Jye Marchant¹ (), Jiahan Hui, Alex O. Holcombe¹; ¹The University of Sydney

Evidence from individual differences suggests that different motion-position illusions are mediated by distinct mechanisms (Cottier et al., 2023). To probe these apparent processing differences, we have been investigating directional asymmetries. The flash-lag effect, where a flashed object is perceived to lag behind a moving one despite spatiotemporal alignment, is larger for motion toward the fovea compared to away (Kanai et al., 2004; Mateeff & Hohnsbein, 1988; Shi & Nijhawan, 2008). Here we investigated the flash-grab effect (FGE), in which the perceived position of a flash coinciding with the reversal of a moving background is displaced in the direction of post-flash motion. Our study used post-flash motion either toward or away from the fovea. Toward-fovea post-flash motion produced a robust displacement toward fixation, consistent with the classic FGE. Unexpectedly, away-from-fovea post-flash motion also resulted in a toward-fixation shift (a negative FGE), although the effect was small. As a control for general flash mislocalization bias, we also included a condition with no background motion; here too, flashes were mislocalized toward fixation, and after subtracting this static bias from the motion conditions the difference between toward- and away-from-fovea motion in the FGE was eliminated. This suggests a dissociation with the flash-lag effect, as Shi & Nijhawan (2008) found that flash mislocalisation cannot fully account for the flash-lag effect’s directional asymmetries. This difference may reflect processes that contribute to the flash-lag effect but not the FGE. Unlike the flash-lag effect, the FGE does not require temporally binding the flash to a specific position in the trajectory of the moving object, so the FGE may be better for isolating the effect of motion on position perception.