Neural numerosity selectivity changes after visual numerosity adaptation

The perception of visual numerosity (i.e. the set size of a group of items) is an evolutionarily preserved ability found in humans and animals. A useful method to infer the neural underpinnings of a given perceptual feature is sensory adaptation. Numerosity is susceptible to adaptation, similarly to other visual features. Recently, we have shown numerosity-selective neural populations with a topographic organisation in the human brain. Here, we investigated whether the numerosity selectivity of these neural populations changes during adaptation to visual numerosity. Using 7 Tesla ultra-high field fMRI, we scanned participants while they viewed stimuli which systematically changed in numerosity (1 to 7 dots with a baseline of 20 dots). In the adaptation conditions, the conventional numerosity stimuli were interleaved with a low or high numerosity adapter, consisting of 1 or 20 dots, respectively. We analysed the responses using custom-build population receptive field neural models of numerosity encoding, and compared estimated numerosity preferences between adaptation conditions. We replicated our previous studies where we found several topographic maps of numerosity-selective responses. During numerosity adaptation, we found that the numerosity preferences within the numerosity maps were biased towards the numerosity of the adapter. Specifically, and after adaptation to a low numerosity (1 dot), the numerosity preference of the numerosity maps was biased towards lower numerosities, whereas after adaptation to a high numerosity (20 dots), the numerosity preference of the numerosity maps was biased towards higher numerosities. These results suggest that the observed changes in numerosity-selective neural populations could contribute to the perceptual effects of numerosity adaptation.