2012 Keynote Speaker: Dr. Stanislas Dehaene
Electrophysiological Signatures of Conscious Processing
Invited address by Stanislas Dehaene, Ph.D., Professor at the College de France and Director of the Cognitive Neuroimaging unit of France's National Medical Research Institute (INSERM)
In an address summarizing his fascinating and ground-breaking work on consciousness and the human brain, Dr. Stanislas Dehaene described research that ranged from basic science questions of perception and stimulus detection to applied, life-saving research with coma patients.
To begin, Dehaene described the fundamental question which has driven much of his academic work—that is, when and how do we become conscious? Dehaene presented evidence that consciousness arises from the ignition of a distributed network of neurons, and he outlined a critical difference between global availability of information as compared to local availability. Local availability essentially refers to the entry of information about a stimulus into some sensory system. What we experience, says Dehaene, is the global availability of information. That is, when important information is presented for a sufficient amount of time, sensory information about that stimulus accumulates, and activates what Dehaene calls a "global neuronal workspace." Consciousness, according to Dehaene, relies on the activation of this global system.
This is perhaps best illustrated through tasks that interfere with conscious access, as Dehaene noted in his talk. One such task involves the presentation of information subliminally, or at very high speeds. When information—e.g., a digit—is presented subliminally, there is evidence that the digit is processed visually, indeed, it does enter the brain, but information about that stimulus is so limited that the signal is not amplified by the connecting regions in the distributed network, and so it dies out rapidly. Another method is "preconsciously,"—presenting a stimulus when the system is already overloaded; though the stimulus may be presented for enough time to enter the system, attentional resources are so consumed by other information that the system is incapable of gathering sufficient information about that stimulus. Dehaene presented multiple studies using multiple methodologies to suggest that consciousness does indeed depend upon this accumulation of information igniting a broad system in the brain.
Dehaene went on to present some of his most intriguing data, related to his use of event-related potentials (ERP) in patients in a persistent vegetative state, in hopes of improving diagnostic capabilities. Because a vegetative state—unlike a coma—can involve motor responses and the appearance of consciousness (e.g., open eyes, the ability to swallow, responsiveness to stimulation), diagnosis has been notoriously difficult, often controversial and occasionally alarmingly inaccurate. Yet Dehaene and his colleagues have developed a battery of tests—relying chiefly on the magnitude of an ERP known as the P300—which have shown discriminative and predictive ability. Because the P300 has been related to conscious awareness of stimuli in healthy patients, Dehaene and his colleagues ran patients in comas and persistent vegetative states through paradigms to elicit it. While patients who are not conscious will display a very early neural response to a novel stimulus—indicating that the stimulus does become locally available, at least for a time—the P300, which develops somewhat later and reflects global, conscious processing of the novel stimulus, is only present in patients who, though they may be immobilized, are in fact conscious or very likely to regain consciousness. Using these tasks and others, Dehaene and his colleagues have been working to develop a classifier which could determine whether a patient is truly vegetative, minimally conscious, or fully conscious but incapable of expression. Incredibly, using the P300, they have been able to predict with fairly high accuracy which patients will recover, and which patients are unlikely to do so.
In closing, Dehane left the audience with the lovely image of the brain as a flickering candle, ignited by information flowing in from the outside world.