The effect of eccentricity on simultaneous performance in position and movement acuity tasks

Abstract

Visual performance can be made equal across the visual field in various tasks simply by an appropriate change of scale, but the speed at which performance declines towards periphery varies greatly between tasks. E2 denotes the eccentricity at which foveal stimulus size must double in order to maintain performance equivalent to that at the fovea. Two tasks, spatial interval discrimination and displacement detection, were presented at the same location to see how simultaneous processing of these two tasks with different E2 values would influence eccentricity dependence in each task. The subject was to solve only the spatial interval task, only the displacement task, or both tasks simultaneously. Eccentricity dependence in each condition was determined by using the method of partial scaling, where thresholds were estimated at several eccentricities for a series of stimuli, all of which were simply magnified or minified versions of one another.With 500 msec stimulus duration, the individual E2 value was found to be 0.17–0.39° for spatial interval discrimination and 1.0–1.2° for displacement detection. The values remained unaffected whether the subject solved one task or two tasks simultaneously. This finding was confirmed with a 50 msec stimulus duration. As there is no interference between tasks, the mechanisms solving the tasks appear to be functionally dependent, i.e. operating in parallel at all eccentricities.