Dark Side of the Moon: Studies in Unilateral Neglect

Abstract

This thesis is an investigation of persisting unilateral neglect (UN). Detailed individual profiles were developed from systematic neuropsychological examination, and used to establish subgroups with specific difficulties that could be linked to lesion sites, and to address theoretical issues in UN, such as: (l) whether there is a gradient of attention; (2) the role of the parietal lobe in disengaging attention; (3) the nature of hemispheric differences. A precueing paradigm (Egly, Rafal, et al., 1994) was used to compare Kinsbourne's gradient of attention with Heilman's hemispace theory of UN. Three UN participants provided support for the gradient theory when tested with valid cues, demonstrating that an inferior-rostral-parietal-lobe lesion leads to an ipsilesional drift of attention. All UN participants had problems with disengaging and shifting attention contralesionally following invalid cues, but two participants did not have the commonly implicated parietal-lobe lesion (Posner et al., 1982). In this condition, both theories were supported because three participants demonstrated the disengage deficit only in the contralesional hemispace, and two demonstrated this deficit in the contralesional position within each hemispace. Despite difficulties with contralesional stimuli, two UN participants processed stimuli in the lateral position of the right visual field normally, suggesting that persisting UN following a right-hemispheric lesion is not simply a matter of reduced ability to sustain attention (Robertson, 1993). The neuropsychological examination revealed that Map Search (TEA) was the single best predictor of persisting UN, and the memory conditions of the Rey and the Global/Local Figures indicated global processing difficulties. To test the role of global cues (Halligan & Marshall, 1994), seven participants with right-hemispheric lesions identified and copied drawings with varying levels of meaningful information presented in the ipsilesional hemispace (Seron et al., 1989). The results showed that the less meaningful the information, the less the neglect, so that when ipsilesional cues induce a voluntary contralesional shift of attention, there follows explicit reporting of contralesional information. All the participants with right-hemispheric lesions had difficulties with shifting attention between objects, which does not support Egly's (1994) theory that the brain mechanism for shifting attention between objects is located exclusively in the left hemisphere. This thesis is dedicated with love to my favourite niece, Natalie Melissa Victoria Aimola.