Eunhye Choe¹ (), Wentao Si¹, Yong Hoon Chung¹, Viola Störmer¹, Patrick Cavanagh^2,1, Peter U. Tse¹; ¹Dartmouth College, ²Glendon College

Where in the brain are perceived object positions represented? We used fMRI and the frame-induced position shift (FIPS) illusion that allows dissociating (mis)perceived positions from physical positions of objects to test this. A pair of probe dots were flashed in succession in the upper and then lower visual fields, displacing vertically up and down with each alternation. At the same time, a surrounding frame moved leftward and rightward in synchrony with the probe presentations. Rather than seeing the probes displaced vertically, they appeared shifted along a slanted trajectory, demonstrating that perceived position is coded relative to the position of the anchoring frame. We collected 3T fMRI data while participants viewed this FIPS stimulus that induced illusory leftward- or rightward-slanted shifts in perceived probe positions. The probes were also shown without the frame, either at the vertically aligned physical positions or shifted corresponding to each participant’s reports of the illusory shift determined in a pre-scan psychophysical experiment. Participants performed a fixation brightness task or a change detection task on the probes, ensuring that probe positions were always task-irrelevant during the fMRI scan. Multivoxel pattern analysis was applied to assess whether neural activity elicited by illusory positions generalized to activity patterns evoked by matched physical positions. Matched physical positions were robustly decoded in early visual cortex, but cross-decoding of illusory position and matched physical position only emerged in dorsal occipital and occipitoparietal regions, suggesting shared representations between illusory and physical locations arise in higher-order visual areas. Moreover, using a searchlight analysis, strongest cross-decoding was found in dorsal anterior cingulate, DLPFC and the anterior temporal pole similar to Liu et al (2019). These findings provide further support for the unorthodox view that perceived position is represented at a much higher stage of the visual hierarchy, well beyond early retinotopic cortex.