Hongyi Guo¹, Alexander Schütz², Robert Allison¹; ¹York University, ²Marburg University

The motion of targets needs to be taken into account for saccades to land accurately on the target. In an optic flow field, a saccade directed toward a briefly presented stationary target is biased in the direction of radial and rotational optic flow (OF), an effect called assimilation of optic flow (Guo, Schütz & Allison, 2024). We observed that this assimilation disappeared when we tested saccade adaptation to intra-saccadic steps. At least two possible causes were proposed: (1) Saccade adaptation diminished the influence of optic flow, and (2) The visible target after the saccade provided a stationary prior. Two experiments with OF were conducted to test these explanations. Experiment 1: subjects performed saccades to a target that remained visible after the saccade. Experiment 2: subjects performed saccades to a target moving at different speeds left or right, or a stationary target and reported the perceived motion direction of the target. In experiment 1, saccades were smaller while viewing inward OF than no OF; conversely, they were larger with outward OF than no OF. Thus, we replicated the assimilation effect, now when the target remained on screen after the saccade. This means the lack of assimilation during intra-saccadic step adaptation cannot be explained by postsaccadic information about the target. Preliminary results of experiment 2 showed that the perceived motion direction of the stationary target was repelled from the optic flow direction, consistent with the well-known induced motion illusion. Surprisingly, saccade amplitudes were still assimilated in the direction of radial optic flow. Our results show a dissociation in motion perception and eye-movement behaviour, which might indicate separate channels or different reference frames for processing optic flow for perception and behaviour.

Acknowledgements: This study was supported by the IRTG The Brain in Action program, funded by NSERC and DFG.