Non-monotonic Contrast Tuning in macaque area V4

摘要

Neurons in the visual system typically exhibit a sigmoidal increase of their firing with luminance contrast of the stimulus. Here we report the first evidence of the existence of visual neurons showing selectivity tuning for contrast. We recorded responses of individual neurons in macaque area V4d to a set of bar stimuli spanning a wide range of contrasts while the animals were engaged in an orientation discrimination task. First we characterized cells as having a monotonic vs. non-monotonic function. Then we explored the temporal dynamics of CRFs, using partially overlapping time windows. We found highly heterogeneous discharge patterns, with around 34% of the recorded cells showing a non-monotonic profile. These cells were selective for a specific range of contrasts (average full bandwidth: 2.79 ?± 0.22 sem octaves), with maximal responses occurring for different contrast levels across the population (range: 1.1-76.55 % Michelson contrast). The collected evidence indicates that in V4 contrast is also encoded by bandwidth filters selective to contrast, similarly to color or orientation. Interestingly, contrast tuning emerged after a delay from stimulus onset, probably reflecting the contribution of a normalization pool of cells - a candidate physiological mechanism mediating automatic contrast gain. We speculate that selective tuning for contrast and the increase in non-monotonicity over time might improve contrast coding at the population level, probably useful to mediate categorization of stimulus contrast.