A psychophysiological examination of memory dysfunction and disrupted distributed cortical processing in Alzheimer's dementia

Abstract

Two studies examined the electrophysiology of cognitive functioning in Alzheimer's disease. In the first study, fifteen AD patients and 26 elderly controls engaged in a lexical decision task with a list of words and non-words while event-related brain potentials were recorded. Embedded in the list were two repetition conditions: one where words were repeated at relatively long lags and one where words were repeated shortly after a brief masked presentation. Although elderly controls displayed behavioral and ERP repetition priming for words repeated at long lags, AD patients did not. In contrast, both controls and AD patients displayed an ERP repetition priming effect for the words repeated shortly after a brief masked presentation. Although the 2 ERP priming effects differ in elderly controls, the masked priming effect was also different between controls and AD patients. The results are used to critically assess the utility of an impaired/intact dichotomy when examining memory performance in brain damaged populations and are interpreted as lending support to dimensional models which postulate complex interactions between brain regions. In the second study utilizing the participants from the first study, eighteen AD patients and 22 elderly controls had their EEG recorded under four conditions: eyes open and closed resting baseline, and a verbal and spatial categorization task. EEG power was examined in 6 spectral bands revealing significant differences in the Theta band across all conditions and in the Alpha band during resting eyes closed and the categorization tasks. Alpha activity was examined utilizing instantaneous frequency analysis (IFA) in order to produce a measure of amplitude per unit time in the eyes open baseline and for the verbal and spatial cognitive tasks. The IFA analysis revealed that AD patients, relative to controls, failed to show an event-related decrease in alpha activity across the entire scalp during the performance of the 2 cognitive tasks. The failure of AD patients to display the expected alpha decrease in the left frontal region was significantly correlated with verbal task performance. These results are discussed with respect to disruptions in sustained and focused attentional mechanisms which appear to occur in AD.