Scientists’ attempts to discern the neurological activities that constitute consciousness have taken a dramatic twist, after a recent study appeared to contradict one of the leading theories on the topic. Until now, it had been widely accepted that conscious thought requires complex, sustained and widespread brain activity, while the mind’s unconscious workings involve much shorter and simpler processes. However, the paper, which appeared in the journal Cortex, seems to show that some of the mechanisms previously considered unique to and constitutive of consciousness can in fact be observed during unconscious processes, too.
In particular, the study challenges the notion that a specific event-related potential (ERP) – an electrophysiological response to external stimuli – called P3b is a key indicator of conscious thought. Because this particular ERP occurs relatively late (about 375 milliseconds) after the occurrence of a stimulus, it had previously been thought that its presence was a neural correlate of consciousness (NCC), i.e. something that constitutes consciousness. In contrast, a related ERP called P3a occurs much earlier (250 milliseconds), and had therefore been associated with simpler, unconscious processes such as the automatic attraction of attention.
Moreover, the widely-accepted global workspace theory, first suggested by Bernard Baars in 1982, states that consciousness occurs when information is made accessible throughout the brain, while unconscious information is processed regionally.
To test these theories, a team of neurologists and psychiatrists from the University of Michigan constructed an experiment using the oddball paradigm, which involves subjecting participants to the same regularly-occurring stimulus, interspersed with one incongruous stimulus. In this case, subjects were repeatedly shown the word “LEFT” for a period of just seven milliseconds at a time. This is long enough for the brain to unconsciously register and respond to the stimulus, but too short for participants to consciously realize what they had seen. The oddball came when they were shown the word “RIGHT,” which was flashed for the same period of time and therefore again processed unconsciously.
Using electroencephalography to measure the neural responses of participants, the researchers found that the oddball stimulus generated a P3b response throughout the brain, despite the whole process being unconscious. In other words, the occurrence of P3b revealed that “at least some forms of widespread, globally distributed, sustained processing can occur unconsciously.” The team therefore concludes that much of what we thought we knew about consciousness is wrong, as neither P3b nor global access to information are NCCs, meaning the great mystery of our cognizance continues to deepen.
[H/T: New Scientist]