Fractal analysis of steady-state-flicker visual evoked potentials: feasibility

Electrophysiological models of visual evoked potential recording have assumed that response variability is caused predominantly by random noise added to a true steady signal. Since neuronal geometry has a fractal structure, neural activity may demonstrate deterministic nonlinear dynamics, i.e., chaos. We recorded several-minute time-series traces of the visual evoked potential magnitude in response to full-field flicker from three glaucoma patients and one normal subject. When plotted in phase space, the steady-state response derived from a lock-in amplifier shows an apparent so-called strange attractor (extended nonrepeating loops) rather than the pattern expected from a signal-plus-noise model (a fuzzy dot). The fractal dimension of this attractor may be a more sensitive indicator of early optic-nerve damage than are visual evoked potential latency or amplitude measures.