Visuomotor 'immunity' to perceptual illusion: A mismatch of attentional demands cannot explain the perception-action dissociation

Recent findings of visuomotor immunity division of labour within two to perceptual illusions have been attributed to a perception-action anatomically segregated streams in the Visual cortex. However, critics argue that such experimental findings are not valid and have Suggested that the perception-action dissociations can be explained away by differential attentional/processing demands, rather than a functional dissociation in the neurologically intact brain: perceptual tasks require processing of the entire illusion display while visuomotor tasks only require processing, the target that is acted upon. The present study examined whether grasping of the Muller-Lyer display would remain immune to the illusion when the task required the direction of attention or a related resource towards both Muller-Lyer shafts. Twelve participants were required to match and gasp two Muller-Lyer shafts bimanually (i.e. one with each hand). It was found that bimanual grasping was not significantly affected by the illusion, while there was a highly significant illusion effect on perceptual estimation by matching. Furthermore, it was established that this dissociation did not result from a differing baseline rate of change in manual estimation and grasping aperture to a change in physical object size. These findings provide further Support for the Postulated perception-action dissociation and fail to uphold the idea that grasping 'immunity' to the Muller-Lyer illusions merely represents an experimental artefact. (c) 2005 Elsevier Ltd. All rights reserved.