Prehension and Perception of Size in Left Visual Neglect

Behavioural Neurology, Sep 2018

Right hemisphere damaged patients with and without left visual neglect, and age-matched controls had objects of various sizes presented within left or right body hemispace. Subjects were asked to estimate the objects’ sizes or to reach out and grasp them, in order to assess visual size processing in perceptual-experiential and action-based contexts respectively. No impairments of size processing were detected in the prehension performance of the neglect patients but a generalised slowing of movement was observed, associated with an extended deceleration phase. Additionally both patient groups reached maximum grip aperture relatively later in the movement than did controls. For the estimation task it was predicted that the left visual neglect group would systematically underestimate the sizes of objects presented within left hemispace but no such abnormalities were observed. Possible reasons for this unexpected null finding are discussed.

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Prehension and Perception of Size in Left Visual Neglect

0953-4180 Prehension and perception of size in left visual neglect R.D. McIntosh 0 C.L. Pritchard 1 H.C. Dijkerman 2 A.D. Milner 0 R.C. Roberts 3 0 Department of Psychology, University of Durham , Uk 1 School of Psychology, University of St. Andrews , Fife , UK 2 Department of Psychology, University of Utrecht , Netherlands 3 Department of Medicine, University of Dundee , UK Right hemisphere damaged patients with and without left visual neglect, and age-matched controls had objects of various sizes presented within left or right body hemispace. Subjects were asked to estimate the objects' sizes or to reach out and grasp them, in order to assess visual size processing in perceptual-experiential and action-based contexts respectively. No impairments of size processing were detected in the prehension performance of the neglect patients but a generalised slowing of movement was observed, associated with an extended deceleration phase. Additionally both patient groups reached maximum grip aperture relatively later in the movement than did controls. For the estimation task it was predicted that the left visual neglect group would systematically underestimate the sizes of objects presented within left hemispace but no such abnormalities were observed. Possible reasons for this unexpected null finding are discussed. Visual neglect; hemimicropsia; perception; prehension; size distortion 1. Introduction There is now abundant evidence that the rightward line bisection errors of left visual neglect are, at least in part, attributable to a distorted perception of horizontal extent. Specifically, stimuli presented in relatively leftward egocentric locations may be perceived as smaller than identical stimuli presented in relatively rightward locations. The methods developed to study this phenomenon include the ?landmark task? [ 28 ] and various psychophysical size matching tasks. In the former, the subject is presented with a transected line and asked to point to the end of the line that lies closer to the transection mark. Critical trials occur when the transection mark actually bisects the line but a forced-choice response is required. On such trials most normal subjects respond randomly left or right but the majority of left neglect patients point leftward, indicating that they perceive the left half of the line as shorter than its rightward counterpart [ 6,7,27,28 ]. Size matching tasks follow a similar logic. Subjects are presented with horizontally aligned pairs of stimuli and asked to make relative size discriminations. Left neglect patients systematically underestimate the extent of stimuli presented in left hemispace relative to those presented on the right and this occurs whether the stimuli are horizontal lines, rectangles, circles or nonsense shapes (although less distortion is generally observed for vertically or radially oriented lines or rectangles) [ 10,26,29 ]. It has been suggested that distortions of perceived size associated with neglect reflect damage to neural systems concerned with the conscious representation and analysis of visual scenes, more closely linked with the ventral stream of visual processing than with the dorsal stream [ 23,24,26,29 ]. This theory predicts that, whilst these distortions should affect explicit size judgements, goal-directed visuomotor acts should be relatively unperturbed as these latter responses are primarily subserved by separate mechanisms within the dorsal stream [25]. This prediction has some empirical support. For instance, Robertson et al. [ 33 ] found that the rightward errors made by left neglect patients when pointing to the centre of a horizontal rod were substantially reduced if the instruction was simply to pick the rod up. The fact that a pointing response used to indicate an explicit spatial judgement revealed greater neglect than a more automatic prehensile act is consistent with the notion that the latter accessed a stream of visual processing relatively unaffected by size distortion. A similar dissociation between perceptual experience and visuomotor guidance was reported by Pritchard et al. [ 31 ]. A neglect patient (EC) was asked to provide manual estimates of the size of objects presented within her left or right body hemispace (by matching her index-finger-thumb separation to the size of the targets) and to reach out and grasp the same objects. EC systematically underestimated the size of objects presented on the left relative to those presented on the right. Despite this misperception she was able to reach out and grasp the same objects with ease, showing normal scaling of grip aperture on both sides of space. EC?s behaviour is consistent with the hypothesis that distortions of perceived size in neglect are independent of the mechanisms underlying visuomotor control. However, in order to substantiate this hypothesis it is important that EC?s dissociated pattern of performance should generalise more widely across neglect patient (...truncated)


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R. D. McIntosh, C. L. Pritchard, H. C. Dijkerman, A. D. Milner, R. C. Roberts. Prehension and Perception of Size in Left Visual Neglect, Behavioural Neurology, 13, DOI: 10.1155/2002/252405