The Effect of Pictorial Illusion on Prehension and Perception’,

Abstract The present study examined the effect of a size-contrast illusion (Ebbinghaus or Titchener Circles Illusion) on visual perception and the visual control of grasping movements. Seventeen right-handed participants picked up and, on other trials, estimated the size of "poker-chip" disks, which functioned as the target circles in a three-dimensional version of the illusion. In the estimation condition, subjects indicated how big they thought the target was by separating their thumb and fore nger to match the target's size. After initial viewing, no visual feedback from the hand or the target was available. Scaling of grip aperture was found to be strongly correlated with the physical size of the disks, while manual estimations of disk size were biased in the direction of the illusion. Evidently, grip aperture is calibrated to the true size of an object, even when perception of object size is distorted by a pictorial illusion, a result that is consistent with recent suggestions that visually guided prehension and visual perception are mediated by separate visual pathways

Attention and grasping in Parkinson's disease: Effects of treatment and disease stages

Objective: To compare the attentional resources devoted to reach-to-grasp movements at different stages of Parkinson’s disease (PD), on and off treatment, relative to intact controls. Background: Reach-to-grasp movements are a critical component of activities of daily living. The grasping movements of patients with PD are characterized by a reliance on cues and visual feedback, not seen in the very automatic movements of intact controls (Azulay et al, 2006). PD patients appear to devote significant attentional resources to their movements; dual-task performance is a well-known area of difficulty for PD patients. To date, the role of attention in everyday reach-to-grasp movements has not been examined, though the implications for performance of activities of daily living are clearly considerable. Methods: The performance of three patient groups [de novo (n=11); moderate (n=24); and surgical (STN DBS; n=14)] was compared with age matched controls (n=24) on a dual task paradigm involving grasping and a concurrent cognitive task designed to consume attention. Attentional resources were measured with standardized tests. Participants reached to grasp functional objects with handles (e.g. a comb). Handles were turned away from participants; an appropriate grasp was scored if they successfully picked up the object by the handle. Between and within group comparisons were made using ANOVA and t-tests. Results: The control group and the de novo group not yet on medication displayed strong evidence of automaticity in their movements; when performing a challenging spatial imagery task, they made the same number of appropriate grasps as when they picked up the objects without a concurrent task. In contrast, for the moderate and the surgical groups, performing the concurrent task resulted in fewer appropriate grasps, and there was no improvement while on treatment (medication and stimulation, respectively). Only the surgical group showed decreased divided attention on standardized measures (>-0.7 SD below their normative group). Conclusions: Grasping appears to shift from an automatic to an attention demanding process by the moderate stages of PD. This may be a coping mechanism designed to safely adapt to reduced motor function, or it may reflect pathology in the mechanisms underlying grasping movements. References: Azulay JP, Mesure S, and Blin O. 2006. J Neurol Sci, 248(1-2): 192-195

The dissociation between perception and action in the Ebbinghaus illusion Nonillusory effects of pictorial cues on grasp

AbstractAccording to a recently proposed distinction [1] between vision for perception and vision for action, visually guided movements should be largely immune to the perceptually compelling changes in size produced by pictorial illusions. Tests of this prediction that use the Ebbinghaus illusion have revealed only small effects of the illusion on grasp scaling as compared to its effect on perception [2–4]. Nevertheless, some have argued that the small effect on grasp implies that there is a single representation of size for both perception and action [5]. Recent findings, however, suggest that the 2-D pictorial elements, such as those comprising illusory backgrounds, can sometimes be treated as obstacles and thereby influence the programming of grasp [6]. The arrangement of the 2-D elements commonly used in previous studies examining the Ebbinghaus illusion could therefore give rise to an effect on grasp scaling that is independent of its effect on perceptual judgements, even though the two effects are in the same direction. We present evidence demonstrating that when the gap between the target and the illusion-making elements in the Ebbinghaus illusion is equidistant across different perceptual conditions (Figure 1a), the apparent effect of the illusion on grasp scaling is eliminated