Localization of objects in the peripheral visual field

Abstract

We investigated visual localization by asking humans to point at visual objects without vision of their hand. The objects were luminous discs, presented stereoscopically at different distances, eccentricities and meridians with respect to the subjects' straight-ahead. Final pointing position was recorded by an electromagnetic search-coil technique. We found that the eccentricity of pointing responses towards peripheral targets was larger when subjects fixated straight-ahead rather than looked at the targets. This outcome confirmed our previous finding that target eccentricity in the peripheral visual field is overestimated. We further found that overestimation increased less than proportionally with target eccentricity, which suggests that the local magnification factor gradually declines in the visual periphery. A quantitative analysis indicated that the magnification factor is about 1.5 at the fovea, and approaches 1.0 at 10° visual angle. Thus, our data support the hypothesis of a peri-foveal magnification effect which gradually subsides with increasing eccentricity. The observed magnification was similar for the horizontal and the vertical meridian. We found that the egocentric distance of pointing responses depends not only on the distance of the object pointed at, but also on the distance of a second object in the visual field. This outcome was in quantitative agreement with the predictions of Foley's model of interactive distance evaluation. Response depth, i.e. the difference in the response distances towards the two objects, was larger if both objects appeared near the center of the visual field rather than if one object appeared in the visual periphery. This outcome was in quantitative agreement with the hypothesis that the peri-foveal magnification effect leads to an overestimation of stereoscopic depth.