Implications of motion detection for early non-linearities

Analysis of motion may be accomplished using the spatiotemporal variations produced when a spatially varying luminance waveform moves across linear receptive fields. Moving contrast-modulated patterns which consist of coarse-scale spatial variations in the contrast of fine-scale luminance patterns cannot be analysed in this way. The human visual system can analyse the motion of contrast-modulated patterns and this suggests it may contain mechanisms that use non-linear transformations. Non-linear transformation of contrast-modulated patterns would give rise to a component (a distortion product) that varies on the same spatial scale as the contrast variation; this can be analysed to extract motion. Is the non-linearity simply an inherent part of the transduction process or is it a characteristic of a mechanism specialized for the analysis of the motion of such patterns? Comparisons of the spatial and temporal limitations of motion discrimination using luminance and contrast-modulated patterns suggest that the mechanisms which analyse the two types of patterns are different, although recent physiological evidence suggests that they may have common elements.