Feb. 17, 11:45, Wei Ji Ma. Optimality and Probabilistic Computation in Visual Categorization

Melchi Michel

Psychology Dept., Rutgers

Psychophysics in the cartesian theater: Examining the perceptual consequences of cortical topography

Sensory cortical areas are topographically organized, yet the contribution of this organization to
perception is largely unknown. I present recent work providing evidence that
the topographical organization of cortical population activity influences perceptual judgments. By
combining computational modeling, real-time optical imaging in behaving monkeys, and behavioral
measurements in humans, my colleagues and I demonstrate that human observers exploit the large-scale topography
of V1 population responses when making shape judgments. Specifically, we used a simple
computational model to design visual stimuli that have the same physical shape but were predicted
to vary in the spread of activity they elicit in V1. We confirmed this novel physiological prediction
with real-time optical imaging. We then reasoned that if subjects use the retinotopic extent of
activity to judge shape, they will misjudge the shape of these stimuli. Behavioral measurements in
humans confirmed this prediction and reveal a novel shape illusion, demonstrating that the
retinotopic spread of activity in V1 contributes to shape perception.