Susana Chung¹, Kathlyn Bako¹, Rijul Soans¹; ¹University of California, Berkeley

Our ability to detect small positional differences between objects is very exquisite. The precision of positional acuity depends on many factors. In this study, we investigated how positional acuity changes with stimulus orientation and locations in the visual field. Positional acuity was assessed using a three-dot bisection task. In each trial, a target dot appeared at one of seven positions between two reference dots. Observers (N=8) judged whether the target dot was closer to either of the reference dots. From the psychometric function relating the percent-response toward one reference dot’s direction to the target dot location, we derived positional acuity (the positional shift of the target dot corresponding to a change in response probability from 50% to 84%) and the point of subjective equity (PSE), representing judgment bias. Measurements were made with stimulus dots aligned horizontally, vertically or obliquely (45° or 135°) at the fovea, and at 4° and 8° eccentricities in the nasal, inferior, and inferior-nasal visual fields. Dot size and separation in the parafoveal locations were scaled according to the cortical magnification factor. Each condition was repeated 3-4 times. As expected, positional acuity was best at the fovea and declined progressively with increasing eccentricity. At the fovea, positional acuities were similar regardless of dot alignments. At 4° and 8° eccentricities, positional acuities were worst when dots were aligned radially relative to fixation and best when dots were aligned tangentially. Across observers, the radial-tangential ratio of positional acuity was 1.01 at the fovea (implying isotropy), 1.79 and 2.14 at 4° and 8° eccentricities (implying anisotropy), respectively. PSE did not vary systematically with any conditions. The shift of positional acuities from being independent of stimulus orientations at the fovea to demonstrating strong radial-tangential anisotropy in the parafovea is consistent with how visual processing differs between the fovea and the parafovea.

Acknowledgements: NIH Research Grant R01-EY036054 and Training Grant T35-EY007139