Emma Neto¹, Allison B. Sekuler^1,2,3, Patrick J. Bennett¹; ¹Department of Psychology, Neuroscience & Behaviour, McMaster University, ²Rotman Research Institute, Baycrest Academy for Research and Education, ³Department of Psychology, University of Toronto

Relative phase discriminations are made with even-symmetric and odd-symmetric visual mechanisms (Field & Nachmias, Vision Res, 1984). In central vision, even- and odd-symmetric mechanisms are approximately equally sensitive, but it is unclear how sensitivity changes in peripheral vision. Using compound gratings, Bennett and Banks (Nature, 1987) found that sensitivity of odd-symmetric mechanisms declined dramatically in the periphery; however, using stimuli consisting of single, isolated features, Morrone et al. (Vision Res, 1989) found that even- and odd-symmetric sensitivity remained approximately equal across the visual field. Neto et al. (JOV, 2025) investigated whether these different results reflected phase-specific effects of visual crowding. Consistent with this idea, they found that increasing the number of stimulus features, and decreasing inter-feature spacing, reduced odd-symmetric but not even-symmetric sensitivity. Here we examine the crowding phenomenon by measuring the effects of adjacent flankers on phase discrimination thresholds. Phase discrimination thresholds were measured in four observers. Target stimuli consisted of a single phase-reversing feature (i.e., bright/dark bars or left/right edges). We found evidence of crowding in both tasks: bar and edge discrimination thresholds increased with increasing flanker contrast and with decreasing target–flanker spacing. However, edge discrimination varied far more drastically with flanker contrast and spacing than did bar discrimination. Importantly, bar and edge detection thresholds varied little with flanker contrast and spacing, so the inability to discriminate relative phase was not due to a failure to detect the target stimuli. These results are consistent with the hypothesis that odd-symmetric mechanisms are especially sensitive to crowding in the visual periphery.

Acknowledgements: NSERC