EEG coherence in Alzheimer's disease
Introduction
Slowing of the electroencephalogram (EEG) is a frequent finding in patients with Alzheimer's disease (AD) and its occurrence is related to the stage of the disease (Coben et al., 1985). An increase of slow bands can be found in over 90% of patients with moderate or severe AD (Soininen and Riekkinen, 1992). In the early stages of the disease, changes are more subtle, as a considerable number of patients may have a normal EEG (Penttila et al., 1985). Spectral analysis of the EEG has greatly contributed to a better definition of the modifications of the EEG in the early stages of the disease: these changes consist in a slowing of the peak of the alpha band and in an increase of the power of the theta band (Coben et al., 1983; Soininen and Riekkinen, 1992). Delta band power increases and alpha band power decreases only in more advanced stages; in slightly demented patients alpha activity can be even higher than in control groups (Dierks et al., 1991). All these EEG changes are not specific to AD, and have been reported also in vascular dementia.
Modifications of spectral parameters have been found to be related to the decrease in metabolic activity assessed by PET (Comi et al., 1992; Szelies et al., 1992).
EEG coherence can be defined as the normalized cross-power spectrum per frequency of two signals recorded simultaneously at different sites of the scalp. It is a measure of the synchronization between the two signals and may be interpreted as an expression of their functional interaction (Sklar et al., 1972; Shaw et al., 1978). EEG coherence decreases with the distance between recording electrodes in normal subjects. Thatcher et al. (1986)proposed a two-compartmental model of EEG coherence in order to separate the contributions of long and short cortico-cortical associative fibers. Excluding volume conductor contribution, coherence between near electrodes is particularly influenced by short connections, as the axonal density per unit volume of cortex is approximately 10–100 times higher for short-axoned stellate and Martinotti cells than for long-axoned pyramidal cells (Braitenberg, 1978). On the contrary, the coherence between distant electrodes is mainly due to long axon connections. The influence of subcortical nuclei, particularly of thalamic nuclei, on both local and long-distance coherence, seems also to be relevant. Leuchter et al. (1992), using bipolar recordings, found an alpha coherence decrease in AD patients; they interpreted changes as related to the impairment of the cortico-cortical connections, particularly of the long fibers that connect temporo-parieto-occipital areas to frontal ones. These findings could be consistent with neuropathological evidence that associative areas lose afferent and efferent connections in parallel with the damage and the death of pyramidal neurons (Pearson et al., 1985).
We studied EEG coherence in a group of probable AD patients in the early phases of the disease. The aims of the study are to define if there are changes of EEG coherence in AD, to evaluate whether short-distance or long-distance coherence is more affected and the possible diagnostic value of coherence analysis in AD.
Section snippets
Methods and materials
Ten patients affected by probable AD (NINCDS-ADRDA criteria) entered the study. The mean age was 67.3 years (range 53–77) and the mean duration of the disease 18 months (range 12–20 months). All patients had a mild or moderate degree of cognitive impairment, having a mini-mental state examination (MMSE)>18 and <24. Informed consent was obtained by patients admitted to the study, and their relatives. All patients were followed at the AD Center of the Department of Neurology of the S. Raffaele
Results
In normal subjects, coherences for the different bands showed a similar topographical distribution, without significant side asymmetries, decreasing as distance increased. All values had gaussian distribution (Kolmogorov Smirnov Test: 0.91<0.97, 0.05<P<0.001 for all the pairs of channels).
Alpha band coherence was decreased in the AD group compared with the control group, more evidently between electrodes over the temporo-parietal regions (Fig. 1); the decrease was more accentuated for the
Discussion
EEG coherence is assumed to give information about coupling between different recording electrodes and is interpreted as the evidence of structural and functional connections between cortical areas underlying the recording electrodes. Our goal was to assess the changes of coherence in AD in the early stages of the disease. Previous authors (Leuchter et al., 1992; Dunkin et al., 1994; Sloan et al., 1994) analyzed EEG coherence as obtained by discrete Fourier transformation and used for their
Acknowledgements
We wish to thank Emanuela Mauri and Aldo Elia who collected EEG recordings for this study. Many thanks also to Dr. Margherita Alberoni who selected the patients to submit to the study and performed neuropsychological testing.
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