Source geometry and dynamics of the visual evoked potential

A method is developed for the analysis of the steady-state visual evoked potential (VEP) recorded at an array of electrodes. The VEP is assumed to result from the sum of a number of independent mechanisms. Each mechanisms proves to have a fixed intracerebral source (independent of stimulus paradigm) and dynamics which depend on the stimulus. The steps in the analysis are: (1) Fourier analysis of the responses and retention of the first 4 even harmonics; (2) factor analysis in the frequency domain; (3) an invariant representation ('the invariant fingerprint') of the geometric information contained in the factor analysis; and (4) interpretation of the invariant fingerprint in terms of a simple biophysical model. This analysis is applied to responses elicited by a contrast-reversing checkerboard. The dependence of the midline VEP wave form on reversal frequency, check size, and area of stimulation (upper, lower, and full-field) reflects a change in the dynamics of the generators, rather than a change in their geometry.