Positive motion after-effect induced by bandpass-filtered random-dot kinematograms

Using spatially filtered random-dot kinematograms (RDK) with a 1 octave bandwidth, the duration and direction of motion aftereffect (MAE) were measured while varying step displacement, and the results were compared to those for motion direction discrimination. We found that MAE and motion discrimination show quite different dependencies on displacement. At displacements around 0.5 cycles of the lowest frequency, while motion discrimination was still almost perfect, normal MAE vanished and MAE in the same direction as that of the adapting stimuli, which we name "positive MAE", was observed. Similar results were obtained for both 1- and 2-dimensional filtered patterns. A theoretical examination based on the Fourier components of the stimuli showed that the displacement dependency of MAE is predictable in terms of adaptation of first-order detectors, but that of motion discrimination is not. The present results indicate that direction perception and MAE for bandpass RDKs are mediated at least partially by separate mechanisms, and that direction perception at larger displacements is mediated by the second-order mechanism which detects movement of contrast modulation in the image.