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Am J Psychiatry 1996; 153:32-40
Copyright © 1996 by American Psychiatric Association
Visual cortical dysfunction in Alzheimer's disease evaluated with a temporally graded "stress test" during PET
MJ Mentis, B Horwitz, CL Grady, GE Alexander, JW VanMeter, JM Maisog, P Pietrini, MB Schapiro and SI Rapoport
Laboratory of Neurosciences, National Institute on Aging, NIH, Bethesda, MD 20892, USA.
OBJECTIVE: Visual-processing abnormalities commonly contribute to typical
Alzheimer's disease symptoms, but their detailed pathophysiology remains
unknown. To investigate why patients with Alzheimer's disease have greater
difficulty performing visuoconstructive (magnocellular-dominated) tasks
than face- or color- perception (parvocellular-dominated) tasks, the
authors measured brain activation in response to a temporally graded visual
stimulus (neural stress test) during positron emission tomography. METHOD:
The stress test measured regional cerebral blood flow (CBF) in response to
a patterned flash stimulus in the resting state (0 Hz in the dark) and at
frequencies of 1, 2, 4, 7, and 14 Hz. Ten patients with Alzheimer's disease
and 12 age- and sex-matched comparison subjects were studied. RESULTS: The
striate response at 7 Hz and 14 Hz (the degree of regional CBF increase
from that at 0 Hz) was significantly less in the patients than in the
comparison subjects, whereas the change in regional CBF at the lower
frequencies did not differ between groups. In bilateral middle temporal
association areas activated by motion and dominated by magnocellular input,
regional CBF at 1 Hz (the frequency with maximal apparent motion) was
significantly greater than at 0 Hz in the comparison subjects but not in
the patients. CONCLUSIONS: The magnocellular visual system normally
responds to high-frequency input and motion; the failure of response in the
striate cortex at high but not low frequencies in the Alzheimer's patients
suggests greater magnocellular than parvocellular dysfunction at these
levels. Activation failure in the middle temporal areas in the patients
supports magnocellular dysfunction. The finding that the Alzheimer's
disease group had abnormal visual cortical function emphasizes the
importance of clinical visuospatial evaluation of patients with Alzheimer's
disease to fully understand symptom production and to plan interventions.
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