Differences in the visual control of pantomimed and natural grasping movements

Visual illusion and action

The role of allocentric cues on movement control was investigated in the present study. Pointing movements directed to the more distant vertex of closed and open configurations of the Muller-Lyer illusion, as well as to the vertex of control lines, were studied in four experimental conditions. In the first (full-vision condition) subjects saw both stimulus and their hand before and during movement, in the second (non-visual feedback condition) they saw the stimulus, but not their hand during movement. In the two remaining conditions (no-vision conditions) vision of the scene and the hand was precluded. Pointing was executed 0 sec (no vision 0 sec delay condition) or 5 sec (no-vision 5 sec delay condition) after the light was switched off. The Muller-Lyer illusion affected pointing kinematics with respect to the control lines. Subjects undershot and overshot the vertex location, respectively, of the closed and open configuration. Correspondingly, the entire kinematics were changed. The main result was, however, a gradually increasing effect of the perceptual illusion when pointing was executed from memory compared to the full-vision condition. These data are discussed according to the hypothesis that the system underlying visual perception in the allocentric frame of reference and that involved in motor action can functionally interact. The strength of this interaction depends upon the efficiency of the egocentric frame of reference by which motor actions are constructed.

Preserved visual imagery in visual form agnosia

We investigated the ability of a patient (D.F.) with profound visual form agnosia to perform a variety of tasks requiring visual imagery. Despite her inability to discriminate between objects and patterns of different shapes, sizes, and orientations, D.F. showed quite normal visual imagery involving these same 'visual' properties when the images were drawn from long-term memory. Thus, she was able both to scan mental images in search of particular features and to form new images by combining several known images. While there is growing evidence that perception and imagery share common neural substrates, the fact that D.F. shows intact visual imagery in the face of a massive perceptual deficit in form vision challenges recent suggestions that these two psychological processes share common input pathways in early vision. It is suggested that regions in the occipitotemporal pathway may be important for the generation of visual images while regions in the posterior parietal system might be involved in the manipulation of these images.

The internal control of action and Parkinson's disease: a kinematic analysis of visually-guided and memory-guided prehension movements

This paper reports two experiments which examined the effects of Parkinson's disease (PD) upon the sensorimotor mechanisms used to control prehension movements. Transport and grasp kinematics for visually-guided and memory-guided prehension movements were examined in healthy control subjects and compared against those of patients with idiopathic PD. Two research questions were addressed: (1) Are patients with PD particularly susceptible to distraction by non-relevant objects? (2) Are patients with PD especially reliant on external feedback when executing goal-directed actions? The results indicated that the patient group were no more susceptible to distraction by non-relevant objects than the control group. In contrast, the patients with PD were shown to be significantly impaired when executing memory-guided reaches. Furthermore, the deficits exhibited by the PD group on memory-guided reaches were confined solely to those markers associated with the transport component of the prehension movement. That is, while both controls and patients with PD widened their grip aperture on memory-guided trials, the magnitude of this adjustment was comparable across the two groups. The implications of these findings for theories of visuomotor processing in sufferers of PD and the control of prehension movements more generally are discussed.