Induced gamma-band oscillations correlate with awareness in hemianopic patient GY.

In normal vision gamma oscillations are involved in object perception, are modulated by attention, and have been linked to awareness by way of their putative role in perceptual integration, arguably as a mechanism for synchronizing activity in separate neural assemblies. We tested the hypothesis that the presence of gamma oscillations (approximately 30-80 Hz) signal the entry of a neural representation into awareness (as indexed by direct report), while attempting to control for other measures of neural information processing such as discrimination accuracy and reaction time. Hemianopic patient GY sometimes reports an awareness "that something happened" in his blind visual hemifield, in response to stimuli of sufficiently high contrast, although he may deny "seeing" anything. At lower contrast levels GY denies any awareness, but may continue to exhibit greater-than-chance accuracy (blindsight). Using a near-threshold level of contrast offers a unique way to test hypotheses concerning correlates of perceptual awareness, since GY's accuracy on certain tasks is independent of awareness. We tested GY on an orientation-discrimination task using stationary stimuli at a fixed near-threshold level of contrast, to which GY sometimes responded "aware" and sometimes "unaware". We recorded brain activity using magnetoencephalography (MEG) in order to determine the relationship between local induced gamma-band oscillations and awareness. GY's accuracy was significantly greater than expected by chance and no different whether or not he reported awareness of the stimulus. Oscillatory activity in the gamma band (44-66 Hz) over the left occipito-parietal region correlated significantly with awareness (but not accuracy), whereas activity in the alpha band (8-12 Hz) did not.