Illusory bending of a pursuit target

Abstract

To pursue a small target moving in front of a drifting background, motion vectors from the target need to be integrated and segmented from those belonging to the background. Smooth pursuit eye movements typically integrate target and background directions initially and after some time shift towards the veridical target direction. The perceived target direction on the other hand is generally stable over time: the target is perceived to move in the same direction as long as the motion information maintains the same properties over time. If illusory target motion is observed, this tends to be shifted away from the background. Here we investigated how initial motion integration and segmentation of such stimuli are modulated by direction cues. We presented a small pursuit target moving along a straight path, in front of a background moving in a different direction. Without a direction cue, initial pursuit was biased towards the background direction before shifting towards the veridical target direction. The target's perceived direction on the other hand was near veridical. A cue in the background direction increased initial pursuit integration but also caused perception to behave in a similar way: the target initially had an illusory motion component in the background direction and after about 200 ms it was perceived to curve towards its veridical direction. This illusion shows that during the initial process of segmenting the direction of a pursuit target from irrelevant background motion, both pursuit and perception can be erroneously influenced by a direction cue and integrate the cued background motion. Both modalities corrected this initial integration error as more information about the target became available.

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