FULL-WAVE AND HALF-WAVE RECTIFICATION IN 2ND-ORDER MOTION PERCEPTION

Microbalanced stimuli are dynamic displays which do not stimulate moti on mechanisms that apply standard (Fourier-energy or autocorrelational ) motion analysis directly to the visual signal. In order to extract m otion information from microbalanced stimuli, Chubb and Sperling [(198 8) Journal of the Optical Society of America, 5, 1986-2006] proposed t hat the human visual system performs a rectifying transformation on th e visual signal prior to standard motion analysis. The current researc h employs two novel types of microbalanced stimuli: half-wave stimuli preserve motion information following half-wave rectification (with a threshold) but lose motion information following full-wave rectificati on; full-wave stimuli preserve motion information following full-wave rectification but lose motion information following half-wave rectific ation. Additionally, Fourier stimuli, ordinary square-wave gratings, w ere used to stimulate standard motion mechanisms. Psychometric functio ns (direction discrimination vs stimulus contrast) were obtained for e ach type of stimulus when presented alone, and when masked by each of the other stimuli (presented as moving masks and also as nonmoving, co unterphase-flickering masks). Results: given sufficient contrast, all three types of stimulus convey motion. However, only one-third of the population can perceive the motion of the half-wave stimulus. Observer s are able to process the motion information contained in the Fourier stimulus slightly more efficiently than the information in the full-wa ve stimulus but are much less efficient in processing half-wave motion information. Moving masks are more effective than counterphase masks at hampering direction discrimination, indicating that some of the mas king effect is interference between motion mechanisms, and some occurs at earlier stages. When either full-wave and Fourier or half-wave and Fourier gratings are presented simultaneously, there is a wide range of relative contrasts within which the motion directions of both grati ngs are easily determinable. Conversely, when half-wave and full-wave gratings are combined, the direction of only one of these gratings can be determined with high accuracy. Conclusions: the results indicate t hat three motion computations are carried out, any two in parallel: on e standard (''first order'') and two non-Fourier (''second-order'') co mputations that employ full-wave and half-wave rectification.