Early neural activity changes associated with stimulus detection during visual conscious perception.

The earliest cortical neural signals following consciously perceived visual stimuli in humans are poorly understood. Using intracranial electroencephalography, we investigated neural activity changes associated with the earliest stages of stimulus detection during visual conscious perception. Participants (N = 10; 1,693 electrode contacts) completed a continuous performance task where subjects were asked to press a button when they saw a target letter among a series of nontargets. Broadband gamma power (40-115 Hz) was analyzed as marker of cortical population neural activity. Regardless of target or nontarget letter type, we observed early gamma power changes within 30-180 ms from stimulus onset in a network including increases in bilateral occipital, fusiform, frontal (including frontal eye fields), and medial temporal cortex; increases in left lateral parietal-temporal cortex; and decreases in the right anterior medial occipital cortex. No significant differences were observed between target and nontarget stimuli until >180 ms post-stimulus, when we saw greater gamma power increases in left motor and premotor areas, suggesting a possible role in perceptual decision-making and/or motor responses with the right hand. The early gamma power findings support a broadly distributed cortical visual detection network that is engaged at early times tens of milliseconds after signal transduction from the retina.