In the late 1800s, psychology began as the study of the mind. Then the behaviorists convinced everyone to abandon the study of mind and consciousness, for good reasons. At the time, the only way to study consciousness was introspection, a source of no reliable information. Through most of the next century, experimental psychologists avoided not only research on consciousness, but also the term itself. Unconscious processes were equally taboo, to avoid any association with Freudian theories.
A joyous exploration of the mind and its thrilling complexities, Consciousness and the Brain will excite anyone interested
in cutting-edge science and technology and the vast philosophical, personal, and ethical implications of finally quantifying consciousness
More recently, Stanislas Dehaene and others have made consciousness not only respectable, but an exciting area of research advances. One key to their success was to adopt an operational definition of consciousness: A cooperative person who reports awareness of one stimulus and not another is conscious of the first and not the second. A second key was a focus on a limited, answerable question: What type of brain activity occurs when we have conscious access to a sensory stimulus that does not occur when we lack access to the same stimulus? A third was the arrival of new methods such as fMRI that can localize brain activity in healthy people. The fourth advance came from presenting a stimulus under two conditions, one that permits conscious access and one that prevents it. For example, a brief stimulus followed by a blank screen is visible, but the same stimulus followed by a masking stimulus is not. In binocular rivalry, the left and right eyes view incompatible scenes, and the viewer alternates between awareness of one and awareness of the other. So, an experimenter presents a stimulus under conditions that do or do not permit consciousness, verifies consciousness by the viewer’s report, and compares the resulting brain activity in the two conditions.
According to the research of Dehaene and others, the initial stages of processing are identical for stimuli that do or do not become conscious. In both cases the stimuli excite retinal receptors that send messages to the thalamus and then to the primary visual cortex. From that point on, the process bifurcates in an all-or-none manner. If interference from previous and subsequent stimuli is great enough, the response to the stimulus weakens as it passes to other cortical areas, where it may subtly bias behavior in ways that we call “subliminal perception.” However, the person has no conscious perception and cannot report the stimulus verbally. In the absence of strong inhibition, the prefrontal and parietal cortices send messages back to the primary visual cortex and the message reverberates and amplifies through other brain areas. If you are recording with an EEG, you see a P300 wave resulting from all this activity. (Dehaene notes that the P300 wave, one of the signatures of conscious processing, occurs by definition about 300 ms after a stimulus. Consistent with other types of research, this finding indicates that our consciousness of something lags almost a third of a second behind the event itself. It lags even further behind in human infants.)
In this lively book, Stanislas Dehaene describes the pioneering work his lab and the labs of other cognitive neuroscientists worldwide have accomplished in defining, testing, and explaining the brain events behind a conscious state. We can now pin down the neurons that fire when a person reports becoming aware of a piece of information and understand the crucial role unconscious computations play in how we make decisions. The emerging theory enables a test of consciousness in animals, babies, and those with severe brain injuries.