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Robert J Green, Susana T L Chung; Radial-tangential anisotropy of bisection thresholds in the normal periphery. Journal of Vision 2019;19(10):67b. doi: https://doi.org/10.1167/19.10.67b.
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© ARVO (1962-2015); The Authors (2016-present)
It is well known that the spatial interaction zone exhibits a radial-tangential anisotropy in the normal periphery, such that the zone is larger along the radial than the tangential meridian. Several reports provide evidence that bisection thresholds (judging whether an object divides a spatial interval into two equal halves) also demonstrate such a radial-tangential anisotropy in the normal periphery. However, most of these data were obtained only along two meridians (radial and tangential), which are insufficient to fully quantify the characteristics of the two-dimensional shape of the bisection zone. Here, we measured thresholds for a three-dot bisection task along four meridians to map out the two-dimensional shape of the bisection zone. Three collinear dots, with the two outer dots separated by 1°, were presented with the center of the ensemble located at 5° eccentricity in the right, lower-right or lower visual field. The three dots were presented along four meridians (radial, tangential and ±45° from the radial). We presented the middle dot at five displacements from the center of the spatial interval (0, ±1, ±2, steps). Observers judged whether the middle dot was closer to either of the two outer dots. A psychometric function was fit to each set of data. The displacement that corresponded to 75% response toward the direction of one of the two outer dots was used to represent the bisection threshold. Consistent across four observers, bisection thresholds were higher along the radial than the tangential meridian, demonstrating a radial-tangential anisotropy, but only in the right and lower visual fields. In the lower-right field, bisection thresholds were similar along all four tested meridians. Our results provide a more complete picture of the two-dimensional shape of the spatial fidelity zone for bisection in the normal periphery and allow us to model small rotations of the zones.
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