Search

Quantitative descriptions, derived from an earlier empirical study, of threshold relationships among the principal stimulus variables for motion discrimination are presented, with emphasis on evaluating the influence of stimulus energy content. The present findings are compared with those of R, H. Brown (1955, 1957, 1958), who reported the applicability of luminance-time reciprocity to motion threshold for exposures shorter than 0.1 sec. It is demonstrated that such reciprocity is atypical of motion threshold, and that it represents limitations imposed by the requirements of target visibility rather than by motion discrimination itself. It is also shown that Brown’s data are predictable from the present equations.

The threshold stimulus for visual motion discrimination was analyzed into the constituent parameters of velocity, i.e., time and distance, with both of these primary variables subject-determined. It was found that, given a constant stimulus luminance, motion threshold was characterized generally by a “trade-off” or inverse power relationship between time and distance of movement. Earlier reports of energy constancy at threshold (R. H. Brown, 1955, 1957, 1958), implying threshold relationships incompatible with these, were confirmed only for the atypical conditions of high-velocity/low-Iuminance stimuli and were attributed to absolute visibility requirements. Under more general and representative conditions, threshold was relatively insensitive to luminance. The present results were also contrasted with earlier findings (Graham. 1968) of distance or “displacement” constancy at threshold, pertaining to movement between stationary start and stop positions.