Brain, Vol. 118, No. 6, 1421-1435, 1995
© 1995 Guarantors of Brain
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Asymmetries in the covert orienting of visual spatial attention to spatial and non-spatial cues in Alzheimer's disease
1The Neurophysiology and Neurovisual Research Unit, Alzheimer's Disease Clinical Research Unit, Mental health Research Institute of Victoria Parkville, Victoria 2Brain Research Unit, Drug and Alcohol Services, Westmead Hospital Westmead, New South Wsles, Australia
Correspondence to:
Correspondence to: Paul Maruff, Neurophysiology and Neurovisual Research Unit, Mental Health Research Institute of Victoria, Parkville, Victoria 3052, Australia
The ability to direct covert visual spatial attention to the left (LVF0 and right visual field (RVF) was examined in 15 patients with mild to moderate Alzheimer's disease and 15 age- and education-matched controls using the covert orienting of visual spatial attention task (COVAT) modified to include both spatial and non-spatial cues. Subjects responded with a button press when they detected a target at a location 8° to either the left or right of fixation. On 70% of trials a spatial cue was flashed at the target location before the target appeared.On 15% of trials the sparial cue was flashed at the target lacation contralateral to where it would appear and on the remaining 15% of trials non-apatial diffuse cue preceded targets.The cue to target interval (CTI) varied between 150 and 550 ms. Mean reaction times for each cyetype in the RVF and LVF were calculated. Compared with controls the percentage of trials excluded because of very slow reaction times was significantly greater in the Alzheimer's disease group for the 550 ms CTI. Analysis of the symmetry of reaction times to LVF and RVF targets for the 150 ms CTI enabled us to classify alzheimer's disease subjects into three subgroups based on the hemifield of abnormally slow attentional biases.The first subgroup showed a significant slowing of reaction time to all LVF tergets, the second showed a significant slowing of reaction time to all RVF targets and the third showed a significant slowing of reaction time to both LVF and RVF targets.Patients with alzheimer's desease who showed an abnormal attentional bias performed significantly better on neuropsychological tests of memory, language and executive function than Alzheimer's disease patients with no attentional bias.Eight of the Alzheimer's disease subjects were assessed serially on at least six occasions over a 12-month period.The initial classification of abnormal attentional bias no attensional bias was reliable for seven Alzheimer's disease subject, initially classified as having a slowed rightward attensional bias, in subsequent testing over the 12-month period was more consistent with symmetrical COVAT performance. Control subjects showed no attensional biases over the 12-month period and the magnitude of asymmetric attensional slowing over the 12-month period was significantly more variable in individual Alzheimer's disease subjects than in controls. The presence of subgroups of patients with Alxheimer's disease with qualitatively different COVAT performance indicates a large between-subject variability in attensional deficits in Alzheimer's disease.The presence of asymmetric attensional slowing and milder neuropsychological deficits in a subgroup of patients with Alzheimer's disease suggestes that in these patients there is functional impairment of attensional areas in only one hemisphere rather than an asymmetric impairment of both hemispheres and that the neurodegenerative disease process amy have been less advanced or in an earlier stage than that present in Alzheimer's disease aptients with symmetric attentional performance and bilateral COVAT impairment. The preservation of asymmetric attensional slowing over time, together with the increased intra-subject variability in the magnitude of these asymmetries, suggests that asymmetrical COVAT performance represents a reliable reflection of underlying hemispheric function in Alzheimer's disease, although designation of asymmetrical attensional biases should be made on the basis of two or more sequential testing sessions.
Alzheimer's disease; attention; visual; asymmetry
Received September 15, 1993. Revised May 29, 1995. Accepted July 22, 1995.
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