Visuomotor 'immunity' to perceptual illusion: a mismatch of attentional demands cannot explain the perception-action dissociation.
Recent findings of visuomotor immunity to perceptual illusions have been attributed to a perception-action division of labour within two 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 Müller-Lyer display would remain immune to the illusion when the task required the direction of attention or a related resource towards both Müller-Lyer shafts. Twelve participants were required to match and grasp two Müller-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 Müller-Lyer illusions merely represents an experimental artefact.
Dewar
Michaela T
MT
Vision Research Laboratories, School of Psychology, The University of Aberdeen, Old Aberdeen, Scotland AB24 2UB, United Kingdom.
Carey
David P
DP
D000328:Adult; D000704:Analysis of Variance; D001288:Attention / Q000502:physiology*; D005260:Female; D018737:Hand Strength / Q000502:physiology; D006801:Humans; D008297:Male; D009903:Optical Illusions / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology; D010775:Photic Stimulation / Q000379:methods; D011597:Psychomotor Performance; D013028:Space Perception / Q000502:physiology