title: Rapid decrement in the effects of the Ponzo display dissociates action and perception.
abstract: It has been demonstrated that pictorial illusions have a smaller influence on grasping than they do on perceptual judgments. Yet to date this work has not considered the reduced influence of an illusion as it is measured repeatedly. Here we studied this decrement in the context of a Ponzo illusion to further characterize the dissociation between vision for perception and for action. Participants first manually estimated the lengths of single targets in a Ponzo display with their thumb and index finger, then actually grasped these targets in another series of trials, and then manually estimated the target lengths again in a final set of trials. The results showed that although the perceptual estimates and grasp apertures were equally sensitive to real differences in target length on the initial trials, only the perceptual estimates remained biased by the illusion over repeated measurements. In contrast, the illusion's effect on the grasps decreased rapidly, vanishing entirely after only a few trials. Interestingly, a closer examination of the grasp data revealed that this initial effect was driven largely by undersizing the grip aperture for the display configuration in which the target was positioned between the diverging background lines (i.e., when the targets appeared to be shorter than they really were). This asymmetry between grasping apparently shorter and longer targets suggests that the sensorimotor system may initially treat the edges of the configuration as obstacles to be avoided. This finding highlights the sensorimotor system's ability to rapidly update motor programs through error feedback, manifesting as an immunity to the effects of illusion displays even after only a few trials.
authors.affiliation: Department of Psychology, University of British Columbia, Vancouver, British Columbia, Canada. rwhitwel@gmail.com.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Whitwell
authors.affiliation: Department of Psychology, Heriot-Watt University, Edinburgh, Scotland, UK.
authors.forename: Gavin
authors.initials: G
authors.lastname: Buckingham
authors.affiliation: Department of Psychology, University of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: James T
authors.initials: JT
authors.lastname: Enns
authors.affiliation: School of Psychology and Public Health, La Trobe University, Melbourne, Victoria, Australia.|Department of Psychology, University of Western Ontario, London, Ontario, Canada.
authors.forename: Philippe A
authors.initials: PA
authors.lastname: Chouinard
authors.affiliation: Department of Psychology, University of Western Ontario, London, Ontario, Canada.|Department of Physiology and Pharmacology/Brain and Mind Institute, University of Western Ontario, London, Ontario, Canada.|The Brain and Mind Institute, The University of Western Ontario, London, Ontario, Canada.
authors.forename: Melvyn A
authors.initials: MA
authors.lastname: Goodale
mesh_terms: D000293:Adolescent; D005260:Female; D005385:Fingers; D006801:Humans; D007088:Illusions; D007600:Judgment; D008297:Male; D012858:Size Perception; D014796:Visual Perception; D055815:Young Adult
keywords: Action; Error minimization; Grasping; Haptic feedback; Illusion decrement; Motor learning; Perception; Ponzo illusion; Sensorimotor; Two-visual-systems hypothesis; Visual feedback; Visuomotor