Evalie C. Rehor¹ (), Daniel R. Coates², Susana T. L. Chung¹; ¹University of California, Berkeley, ²University of Houston

Crowding, the impaired ability to recognize an object in clutter, has been suggested as the bottleneck of object recognition, especially in peripheral vision. A signature of crowding is its radial-tangential anisotropy in peripheral vision —the extent of crowding is larger along the radial than the tangential direction. Nandy and Tjan (2012) proposed that this anisotropy is a result of the saccade-confounded image statistics in peripheral vision, which affects the direction along saccades (radial with respect to fixation) more than other directions. Because saccades made by humans are usually <15°, these authors predicted that the radial-tangential anisotropy would be smaller at eccentricities >15°. We tested this prediction by comparing the crowding extent (critical spacing) along the radial and tangential directions at 5°, 10°, 20° and 30° eccentricities in the right visual field. Participants (n=4) identified the orientation of a Tumbling-E presented alone, or flanked by two other Tumbling-Es positioned either radially or tangentially relative to the target-E, at a range of target-flanker spacings and letter sizes. The critical target-flanker spacing yielding 79%-correct was determined psychophysically, for each combination of eccentricity × radial/tangential conditions. Fixation was monitored using an eyetracker. As expected, the critical spacing increased with eccentricity. However, the increase was not isotropic: 5.5× radially and 7.8× tangentially (5° vs. 30° eccentricity). Moreover, the radial-tangential ratio of critical spacing was not a constant across eccentricity (p=0.005); instead, it increased slightly from 5° to 10° eccentricity, then diminished at larger eccentricities. Post-hoc analyses showed that the ratio at 30° eccentricity (1.50±0.15) was significantly different from the ratios at 5° (2.13±0.21) and 10° (2.31±0.31) eccentricities. Our finding of a reduction in the radial-tangential anisotropy of the crowding extent at large eccentricities is consistent with the prediction of Nandy and Tjan (2012) on the role of saccadic eye movements in modulating the crowding extent.

Acknowledgements: Grant Support: NIH Grant T32-EY007043