Responses of a specific frequency after stimulation can be identified by computing the amplitude-frequency characteristic (AFC) of the averaged ERPs,15,22-23 or event-related and evoked power spectra. The AFC and event-related power spectra describe the brain system's transfer properties, eg, excitability and susceptibility to respond, by revealing resonant as well as salient frequencies. The AFC does not simply represent the spectral power density characterizing the transient signal in the frequency domain but the predicted behavior of the system (brain) assuming sinusoidal modulated input signals of defined frequencies were applied as stimulation. Since it reflects amplification in a given frequency channel, the AFC is expressed in relative units. Hence, the presence of a peak in the AFC or post-stimulus spectra reveals the resonant frequencies interpreted as the most preferred oscillations of the system during the response to a stimulus.
