DETECTION OF GEOMETRIC IMAGE DISTORTIONS AT VARIOUS ECCENTRICITIES

Purpose. Human ability to perceive spatial stimuli declines with incre asing eccentricity. To study this phenomenon with natural images, the authors applied the spatial scaling method by measuring the smallest d etectable amount of geometric change in a human face at several eccent ricities for a series of stimulus magnifications to find out whether p erformance could be made equal across the visual field simply by an ap propriate enlargement. Methods. The authors used a novel method to pro duce subtle changes to an image of a face. The smallest change recogni zed was determined using a two-alternative forced-choice method and ex pressed in terms of correlation sensitivity, the inverse of the correl ation between the images that just could be discriminated. Results. Th e detection of changes in the facial features, presumably a spatially complex task, became equal across the visual field simply by an approp riate change of scale. The E-2 value represents the eccentricity at wh ich the foveal stimulus size must double to maintain performance at th e foveal level. The E-2 values, found to be 1.73 degrees to 2.45 degre es, were similar to our previously measured values for vernier acuity, orientation discrimination, and curvature detection and discriminatio n, obtained with the same method of spatial scaling. Results. The dete ction of changes in the facial features, presumably a spatially comple x task, became equal across the visual field simply by an appropriate change of scale. The E-2 value represents the eccentricity at which th e foveal stimulus size must double to maintain performance at the fove al level. The E-2 values, found to be 1.73 degrees to 2.45 degrees, we re similar to our previously measured values for vernier acuity, orien tation discrimination, and curvature detection and discrimination, obt ained with the same method of spatial scaling. Conclusions. The author s' results indicate that with adequate stimulus magnification, one is capable of detecting geometric changes in complex images such as face equally at the fovea and in the periphery. In this task, there seems t o be no qualitative difference between the accuracy of foveal and peri pheral processing.