Behavioral effects of subthalamic deep brain stimulation in Parkinson's disease.

To date, few studies have utilized standardized measures to assess the neurobehavioral changes that can accompany deep brain stimulation (DBS) of the subthalamic nuclei (STN) for the treatment of Parkinson's disease (PD), yet behavioral changes are the most debated among practitioners. We evaluated behavior with the Frontal Systems Behavior Scale (FrSBe), which includes a large-scale normative sample for self- and collateral ratings and is particularly relevant to PD with subscales assessing Apathy, Disinhibition, and Executive Dysfunction. Data were collected from 16 (11 males) PD patients. All FrSBe subscale scores increased significantly when retrospective preoperative scores and current (postoperative) scores were compared. Self- and collateral FrSBe ratings were not significantly correlated with each other, though for both scores at least half of the group met criteria for a clinically significant level of symptoms postoperatively. No significant correlations were seen for collateral current FrSBE ratings with cognitive or motor variables. Higher self-ratings of behavior characteristic of apathy were related to higher self-ratings of depressive symptoms, and to a smaller decrease in antiparkinsonian medications following surgery. We propose that the standardized assessment of behavioral aspects of executive dysfunction adds information that is largely dissociable from the motor and cognitive assessment of function in PD patients undergoing STN DBS. In future, prospective standardized measurement of behavior may allow for better prediction of which patients will experience significant behavioral issues postoperatively.

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

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.

Independent effects of pictorial displays on perception and action.

Although pictorial illusions have been used to study perception for a long time, the effects of such displays on the visual control of actions has recently been the matter of some debate. Evidence from a re-analysis of an earlier study is presented that suggests pictorial displays can exert opposite effects on perceptual size judgements and grip scaling, perhaps because the two-dimensional elements surrounding the target for a grasp are treated as potential obstacles. This interpretation was supported by the results of an experiment in which the relative position and distance of two-dimensional elements flanking a target had differential effects on perceptual judgements of size and the scaling of grip aperture.

The effect of learned perceptual associations on visuomotor programming varies with kinematic demands.

The present set of experiments investigated the possibility that learned perceptual information can, under certain circumstances, be utilized by visuomotor programming. In Experiment 1 (N = 28), an association was established between the color and size of square wooden blocks (e.g., red = large; yellow = small, or vice-versa). In Experiment 2 (N = 28), an association was established between the shape and size of plastic objects (e.g., hexagon = large; circle = small, or vice-versa). It was expected that the learned associations would change the perceived size of two probe objects halfway in size between the large and small objects (the probe object matched by color or shape to the large group of objects would appear smaller than the probe object matched to the small group of objects as a result of within-group relative size comparisons). In both experiments, half of the participants grasped the target objects, and the other half estimated the size of the objects by opening their thumb and finger a matching amount. For Experiment 1, it was predicted that an influence of the learned association on the treatment of the probe objects would be seen in manual estimations and in grip scaling because the kinematics of the grasping movement were very similar across trials. As predicted, the learned association between size and color was as easily incorporated into visually guided grasping as it was into visual perceptions. In Experiment 2, it was predicted that an influence of the learned perceptual association would be seen only in manual estimations, and not in grip scaling, because the variability in target object shape from trial to trial would demand changes in precontact finger posture across trials. Despite the significant effect of the size-shape association on size estimations, no influence was seen in preparatory grip scaling, probably because varying shape increased the metrical demands on visuomotor programming from those in Experiment 1. Together, the results suggest that visuomotor programming can make use of learned size information under some, but not all, conditions.

Does a monocularly presented size-contrast illusion influence grip aperture?

The present study tested the idea that if subjects rely more on scene-based pictorial cues when binocular cues are not available, then both their perceptual judgements and their grasp might be influenced by pictorial illusions such as the Ebbinghaus (Titchener) Circles Illusion under monocular viewing conditions. Under binocular viewing conditions, subjects were always able to scale their grip accurately to the true size of the target disc and were unaffected by the illusion. Under monocular viewing, however, subjects appeared to be influenced by the illusion. Thus, when confronted with physically different target discs displayed on backgrounds that made them appear equivalent in size, subjects treated the two discs as equivalent--even when picking them up. These results, combined with earlier work from our laboratory suggests that binocular information plays a critical role in normal human prehension but when this information is not available the visuomotor system is able to "fall back" on the remaining monocular cues, which can cause the visuomotor system to be more susceptible to pictorial illusions.

The effect of pictorial illusion on prehension and perception.

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.