Visibility and visual awareness in the primate

Steve Macknick

Harvard University

What are the processes that lead to a visible or invisible percept? One way to address this is to vary the visibility of the stimulus and examine its correlated neural activity. We found that spatiotemporal edges of stimuli cause strong neural signals. These signals are represented by transient bursty firing that occurs when the stimulus turns on and off. When we suppressed transient bursty firing (using visual masking stimuli), the stimulus disappeared perceptually, even though it remained physically extant on the retina. By using dichoptic stimuli (stimuli that are different in each eye), we addressed where in the brain activity must occur for a perception to form. We found that robust responses to a stimulus can occur in both the LGN and in V-1, even when that stimulus is perceptually invisible. This suggests that neither activity in the LGN nor in V-1 is sufficient by itself (or together) to generate perception of the stimulus, and so visual awareness must be a function of activity in higher cortical circuits.