Abstract
Neural responses in early visual cortex are regulated by gain control mechanisms. Divisive normalization has been put forth as a unifying explanation for phenomena such as contrast response saturation, cross-orientation suppression, and surround suppression. The magnitude of suppression evoked by different visual stimuli determines the strength of neural responses in early visual cortex, and in turn, the subjective salience of the visual stimulus. Notably, suppression strength is modulated by feature similarity; for instance, responses to a center-surround stimulus in which the components are collinear to each other are weaker than when they are orthogonal. However, this feature-tuned aspect of normalization has been understudied, and we lack a clear picture of how the gain of responses is affected by feature-tuned normalization. Here, we examine the contribution of the tuned component of suppression to contrast response modulations. To do so, we used functional magnetic resonance imaging (fMRI) to measure contrast response functions (CRFs) in early visual cortex (areas V1 – V3) in human observers while they viewed full-field center-surround gratings. The surround (contrast: 100%) was either collinear or orthogonal with respect to the center, which varied in contrast between 2.67-96%, after initial adaptation to low-contrast (16%) stimulus to promote non-linear CRFs. We found substantially stronger suppression of responses when the surround was collinear configured, manifesting as shifts in the population CRF. The measured CRFs were fit with a variant of the normalization model at the individual voxel level, which revealed distinct changes in the gain of the CRFs with tuned suppression. Furthermore, the magnitude of the CRF shift was strongly dependent on voxel spatial preference, and seen primarily in voxels whose receptive field spatial preference corresponds to the area straddling the center-surround boundary in our display, with little-to-no modulation elsewhere.