Feiyi Wang¹, Emily Wiecek^1,2,3, Sam Ling¹; ¹Department of Psychological & Brain Science, Boston University, ²Department of Ophthalmology, Boston Children’s Hospital, ³Harvard Medical School

Visibility of a target can be impaired when closely flanked by other stimuli, a phenomenon known as crowding. Crowding tends to be feature-tuned, wherein suppression is more potent when the target and flankers share similar features, such as spatial frequency (SF). Here, we used fMRI to assess the visuocortical mechanisms underlying crowding, and its impact on SF processing. To do so, we leveraged population spatial frequency tuning (pSFT), a model-based technique for characterizing voxel-wise SF tuning, under crowded and uncrowded conditions. In the uncrowded condition, participants viewed targets (eccentricity: 7.85 dva; subtending 7.5 x 3.5 dva), which swept through a range of SF’s (bandpass-filtered noise, 0.5 cpd-12 cpd) to map voxel-wise pSFT tuning, across V1-V3. In the crowded condition, participants were shown the same SF-filtered noise targets but surrounded by two flankers (center-to-center distance: 3.6 dva). To examine the influence of flanker SF on target tuning, flankers were SF-filtered to be either above (high-SF: 3 cpd) or below (low-SF: 1 cpd) the average preferred SF of voxels corresponding to the target (2 cpd). We found that crowding shifted population SF tuning preferences: a majority of the voxels within the target area shifted their peak tuning to lower SF’s when the target was crowded by high-SF flankers. In extrastriate cortex, tuning shifted to higher SF’s when the target was crowded by low-SF flankers. Our results show that crowding can shift population tuning profiles, but that this shift is dependent on match between flanker SF content and the preferences of a target, as well as visual area. This tradeoff in modulatory effects as a function of area and flanker SF may be driven by the systematic drop-off in SF sensitivity along the visuocortical hierarchy.