Contrast is a key ingredient of the visual world and it is indispensable for image perception. A particular form of it is luminance contrast, the physical quantity that specifies the relative level of luminance of a visual stimulus compared to the background.

Most cortical visual neurons display a monotonic increase of their firing with respect to stimulus contrast. Psychophysical, neurophysiological and computational evidence suggests that at least a fraction of cortical visual neurons might show a non-monotonic tuning profile in response to varying contrast. However, the phenomenon has not been quantified and mechanisms underlying non-monotonicity remain unknown.

We recorded responses of individual neurons in macaque area V4d to a set of stimuli spanning the complete range of stimulus contrasts. We studied their contrast response functions (CRFs) and characterized cells as having a monotonic vs. non-monotonic function. To gain a better understanding of the mechanisms leading to such divergent patterns, we explored the temporal dynamics of CRFs.

The neurons with non-monotonic CRFs were selective to a particular range of contrasts, with peak contrast varying from neuron to neuron. Interestingly the 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 mechanisms.