Steady-state visual evoked potentials and travelling waves

Objective: The amplitude and phase of the steady-state visual evoked potent ial (SSVEP) is sensitive to cognition and attention but the underlying mech anism is not well understood. This study examines stimulus evoked changes i n the SSVEP phase topography and the putative role of travelling waves. Methods: Eighteen subjects viewed a central-field checkerboard and full-fie ld flicker stimulus temporally modulated at the peak alpha rhythm frequency . EEC was recorded from 10 midline scalp sites and the bipolar SSVEP obtain ed from differences between adjacent electrodes. Results: The SSVEP phase comprised either progressive variations consistent with travelling waves or a phase reversal consistent with standing waves. The checkerboard pattern elicited travelling wave patterns in 14 subjects w ith estimated phase velocities ranging from 7 to 11 m/s after correcting fo r folded cortex. The flicker stimulus elicited phase reversals in 9 subject s, suggesting standing waves. Six subjects demonstrated a phase topography specific to the stimulus with travelling wave patterns associated with the checkerboard and standing wave patterns associated with the flicker. Conclusions: These differences suggest the emergence of travelling and stan ding waves under different spatial configurations of visual input to the co rtex and that wave phenomena contribute to the spatiotemporal dynamics of t he SSVEP. (C) 2000 Elsevier Science ireland Ltd. All rights reserved.