Abnormal fronto-parietal coupling of brain rhythms in mild Alzheimer's disease: a multicentric EEG study

Cholinergic deafferentation/recovery in rats mainly impinges on the fronto-parietal coupling of brain rhythms [D. P. Holschneider et al. (1999) Exp. Brain Res., 126, 270-280]. Is this reflected by the functional coupling of fronto-parietal cortical rhythms at an early stage of Alzheimer's disease (mild AD)? Resting electroencephalographic (EEG) rhythms were studied in 82 patients with mild AD and in control subjects, such as 41 normal elderly (Nold) subjects and 25 patients with vascular dementia (VaD). Patients with AD and VaD had similar mini-mental state evaluation scores of 17-24. The functional coupling was estimated by means of the synchronization likelihood (SL) of the EEG data at electrode pairs, accounting for linear and non-linear components of that coupling. Cortical rhythms of interest were delta (2-4 Hz), theta (4-8 Hz), alpha (1 8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), beta 2 (20-30 Hz) and gamma (30-40 Hz). A preliminary data analysis (Nold) showed that surface Laplacian transformation of the EEG data reduced the values of SL, possibly because of the reduction of influences due to head volume conduction. Therefore, the final analysis was performed on Laplacian-transformed EEG data. The SL was dominant at alpha 1 band in all groups. Compared with the Nold subjects, patients with VaD and mild AD presented a marked reduction of SL at both fronto-parietal (delta-alpha) and inter-hemispherical (delta-beta) electrode pairs. The feature distinguishing the patients with mild AD with respect to patients with VaD groups was a more prominent reduction of fronto-parietal alpha 1 SL. These results suggest that mild AD is characterized by an abnormal fronto-parietal coupling of the dominant human cortical rhythm at 8-10.5 Hz.