Quadratic phase coupling in auditory evoked potentials from healthy old subjects and subjects with Alzheimer´s dementia

Auditory evoked potentials (EPs) were recorded from healthy old (mean age 70) and Alzheimer´s (mean age 75) subjects in four experimental oddball series. In three series, targets occurred randomly with global probabilities of 20, 50 and 80%. In the fourth series, targets occurred predictably every fifth tone (20% global probability). EPs were recorded at Fz, Cz, Pz, and left and right temporo-parietal sites. Bicoherence analyses revealed that quadratic phase coupling occurred in the EPs in the classic delta, theta, alpha, and beta EEG frequency bands. Quadratic phase coupling patterns displayed a fronto-central topography and varied directly with global stimulus probability. The data suggest an inverse relationship between P300 amplitude and degree of quadratic phase coupling in the delta band. Quadratic phase coupling patterns were altered in Alzheimer´s patients. This is the first demonstration that task-dependent quadratic nonlinearities can be detected in transient evoked potentials.