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)