Elsevier

Neuropsychologia

Volume 40, Issue 9, 2002, Pages 1577-1585
Neuropsychologia

Task-dependent differences in the exploratory behaviour of patients with spatial neglect

https://doi.org/10.1016/S0028-3932(02)00020-9Get rights and content

Abstract

The present study analysed task-dependent effects on the exploratory behaviour of neglect patients during their spontaneous search of the surroundings. We were asking whether different tasks would be associated with different structuring of the visual display and, therefore, would result in different forms of neglect in one and the same brain-damaged subjects. Neglect patients’ eye and head movements were recorded when they searched for a target within a homogeneous stimulus array surrounding the subjects. Subsequently, they explored the same array which was now segmented into different areas. When the patients’ attention was allocated to the whole surrounding space, all patients completely neglected the left hemispace and spontaneously attended to the right hemispace. No significant left–right asymmetry was detected in a selected segment located in the periphery of the attended, right hemispace. However, all patients completely ignored the left part of this segment when they had to concentrate visual search on this segment alone. The results suggest an important influence of task-dependent effects on the exploratory behaviour of neglect patients. They show that one and the same physical stimulus at one and the same location in a scene might be attended or, in another situation, neglected, just depending on the behavioural goal of the subject. The findings support the idea that the brain organises and reorganises continuously the representation of the same physical input according to the changing task requirements.

Introduction

Patients with spatial neglect show a disturbed ability to find target stimuli located on the contralesional side in the surroundings [2], [11], [21]. This spatio-behavioural deficit was assumed to be based on a disturbed cortical representation of the presented scenery (e.g. [3]). Given such a representation that guides behaviour, an economical implementation would be a flexible neural matrix that encodes stimuli depending on the task that the subject is currently dealing with. The notion that the brain uses context-dependent representations of the stimulus displays is supported by single-unit studies recording from the parietal and the temporal cortex in monkey. These studies found neurons that differentially responded to identical visual stimuli merely depending on the task-related significance of the stimuli [5], [8]. Consequently, if a disruption of such representations is the agent of neglect, one should expect that the behaviour in neglect patients is influenced by the respective task the subject is dealing with.

Indeed, it is known that various kinds of manipulations can modulate neglect behaviour. For example, cues directing the patients’ attention to the contralesional side can temporarily reduce the severity of neglect symptoms [6], [17]. However, it is not clear whether such cues have a direct impact on the supposed disturbed representation or influence the patients’ performance indirectly, e.g. via an additional compensation effect relying on mechanisms independent from the defect causing spatial neglect.

Marshall and Halligan [14] used explicit verbal instructions to bias a neglect patient’s figure-ground perception of ambiguous figures. Depending on these instructions, the patient either perceived or neglected a contour in the stimulus display. But this effect might be restricted to situations where ambiguous figure-ground perception supports switching between different ways to perceptually structure the visual image. However, it might not occur with less ambiguous stimulus arrays allowing perception to converge on one robust ‘interpretation’ of the visual scene. Explicit verbal task instructions requesting either to mentally rotate the stimulus material or to ignore the display’s orientation also have been shown to determine the spatial distribution of detected stimuli in neglect [4]. However, it might be that the cognitive processes exogeneously driven by such instructions do not adequately match the strategies a subject would employ when spontaneously performing the task.

The goal of the present study was to analyse task-dependent effects on the exploratory behaviour of neglect patients during their spontaneous search of the surroundings. We were asking whether different tasks would be associated with different structuring of the visual display and, therefore, would result in different forms of neglect in one and the same brain-damaged subjects. To test this hypothesis, we employed a visual search task in a random stimulus array and manipulated the subjects’ task by instructing them to either focus on the whole surrounding space or only on a certain part of it. During the tasks we recorded the subjects’ exploratory eye movements. Since eye movements and attentional mechanisms are closely related (e.g. [12], [13], [20]), this method provides a spatially fine grained mean to estimate the spatial pattern of salience distribution across space and also has been shown to reflect neglect related deficits in tests other than visual search [9].

Section snippets

Subjects

Exploratory gaze movements of three unselected, consecutively admitted patients with an acute right hemispheric lesion and severe neglect were recorded. Demographic and clinical data are given in Table 1; lesion location in Fig. 1. For control, we studied gaze movements in four neurological patients without brain lesions (one woman, three men, aged from 53 to 71 years, median=64 years). All subjects gave informed consent to participate in the study. None of the subjects had oculomotor palsies

Results

Fig. 3 illustrates the frequency distribution of horizontal gaze positions during the four conditions of the visual search task. During the ‘global’ and the ‘global segmented’ conditions, the control subjects showed a flat, symmetrical distribution of exploratory movements, covering a broad part of space up to about 140° on the left and the right side of the body’s mid-sagittal trunk plane with only a slight increase for the more central parts of the letter array. These data confirm previous

Discussion

The goal of the present study was to analyse task-dependent effects on the exploratory behaviour of neglect patients. Exploration was recorded when their attention was directed to the whole surrounding space and was compared with conditions when it was drawn to a certain, well-defined segment of space. As in previous studies [10], [11], the first condition revealed contralesional neglect with respect to the body in all patients. When they had to attend to the whole surrounding environment, the

Acknowledgements

This work was supported by grants from the Deutsche Forschungsgemeinschaft and the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie awarded to the first author.

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