Dual Processes Underlie the Effect of the Ebbinghaüs Illusion on Control of Grasping

Previous studies have shown that control of grasping is affected by the Ebbinghaüs illusion. However, there is debate about whether effects on grasping are solely due to the illusion or involve other processes. The aim of this study was to distinguish the influences of the size illusion and obstacle avoidance on control of grip aperture. We compared size perception and grip aperture during grasping for targets in Ebbinghaüs contexts that varied in the size, distance and density of flankers. The size illusion is affected by all of these flanker parameters, while effects due to obstacle avoidance would depend primarily on flanker-target distance. We found that flanker size had consistent effects on perceptual estimation and grip control during grasping: larger flankers caused the target to appear smaller, and the maximum grip aperture (MGA) during grasping was reduced. However, the effects of flanker-target distance were more complicated. Increasing flanker-target distance generally caused the target to appear smaller but this effect became weaker with sparse flankers. For grip control, the flanker-target distance effect had opposite directions in two flanker density conditions: Increasing flanker-target distance caused MGA to decrease with dense flankers and to increase with sparse flankers. These findings can be explained by a combination of influences from size illusion and obstacle avoidance on grasping. Our results do not support that visuomotor control is immune to visual illusions, such as the Ebbinghaüs illusion. Apparent discrepancies between perception and visuomotor control with visual illusions could be explained by additional influence of obstacle avoidance mechanisms.