Grasping and Manual Estimation of the Muller-Lyer Illusion at Different Exposure Times

The two-stream model of visual processing describes a distinction between vision for ‘perception’ and vision for ‘action’. The inability of visual illusions (for example, the Ebbinghaus illusion) to affect grasping actions despite affecting manual estimation has been used as evidence in support of this model, but there remains disagreement about the phenomenon. Moreover, it is unclear how long it takes the brain to process visual illusory information for both grasping and perception. The present study explores these temporal dynamics in the context of the Muller-Lyer illusion. Participants were presented with rectangular objects surrounded by Muller-Lyer illusory wings on a screen for either 50 or 1500 ms, after which visual feedback was removed. In grasping conditions, participants proceeded to reach out and pick up the object, and in perception conditions participants used their thumb and index fingers to indicate the size of the object. Our data show that grasping (maximum grip aperture) was not affected by the Muller-Lyer illusion regardless of exposure duration but manual estimation was affected only after a long exposure duration. This suggests the existence of a threshold above 50ms at which the perceptual system begins to integrate stimulus information with surrounding contextual information. Additionally, the inability of the Muller-Lyer illusion to affect prehension and not estimation provides further evidence for the two-stream model of visual processing