Visibility and visual awareness in the primate
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.