P. Maruff et al., ASYMMETRIES IN THE COVERT ORIENTING OF VISUAL-SPATIAL ATTENTION TO SPATIAL AND NONSPATIAL CUES IN ALZHEIMERS-DISEASE, Brain, 118, 1995, pp. 1421-1435
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.