conscious elements, including conscious perception, inner speech, and visual imagery, each mobilizing widespread functions.
- Conscious information enables many types of learning, using a variety of different brain mechanisms.
- Voluntary control is enabled by conscious goals and perception of results.
- Selective attention enables access to conscious contents, and vice versa.
- Consciousness enables access to ‘self’: executive interpretation in the brain.
A wealth of data bolsters the above theses; I would point the interested reader to the article.
Baars goes on to claim that his GWT explains the above seven evidences. If GWT is to be overturned, its replacement must do even better.
Mechanisms of Brain Access
So, we see evidence of conscious activity being correlated with full-brain activation. But what mechanisms might produce full-brain activation? Baars identifies several research traditions exploring different (potentially complementary) answers to the question:
- Dehaene and Changeux have focused on frontal cortex 
- Edelman and Tononi on complexity in re-entrant thalamocortical dynamics 
- Singer and colleagues on gamma synchrony 
- Flohr on NMDA synapses 
- Llinas on a thalamic hub 
- Newman and Baars on thalamocortical distribution from sensory cortex 
Baars notes in his article that efforts to integrate research on attention and consciousness are long overdue. I would go a step further. His theory of consciousness also ought to be integrated with:
- dual-process theory (theoreticians have already correlated System 2 with conscious awareness)
- working memory (Alan Baddeley is already struggling to integrate his Central Executive with conscious awareness)
1. Dehaene, S. et al (2001) Cerebral mechanisms of word masking and unconscious repetition priming.
2. Tononi, G. and Edelmen, G.M. (1998) Consciousness and complexity.
3. Engel, A.K and Singer, W (2001) Temporal binding and the neural correlates of sensory awareness.
4. Flohr, H et al (1998) The role of the NMDA synapse in general anesthesia.
5. Llinas, R et al (1998) The neuronal basis for consciousness.
6. Newman, J and Baars, B.F. (1993) A neural attentional model for access to consciousness: a global workspace perspective.