NONLINEAR PREPROCESSING IN SHORT-RANGE MOTION

The phenomenon of non-Fourier motion (visually perceived motion that c annot be explained simply on the basis of the autocorrelation structur e of the visual stimulus) is well recognized, and is generally conside red 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 P -n will generate a perception of motion if it is preprocessed by a non linearity of polynomial order n or greater, but not if preprocessed by a nonlinearity of polynomial order less than n. We found that unambig uous motion direction was perceived for P-2, P-3, and P-4, but not for higher-order stimuli, and we measured the contrast thresholds for dir ection discrimination with superimposed noise, We found that an asymme tric compressive nonlinearity can, in a unified fashion, account for t hese results, while a purely quadratic nonlinearity or a rectification of the form T(p) = \p\ cannot. We compared velocity discrimination ju dgements for second-order non-Fourier stimuli (P-2) with standard drif ting gratings. Although velocity comparisons were veridical, uncertain ties were greater for the non-Fourier stimuli. This could be reproduce d by substituting a Fourier grating with superimposed noise for the no n-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 i nterpreted as demonstrating separate pathways. (C) 1997 Elsevier Scien ce Ltd.