Spatiotemporal characterization of a Fourier and non-Fourier motion system.

Direct-phi perception elicited by a reverse-phi (i.e. reversed-polarity) stimulus may well be accounted for, if the front-end filters of a classical Reichardt unit are full-wave rectifiers. It is shown that reverse-phi perception is progressively replaced by direct-phi perception when either the spatial or the temporal modulation of the reversed-polarity stimuli are decreased. Reverse-phi perception is very weak or absent for spatial and temporal frequencies < or = 1 c/deg and < or = 3.75 Hz, respectively, indicating that the sensitivity of a linear mechanism is weak or null within this frequency range. By pitting against each other reverse- and direct-phi stimuli, the relative sensitivities of the putative motion mechanisms with linear (Fourier) and full-wave rectified (non-Fourier) front-end filters were assessed for a large range of spatial and temporal frequencies. Absolute sensitivities of the two mechanisms were estimated on the assumption that they contribute through probability summation to the overall spatiotemporal sensitivity surface described by Kelly [(1979) Journal of the Optical Society of America, 69, 1340-1349]. In conjunction with related evidence it is suggested that the Fourier/non-Fourier distinction may be generalized in terms of a specific/unspecific dichotomy in motion processing.