Abstract
When multiple visual stimuli are presented simultaneously in the receptive field, the neurophysiological response is surprisingly lower than when the identical stimuli are presented sequentially (Kastner et al. 1998, 2001, 2021; Reynolds et al. 1999). However, the underlying mechanism of this suppression effect is not well-understood. Here we collected fMRI data in two separate experiments and computationally tested simultaneous suppression using population receptive field (pRF) models. First, we mapped each voxel’s spatial pRF and defined visual areas using cartoon stimuli (Toonotopy; Finzi et al. 2020). Second, we presented colorful square stimuli either sequentially, where four stimuli appeared one at a time, in random order, or simultaneously, where identical stimuli appeared for the same duration but all at once. To examine how temporal and spatial summation contribute to brain responses, both conditions used two durations (200 and 1000 ms) and two stimuli sizes (4 and 16 deg2). We found that V1-V2 voxel responses were similar for simultaneous and sequential conditions, and larger for longer and bigger stimuli. This response pattern was well-predicted by a linear pRF model (Dumoulin & Wandell, 2008), as pRFs were small and typically covered one square. However, in V3 and higher visual areas, responses were lower for simultaneous than sequential presentations, higher for shorter than longer durations, and did not increase much with stimulus size. While pRFs in these regions covered multiple stimuli, the linear pRF model failed to predict our observations. A compressive spatial summation pRF model (Kay et al. 2013) predicted the modest increase with stimulus size, but overpredicted simultaneous suppression and did not predict larger responses for shorter durations. Our results indicate that sensory suppression cannot be explained by spatial pRFs only and that spatiotemporal pRFs with nonlinearities are necessary for predicting responses beyond early visual areas.