Influences of intra- and crossmodal grouping on visual and tactile Ternus apparent motion.

Previous studies of dynamic crossmodal integration have revealed that the direction of apparent motion in a target modality can be influenced by a spatially incongruent motion stream in another, distractor modality. Yet, it remains to be examined whether non-motion intra- and crossmodal perceptual grouping can affect apparent motion in a given target modality. To address this question, we employed Ternus apparent-motion displays, which consist of three horizontal aligned visual (or tactile) stimuli that can alternately be seen as either 'element motion' or 'group motion'. We manipulated intra- and crossmodal grouping by cueing the middle stimulus with different cue-target onset asynchronies (CTOAs). In unimodal conditions, we found Ternus apparent motion to be readily biased towards 'element motion' by precues with short or intermediate CTOAs in the visual modality and by precues with short CTOAs in the tactile modality. By contrast, crossmodal precues with short or intermediate CTOAs had no influence on Ternus apparent motion. However, crossmodal synchronous tactile cues led to dominant 'group motion' percepts. And for unimodal visual apparent motion, precues with long CTOAs shifted apparent motion towards 'group motion' in general. The results suggest that intra- and crossmodal interactions on visual and tactile apparent motion take place in different temporal ranges, but both are subject to attentional modulations at long CTOAs.