Detection of geometric image distortions at various eccentricities

Human ability to perceive spatial stimuli declines with increasing eccentricity. To study this phenomenon with natural images, the authors applied the spatial scaling method by measuring the smallest detectable amount of geometric change in a human face at several eccentricities for a series of stimulus magnifications to find out whether performance could be made equal across the visual field simply by an appropriate enlargement.

The authors used a novel method to produce subtle changes to an image of a face. The smallest change recognized was determined using a two-alternative forced-choice method and expressed in terms of correlation sensitivity, the inverse of the correlation between the images that just could be discriminated.

The detection of changes in the facial features, presumably a spatially complex task, became equal across the visual field simply by an appropriate change of scale. The E2 value represents the eccentricity at which the foveal stimulus size must double to maintain performance at the foveal level. The E2 values, found to be 1.73 degrees to 2.45 degrees, were similar to our previously measured values for vernier acuity, orientation discrimination, and curvature detection and discrimination, obtained with the same method of spatial scaling.

The authors' 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 to be no qualitative difference between the accuracy of foveal and peripheral processing.