ASYMMETRIES IN THE COVERT ORIENTING OF VISUAL-SPATIAL ATTENTION TO SPATIAL AND NONSPATIAL CUES IN ALZHEIMERS-DISEASE

The ability to direct covert visual spatial attention to the left (LVF ) and right visual field (RVF) was examined in 15 patients with mild t o moderate Alzheimer's disease and 15 age- and education-matched contr ols using the covert orienting of visual spatial attention task (COVAT ) modified to include both spatial and non-spatial cues. Subjects resp onded with a button press when they detected a target at a location 8 degrees to either the left or right of fixation. On 70% of trials a sp atial cue was flashed at the target location before The target appeare d On 15% of trials the spatial cue was flashed at the location contral ateral to where it would appear and on the remaining 15% of trials non -spatial diffuse cue preceded targets. The cue to target interval (CTI ) varied between 150 and 550 ms. Mean reaction times for each cuetype in the RVF and LVF were calculated. Compared with controls, the percen tage of trials excluded because of very slow reaction times was signif icantly greater in the Alzheimer's disease group for the 550 ms CTI. A nalysis of the symmetry of reaction times to LVF and RVF targets for t he 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 showing of reaction t ime to all LVF targets, the second showed a significant slowing of rea ction rime to all RVF targets and the third showed a significant slowi ng of reaction time to both LVF and RVF targets. Patients with Alzheim er's disease who showed art abnormal attentional bias performed signif icantly better on neuropsychological tests of memory, language and exe cutive function than Alzheimer's disease patients with no attentional bins. Eight of the Alzheimer's disease subjects were assessed serially on at least six occasions over a 12-month period. The initial classif ication of abnormal attentional bias or no attentional bias was reliab le for seven Alzheimer's disease subjects. One Alzheimer's disease sub ject, initially classified as having a slowed rightward attentional bi as, in subsequent testing over the 12-month period was more consistent with symmetrical COVAT performance. Control subjects showed no attent ional biases over the 12-month period and the magnitude of asymmetric attentional slowing over the 12-month period was significantly more va riable in individual Alzheimer's disease subjects than in controls. Th e presence of subgroups of patients with Alzheimer's disease wish qual itatively different COVAT performance indicates a large between-subjec t variability in attentional deficits in Alzheimers disease. The prese nce of asymmetric attentional slowing and milder neuropsychological de ficits in a subgroup of patients with Alzheimer's disease suggests tha t in these patients there is functional impairment of attentional area s in only one hemisphere rather than an asymmetric impairment of both hemispheres and that the neurodegenerative disease process may have be en less advanced or in an earlier stage than that present in Alzheimer 's disease patients with symmetric attentional performance and bilater al COVAT impairment. The preservation of asymmetric attentional slowin g over time, together with the increased intra-subject variability in the magnitude of these asymmetries, suggests that asymmetrical COVAT p erformance represents a reliable reflection of underlying hemispheric function in Alzheimer's disease, although designation of asymmetrical attentional biases should be made on the basis of two or move sequenti al testing sessions.