Bivectorial transparent stimuli simultaneously adapt mechanisms at different levels of the motion pathway

Abstract

The motion aftereffect (MAE) to drifting bivectorial stimuli, such as plaids, is usually univectorial and in a direction opposite to the pattern direction of the plaid. This is true for plaids that are perceived as coherent, but also for other plaids which are seen as transparent for most or all of the adaptation period. The underlying mechanisms of this MAE are still not well understood. In order to assess these mechanisms further, we measured static and dynamic MAEs and their interocular transfer (IOT). Adaptation stimuli were plaids with small (coherent) and large (transparent) angles between the directions of the component gratings and a horizontal grating, which were adjusted in spatial frequency and drift velocity so that the pattern speed and vertical periodicity remained constant. Test stimuli were horizontal static or counterphasing gratings with the same periodicity as the adaptation stimuli. MAE duration was measured for monocular, binocular and IOT conditions. All static MAEs were smallest for the transparent plaid and largest for the grating, while all dynamic MAEs were constant across adaptation stimuli. IOT was twice as big for dynamic MAEs as for static MAEs, and did not vary with the adaptation stimuli. Other adaptation stimuli were plaids that differed in intersection luminance, contrast or spatial frequency, resulting in different amounts of perceived coherence. MAEs and IOT did not vary with perceived coherence. The results suggest that the MAE for bivectorial stimuli consists of low-level adaptation (dependent on local component properties, small IOT), as well as high-level adaptation (dependent on global integrated pattern properties, large IOT), which can be measured independently with static and dynamic test stimuli.