Stephen L. Macknik

Biography

My early vision research was on an intriguing illusion called backward masking, in which a visual target is rendered invisible by a masking stimulus presented 100 msec after the target is extinguished. It was as if the mask traveled back in time to destroy the target, before the brain could achieve conscious awareness of it. I employed a multidisciplinary approach that combined monkey electrophysiology, human psychophysics, and human fMRI to determine its neural mechanisms. I found that backward masking occurred when the mask inhibited the target's termination response–the response that occurs to the turning off of the target–about 100 msec after the stimulus is extinguished. So, once we realized that termination responses were critical to visibility, the mysterious timing of backward masking finally made sense: when the mask has just the right timing, it inhibits the target's termination response through classical lateral inhibition mechanisms. Without the termination response, the brain has a diminished signal from which to produce the percept of the target, and the target is therefore less visible. Previous work on neural coding in the brain had favored stimulus onset responses over termination responses. We published these studies, and follow up optical imaging studies of cortical activity, in Nature Neuroscience and PNAS. My colleagues in these studies were Margaret Livingstone, David Hubel, Susana Martinez-Conde, and Michael Haglund.

Research Interest

The neurobiology and perception of brightness and flicker, Abnormal blood flow in neurological and ophthalmological disease, Attention and cognition in mental disease.