Numerosity adaptation suppresses early visual responses

Humans and many other animals possess an innate ability to rapidly perceive numerosity: the number of objects in a visual scene. Numerosity perception is influenced by adaptation, whereby previously viewed numerosities affect perception of the current image’s numerosity. Parietal and frontal neural populations are tuned to specific preferred numerosities, and this tuning is affected by adaptation. A parallel line of research has revealed that early visual responses monotonically increase with numerosity. Here we use ultra-high field (7T) fMRI, to show that these monotonic responses become less pronounced after adaptation to higher numerosities. This neural adaptation effect is consistent with a reduction in perceived numerosity during adaptation to high numerosities. Moreover, it becomes stronger as we progress through the early visual hierarchy (V1-V3, hV4, LO1-LO2 & V3A/B). Together, these findings show that numerosity adaptation effects, while impacting higher order cognition, originate in early visual responses to image contrast.