Cognitive decline unlike normal aging is associated with alterations of EEG temporo-spatial characteristics.

The diagnosis of beginning dementia is based mainly on neuropsychological testing. Several measures of EEG spectral composition, coherence and complexity (correlation dimension) have been shown to correspond to cognitive function. Only a few studies have evaluated EEG changes in normal aging, and no quantitative study has addressed changes in EEG during cognitive tasks in demented elderly. In this study the quantitative descriptors of EEGs from 31 demented or cognitively impaired elderly persons, 30 healthy elderly (mean age 69 years) and 35 young controls (mean age 31 years) were compared. The EEGs were recorded during two resting conditions (eyes closed and eyes opened) and two tasks (mental arithmetics and a lexical decision). The goal of the study was to evaluate which temporal and spatial EEG descriptors change with cognitive decline and with normal aging, respectively. Cognitive categories (unimpaired, impaired, demented) were based on Structured Interview for the Diagnosis of Dementia of Alzheimer Type (SIDAM) scores. The EEGs were analysed using adaptive segmentation of continuous EEG, which quantifies the succession of distinct stable topographic voltage patterns (EEG microstates). The main findings were a significant increase in the number of ultra-short EEG microstates and, independently, a reduction in the average duration of EEG microstates in the cognitively impaired and demented patients. In addition, cognitive impairment was associated with a reduction or loss of EEG reactivity normally observed when the resting states with closed and with opened eyes are compared. No alterations in temporal or spatial EEG descriptors were found in normal aging. Cognitive tasks did not add to information already obtained during the resting states. The reduction in EEG microstate duration correlated with loss of cognitive function. Temporo-spatial analysis of EEG therefore is a useful indicator of cortical dysfunction in dementia, correlating with the degree of cognitive impairment. Normal aging seems not to be accompanied by changes in temporo-spatial EEG patterns. The data suggest that fragmentation of the electrophysiological processes underlies cognitive dysfunction in Alzheimer's disease.