Decreased alpha bandwidth responsiveness to photic driving in Alzheimer disease.

The power spectra of the photically activated occipital EEGs of 9 mildly to moderately demented probable Alzheimer disease (AD) patients (according to NINCDS-ADRDA criteria), 9 normal age-matched control and 27 normal subjects of different ages were compared. In normal subjects, photic stimulation with rhythmic flashes ranging between 2 and 20 Hz elicited a characteristic response in each EEG bandwidth (delta, theta, alpha, beta1 and beta2). The magnitude of each bandwidth response was a function of the frequency of the photic stimulus. In AD patients the alpha bandwidth response curve was significantly smaller than that of age-matched controls (MANOVA main effect of group, P = 0.018); all the other bandwidth response curves were normal. Therefore, in AD there is a selective abnormality in the alpha bandwidth responsiveness to photic stimulation, probably due to AD pathology in the neuronal generator of the alpha rhythm.