Spectro-temporal representations and time-varying spectra of evoked potentials. A methodological investigation.

Evoked potential waveforms are generally of a dynamic, transient character. Consequently, their spectral energy distribution cannot be adequately described by time-invariant representations, such as the power density spectrum. Obviously, a spectro-temporal description is needed. Appropriate means for obtaining such descriptions are discussed, on the basis of theoretical considerations concerning simultaneous time-frequency representations and methods of short-time spectral analysis. With reference to the "uncertainty principle", particular attention is paid to time-bandwidth products of various filter types, used in relation with the latter technique. It is concluded that the method of bandpass filtering with proportional bandwidth filters, having cosine transfer functions, arises as a suitable solution in evoked potential analysis. The results of applying this method to somatosensory, visual, and brainstem auditory evoked potentials are presented.