Eunhye Choe¹ (), Patrick Cavanagh^1,2, Peter U. Tse¹; ¹Dartmouth College, ²Glendon College

In apparent motion quartets, dots at two diagonally opposite corners of an imaginary rectangle are replaced by two dots at the other two corners, causing subjective perception to shift between horizontal and vertical motion percepts. When the imaginary rectangle is a square (i.e. aspect ratio or AR = 1), there is typically a bias toward perceiving vertical more often than horizontal apparent motion. This may arise from a tendency to match successive dots within a hemifield, as such matching occurs within rather than between brain hemispheres. This bias can be counteracted by increasing the within-hemifield matching distance relative to the cross-hemifield distance (i.e. AR > 1). The within-hemifield matching bias is expected to be greater in peripheral vision, where hemisphere-specific processing occurs, than in foveal vision which is processed to some extent in both hemispheres. In this study, apparent motion quartets were presented on a virtual circle with radii of 0.25, 0.5, 1, 2, 4, or 8° of visual angle relative to fixation. Each quartet consisted of two diagonal dot pairs, with aspect ratios gradually shifting from 3 to 1/3 or the reverse. Participants reported the aspect ratio at which they perceived a change in motion direction. The results show that the threshold aspect ratio of horizontal to vertical distances increased significantly with greater eccentricity (0.25°, AR=1.12; 0.5°, AR=1.14; 1°, AR=1.16; 2°, AR=1.19; 4°, AR=1.22; 8°, AR=1.21). Our findings suggest that the bias to match within-hemifield becomes more pronounced with increasing eccentricity.