Facilitation beyond suppression: Enhanced direction discrimination after bidirectional motion adaptation

This study investigates how unidirectional and bidirectional motion adaptation affects direction discrimination sensitivity using a rotational motion paradigm. Participants adapted to rotating log-spiral stimuli under three conditions: clockwise, counterclockwise, or bidirectional (opposing directions simultaneously). They then performed a direction discrimination task, judging whether a briefly presented test stimulus rotated clockwise or counterclockwise. Unidirectional adaptation decreased sensitivity and induced a directional bias, consistent with the suppressive effects of adaptation and the waterfall motion illusion. In contrast, bidirectional adaptation enhanced direction discrimination sensitivity without introducing directional bias. These findings challenge traditional models of motion adaptation that emphasize direction-specific suppression. Instead, they support a dynamic, resource-based account in which the visual system actively recalibrates sensitivity to optimize performance based on the distribution of motion stimuli in the environment.