PITFALLS IN ESTIMATING MOTION DETECTOR RECEPTIVE-FIELD GEOMETRY

A number of psychophysical investigations have used spatial-summation methods to estimate the receptive field (RF) geometry of motion detect ors by exploring how psychophysical thresholds change with stimulus he ight and/or width, This approach is based on the idea that an observer 's ability to detect motion direction is strongly determined by the re lationship between the stimulus geometry (height and width) and the RF of the activated motion detectors. Our results show that previous est imates of RF geometry can depend significantly on stimulus position in the visual field as well as on the stimulus height-to-width ratio, Th e data further show that RF estimates depend on the stimulus in a mann er that is inconsistent with basic predictions derived from current mo tion detector models, Hence previous estimates of height, width, and h eight-to-width ratios of motion detector RFs are inaccurate and unreli able, This inaccurac/unreliability is attributed to a number of source s, These include incorrect fixed-parameter values in model fits, as we ll as the confounding of physiological spatial summation area through combined use of contrast thresholds and Gaussian-windowed stimuli, A t hird source of error is an asymmetric variation of spatiotemporal corr elation in the stimulus as either its height or width is varied (and t he other dimension held constant), Most importantly, a fourth source o f unreliability is attributed to the existence of a nonlinear, nonmono tonic distribution of motion detectors in the visual field that has be en previously described and is a natural result of visual anatomy. Cop yright (C) 1996 Elsevier Science Ltd