Remapping of the line motion illusion across eye movements.

Although motion processing in the brain has been classically studied in terms of retinotopically defined receptive fields, recent evidence suggests that motion perception can occur in a spatiotopic reference frame. We investigated the underlying mechanisms of spatiotopic motion perception by examining the role of saccade metrics as well as the capacity of trans-saccadic motion. To this end, we used the line motion illusion (LMI), in which a straight line briefly shown after a high contrast stimulus (inducer) is perceived as expanding away from the inducer position. This illusion provides an interesting test of spatiotopic motion because the neural correlates of this phenomenon have been found early in the visual cortex and the effect does not require focused attention. We measured the strength of LMI both with stable fixation and when participants were asked to perform a 10° saccade during the blank ISI between the inducer and the line. A strong motion illusion was found across saccades in spatiotopic coordinates. When the inducer was presented near in time to the saccade cue, saccadic latencies were longer, saccade amplitudes were shorter, and the strength of reported LMI was consistently reduced. We also measured the capacity of the trans-saccadic LMI by varying the number of inducers. In contrast to a visual-spatial memory task, we found that the LMI was largely eliminated by saccades when two or more inducers were displayed. Together, these results suggest that motion perceived in non-retinotopic coordinates depends on an active, saccade-dependent remapping process with a strictly limited capacity.