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 forefinger 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.

According 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.

Deep brain stimulation (DBS) of the globus pallidus pars interna (GPi) is an effective treatment for generalized dystonia. Its role in the management of other types of dystonia is uncertain. Therefore we performed a prospective, single-blind, multicentre study assessing the efficacy and safety of bilateral GPi-DBS in 10 patients with severe, chronic, medication-resistant cervical dystonia. Two blinded neurologists assessed patients before surgery and at 6 and 12 months post-operatively using the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS). The primary outcome measure was the severity subscore (range 0-30, higher scores indicating greater impairment). Secondary outcomes included disability (0 to 30), pain (0 to 40) subscores and total scores of the TWSTRS, Short Form-36 and Beck depression inventory. Swallowing and neuropsychological assessment were also performed at baseline and 12 months. One-way repeated measures analysis of variance was used to analyse the data. The TWSTRS severity score improved from a mean (SD) of 14.7 (4.2) before surgery to 8.4 (4.4) at 12 months post-operatively (P = 0.003). The disability and pain scores improved from 14.9 (3.8) and 26.6 (3.6) before surgery, to 5.4 (7.0) and 9.2 (13.1) at 12 months, respectively (both P < 0.001). General health and physical functioning as well as depression scores improved significantly. Complications were mild and reversible in four patients. Some changes in neuropsychological tests were observed, although these did not impact daily life or employment. Our results support the efficacy and safety of GPi-DBS for the treatment of patients with severe and prolonged cervical dystonia who have failed medical management.

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