An optical illusion can help define which parts of the brain are responsible for human consciousness. People cannot consciously perceive a number flashed on a screen for 16 ms if it is quickly followed by another stimulus in the same area. As the time between the two stimuli increases, the first stimulus becomes visible; that is, it is accessible to the person's consciousness. Del Cul et al. recorded electrical brain waves from people's scalps as they were shown these stimuli and reported to the investigators whether they were visible or invisible. One brain wave in particular, P3, occurring 270 to 400 ms after the beginning of the trial, correlated with conscious perception of the stimulus. This wave seems to arise from sudden simultaneous activity in several parts of the brain, specifically the frontal, parietal, and temporal cortices of both hemispheres. These data are inconsistent with several proposed correlates of consciousness, including the rapid induced activity in the visual areas of the brain and the later more distributed, but still local, neural reverberations. Rather, they suggest that conscious perception is associated with a sudden global reverberation of neural activity, about 300 ms after the stimulus, encompassing several cortical areas bilaterally. — KK
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