Over the past decade scientists have made progress in identifying how consciousness arises in the brain, but a new study calls into question one of the leading models that has come out of this research.
The challenged idea—known as the global neuronal workspace theory of consciousness—proposes that conscious perception gives rise to activity across many regions of the brain. This is contrasted with unconscious—or subliminal—perception which produces only local activity.
Previous studies, which involved measuring the EEG activity of volunteers’ brains, showed distinct differences between conscious and unconscious perception of visual stimuli. Both types of perception started with identical brain activity that lasted 270 milliseconds. After that, unconscious perception resulted in a drop in brain activity.
When a participant became conscious of the stimulus, however, there was a sudden increase in activity over many parts of the brain. This occurred 300 milliseconds after the stimulus that triggered it. This activity had a characteristic EEG signal that scientists called P3b. This, they proposed, is the neural fingerprint of consciousness.
A new study, published online September 25 in the journal Cortex, suggests that P3b might also be present during unconscious perception.
In the study, researchers flashed one of two words (“LEFT” or “RIGHT”) for 7 milliseconds on a screen, followed by a longer mask to keep the participants from registering the word consciously. One word appeared more frequently and the other only rarely.
Other studies have shown that a rare stimulus can trigger a larger response in the brain. In this case, the rare subliminal stimulus triggered the typical P3b signal, with widespread activity in the brain—something that was previously thought to be a sign of conscious perception.
“Even though they don’t know [what] the stimuli are, the brain is still able to recognize that there is something unexpected that occurs,” study author Brian Silverstein at the University of Michigan in Arbor told New Scientist.
The authors suggest that P3b may not be a definitive neural signal of consciousness. However, they don’t rule it out as an indication of some form of consciousness, suggesting that it may depend more upon mental attention rather than conscious awareness.