Regarding the evoked potential (EP) as a superposition of evoked EEG rhythms in several frequency ranges, we investigated the following issue: Are distinct evoked rhythms, in particular the alpha (8-15 Hz) response, related to separable physiological processes? Frequency domain analysis of EPs was used to evaluate results of cross-modality experiments, i.e.: responses to auditory stimuli were simultaneously recorded from the auditory cortex (adequate stimulation) and from the visual cortex (inadequate stimulation). Responses to visual stimuli were recorded from the same sites. The results of these experiments and further measurements (EEG and MEG responses in humans, among them multiple sclerosis patients) are consistent in the following respect: The amplitudes of alpha responses are dependent on whether or not a stimulus applied is adequate. Alpha responses may thus be related mostly to primary sensory processing. In contrast, theta responses (4-7 Hz) were observed for adequate as well as inadequate stimuli. They may be related rather to associative and cognitive processing than to primary sensory processing. Thus frequency responses, in particular the alpha response, are not artificial results of digital filtering, but functionally significant brain responses.