Abstract
Humans and many animals can perceive numerosity, the number of visual objects. Recently-viewed numerosities can influence numerosity perception and cause numerosity adaptation aftereffects. In the brain, numerosity produces both early visual responses that monotonically increase with numerosity and later tuned response that peak at different (preferred) numerosities in different neural populations. We have recently shown that the preferred numerosity of numerosity-tuned neural populations is affected by numerosity adaptation. We have also shown that early visual monotonic responses reflect features of spatial frequency domain image contrast that closely follow numerosity. We therefore hypothesized that adaptation’s effects on early visual, contrast-driven monotonic responses to numerosity may precede adaptation’s effects on numerosity-tuned response preferences. We measured early visual responses to numerosity during adaptation to both a high numerosity (20) and a low numerosity (1) using ultra-high field (7T) fMRI. We also measured the spatial population receptive fields (pRFs) to determine the visual field maps of the responsive voxels. During adaptation to high numerosities, the monotonic increase in response amplitudes with increasing numerosity is lower than during adaptation to low numerosities throughout early visual cortex (V1-V3, hV4 & LO1). This is consistent with perceived numerosity decreasing during high numerosity adaptation. Furthermore, baseline responses during high numerosity adaptation are higher than during low numerosity adaptation, consistent with a monotonic response to the adapter itself. Therefore, numerosity adaptation effects begin in the earliest, contrast-driven stages of vision, and may depend on contrast normalization mechanisms. The resulting changes in early monotonic responses may underlie later effects on numerosity-tuned responses.