Mechanisms of unilateral spatial neglect in copying a single object

doi:10.1016/0028-3932(96)00017-6

Neuropsychologia

Volume 34, Issue 10, October 1996, Pages 965-971

https://doi.org/10.1016/0028-3932(96)00017-6 Get rights and content

Abstract

The present study examined 13 patients with typical left unilateral spatial neglect to explore the mechanisms of neglect in copying a single object. All patients identified the figure of a sunflower as a flower that had petals all around the central circle. Besides, in the free viewing of the flowers prepared by the examiner, 10 of the 13 patients could discriminate between the presence and absence of the left-sided petals. When copying the flower, however, they always left the left side unfinished without noticing the incompleteness. Whether perception in the left visual field was impaired or not was irrelevant to the performances in the copying and the free viewing of the flowers. We consider the mechanism of left unilateral spatial neglect in copying a flower as follows. Small leftward shift of attention may take place during the free observation of the stimulus flower. While copying, however, neglect patients pay attention exclusively to the right-sided petals they are drawing. The strong engagement of attention to the right side results in the lack of discovery of the left-sided omission. The close integration of attentional and premotor mechanisms seems to be crucial for the appearance of unilateral spatial neglect in copying a single object. Copyright © 1996 Elsevier Science Ltd

Introduction

Copying of a figure that involves a single object or multiple objects has been used for a sensitive measure of unilateral spatial neglect 4, 15, 28. A flower like a daisy is one of the most popular stimulus objects used for copying. Patients with left unilateral spatial neglect typically copy the right half of the flower and leave the left side unfinished. Gainotti et al.[8] reported that inability to extract visual information from one side of the stimuli during single eye fixations may be the most characteristic feature of unilateral spatial neglect due to right hemisphere lesions. In patients who show this type of neglect, exploration of the left space may sometimes be preserved. In the copying test, they may reproduce parts of the model placed even in the left space but leave unfinished the left side of various elements. Seki and Ishiai [34] studied neglect patients focusing on the copying pattern in which the left-sided object was copied but the left side of the central flower was typically neglected. This type of ‘object-centred’ neglect 6, 23 in copying was frequently observed in neglect patients who could explore into the left space in the line cancellation test.

The present study aimed to explore the mechanisms of unilateral spatial neglect in copying a single object. We used a figure of a flower as the stimulus for copying. In the appearance of neglect, there seems to be at least two hypothetical levels of visuo-spatial processes in the copying of the flower. First, neglect patients may identify the flower as a flower with petals only on the right side, failing to perceive the left side, and draw the right half as identified. Second, neglect patients recognize the stimulus correctly as a flower with petals all around the central circle, but neglect of the left half occurs in the drawing process. In the latter case, combination of the attentional 19, 22, 29, 30 and intentional 2, 14 mechanisms may explain the left-sided omission in copying [26]. The premotor theory of attention proposed by Rizzolatti and Gallese [31] may be more appropriate, as the action of copying and the direction of attention seems to be closely related to each other.

We examined neglect patients who showed typical neglect in copying the figure of a flower. In an attempt to know their identification of the flower, we gave them two tasks. First, the patients were asked what the stimulus looked like before each copying trial. Second, they were tested the ability to discriminate between the presence and absence of the left petals when presented the flowers of the same size in free viewing. The figure of the flower may be recognized in a single eye fixation [8]. We therefore gave the patients another task to discriminate between the presence and absence of the left petals when the flowers were presented tachistoscopically at the fixation point. The responses to the left part of the flower falling in the left visual field were studied in relation to the performances in the copying and the perception of the flower in free viewing. Central field of vision was also examined to determine if the patients had impaired perception in the area that encompassed the whole flower.

Section snippets

Subjects

The subjects were 13 right-handed patients with a right hemisphere stroke who showed obvious left unilateral spatial neglect in the copying of a sunflower-like flower (Fig. 1A). They were also evaluated with a screening battery including the line bisection 16, 33 and line cancellation 1, 17 tests, as well as the copying of a daisy with blades of grass 6 cm away from it on both sides [34]. In these three tests, their neglect was scored as 0 (absent), 1 (mild), 2 (moderate) or 3 (severe)

Copying of flower

Before copying, all subjects identified the stimulus figure as a flower that had petals all around the central circle, such as a sunflower, a chrysanthemum, a dahlia and a dandelion. They drew the right side satisfactorily, starting with the right-sided petals, while they always showed neglect on the left side. At the end of each trial, none of them noticed the omission of the left petals. Unilateral spatial neglect was rated 3 (severe) if the left-sided four petals were all omitted; neglect

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In a copying task the model is given by the examiner, and omissions may derive from a faulty perceptual appreciation of its figural components. However, such an explanation cannot account for a range of phenomena; for example, in a study by Ishiai et al. (1996) neglect patients were able to detect the absence of left-sided leaves on a sunflower, and yet failed to draw them in a subsequent copying task. These findings are not consistent with the presence of a visuoperceptual defect and might suggest an interaction between attentional mechanisms and the process of drawing.
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Left visual neglect following right hemisphere damage is a heterogeneous phenomenon, in which several underlying impairments have been identified. Despite recent advances in understanding the neural and cognitive bases of these impairments, current theories of neglect, particularly those that emphasise attentional deficits, do not explain a number of phenomena, including:
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Mechanisms of unilateral spatial neglect in copying a single object

Abstract

Introduction

Section snippets

Subjects

Copying of flower

Cortex

Neuropsychologia

Neuropsychologia

Neuropsychologia

Cortex

Neuropsychologia

Neuropsychologia

Neuropsychologia

Neuropsychologia

A simple test of visual neglect

Neurology

Perceptual and premotor factors of unilateral neglect

Neurology

Cross-over, completion and confabulation in unilateral spatial neglect

Brain

Can visual neglect operate in object-centred co-ordinates? An affirmative single-case study

Cognitive Neuropsychology

Early orientation of attention toward the half space ipsilateral to the lesion in patients with unilateral brain damage

Journal of Neurology, Neurosurgery, and Psychiatry

Mechanisms of unilateral spatial neglect in relation to laterality of cerebral lesions

Brain

Toward a principled explanation of unilateral neglect

Cognitive Neuropsychology

Do visual field deficits exacerbate visuo-spatial neglect?

Journal of Neurology, Neurosurgery, and Psychiatry