Nonlinear preprocessing in short-range motion.

The phenomenon of non-Fourier motion (visually perceived motion that cannot be explained simply on the basis of the autocorrelation structure of the visual stimulus) is well recognized, and is generally considered to be due to nonlinear preprocessing of the visual stimulus prior to a stage of standard motion analysis. We devised a sequence of novel visual stimuli in which the availability of a motion stimulus depends on the nature of the nonlinear preprocessing: an nth order stimulus Pn will generate a perception of motion if it is preprocessed by a nonlinearity of polynomial order n or greater, but not if preprocessed by a nonlinearity of polynomial order less than n. We found that unambiguous motion direction was perceived for P2, P3, and P4, but not for higher-order stimuli, and we measured the contrast thresholds for direction discrimination with superimposed noise. We found that an asymmetric compressive nonlinearity can, in a unified fashion, account for these results, while a purely quadratic nonlinearity or a rectification of the form T(p) = magnitude of p cannot. We compared velocity discrimination judgements for second-order non-Fourier stimuli (P2) with standard drifting gratings. Although velocity comparisons were veridical, uncertainties were greater for the non-Fourier stimuli. This could be reproduced by substituting a Fourier grating with superimposed noise for the non-Fourier grating. These findings are consistent with a single pathway which processes both Fourier and non-Fourier short-range motion, and are discussed in the context of other investigations which have been interpreted as demonstrating separate pathways.