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Visual neglect and visual field deficits commonly co-occur after unilateral brain damage such as stroke. The conditions, however, are recognised as operationally and conceptually distinct.
A visual field defect describes sensory loss restricted to the visual field and arises from damage to the primary visual pathways linking optic tract and striate cortex. Patients with visual neglect fail to orient or attend predominately to contralesional space and this is thought to result from disorders involving various different attentional systems. Visual field assessment typically employs confrontation and requires fixation whereas visual neglect is normally assessed in free vision and typically equires a manual response.
Although, it is possible to confound visual field defects and visual neglect,1 it is generally accepted that the conditions represent functionally unrelated disorders that differ in terms of lesion location, eye movements, prognosis, and patterns of recovery. In addition, the conditions have been shown to double dissociate.2
Because both conditions ultimately involve impairments to visual processing, it is not clear how patients with visual field defects (but without visual neglect) perform on neglect tests such as line bisection. The answer was originally described in German clinical reports at the turn of the century.3 4 These reports, recently confirmed by Barton and Black5 found that right brain damaged patients with left visual field defects showed reliable contralesional displacement (to the left) on line bisection, whereas right brain damaged patients with neglect showed typical ipsilesional displacement (to the right).
Given these reports of opposing directional displacements, Ferber and Karnath6 (this issue, pp 572–8) hypothesised that the presence of a visual field defect in patients with visual neglect could act to reduce or even cancel the traditional rightward deviation found. Such a prediction if confirmed would constitute a novel form of functional facilitation not unlike that reported by Sprague7 in cats. The “Sprague effect” describes the striking behavioural finding in which the effects of a visual field deficit after a right hemispheric lesion are subsequently abolished by a second lesion in the left hemisphere. Clinical analogues of this effect are comparatively rare.8 Using a simple test that required right brain damaged patients to indicate verbally the subjective straight ahead (SSA) position, Ferber and Karnath convincingly show that the interaction of these two conditions in patients with circumscribed lesions can produce significantly reduced deviations when compared with patients with visual neglect alone. Unlike Sprague’s finding their results suggest that the interaction of two functionally unrelated but simultaneously present deficits within the same hemisphere act to neutralise opposing deviations on this task.
It would be misleading to conclude on the basis of these findings, however, that the effects of visual field defects in these patients had abolished neglect. The results of the clinical cancellation task in the same study clearly show that patients with visual neglect and hemianopia perform worse on the contralesional side of the page that those with neglect alone. Unfortunately the performance on line bisection is not reported. Therefore, whereas the effects of the visual field defects may reduce the directional orientation of neglect’s rightward bias on the subjective straight ahead (SSA), it does not generalise to more traditional clinical tasks and may depend on the nature and demands of the tasks involved. A previous study by Halliganet al,2 using six clinical tests of visuospatial neglect failed to find a significant difference between visual neglect patients with and without visual field defects. Furthermore, the same study did not find any clinical evidence of an opposing directional bias in patients with visual field defects only. Caution is also required in interpreting the findings as it not clear that displacement on the SSA is necessarily the result of visual neglect.8 Notwithstanding such reservations, the study provides a compelling and original illustration of how “paradoxical functional facilitation” can motivate theoretical insights into the behavioural effects of traditionally distinct disorders.9
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