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 variation in luminance of a visual stimulus compared to the background.

Most cortical visual neurons display a sigmoidal increase of their firing with respect to stimulus contrast, which has thus been conceived as a modulation of stimulus energy. However, psychophysical evidence shows that human observers are able to attentively select items of non-maximal contrasts, suggesting that at least a fraction of cortical visual neurons might show a non-monotonic tuning profile in response to varying contrast levels.

To explore this possibility, we recorded responses of individual neurons in macaque area V4d to a set of stimuli spanning the complete range of stimulus contrasts, and we finely explored their contrast response function (CRF) - a critical step in order to achieve a full understanding of the way in which luminance contrast is encoded by the given neuron.

Besides neurons with traditional CRFs, we found neurons which were not appreciably modulated by contrast - a candidate correlate for the phenomenon of contrast invariance in object recognition - and, most crucially, neurons showing a non-monotonic tuning profile in response to varying stimulus contrast – for which contrast is to be considered as a feature of the stimulus instead of a factor determining stimulus strength.