Identification of wave-like spatial structure in the SSVEP: comparison of simultaneous EEG and MEG

Steady-state visual-evoked potentials/fields (SSVEPs/SSVEFs) are used in cognitive and clinical electroencephalogram (EEG) and magnetoencephalogram (MEG) studies because of their excellent signal-to-noise ratios and relative immunity to artifact. Steady-state paradigms are also used to characterize preferred frequencies of dynamic neocortical processes. In this study, SSVEPs and SSVEFs were simultaneously recorded while subjects viewed checkerboard patterns alternating (black to white, white to black) with fixed driving frequency between 2 and 20 Hz. Distinct peaks in SSVEP/SSVEF power were observed in the theta (4-8 Hz) and upper alpha (10-14 Hz) bands. A distinct peak in SSVEP power was also observed in the beta band (between 15 and 20 Hz) which had no counterpart in the MEG. One-dimensional spatial spectra indicate that distinct large-scale source distributions contribute to SSVEP power in the upper alpha band in the form of long wavelength (lambda>20 cm) traveling waves propagating from occipital to prefrontal electrodes. In the beta band, spatial spectra and SSVEF indicate that long-wavelength source distributions over posterior and anterior regions form standing wave patterns. These results suggest that simple models of SSVEP based on a single dipole source in the occipital lobe are inadequate to explain the dynamic spatial patterns of SSVEP magnitude and phase. Theoretical models of SSVEP should include multiple local and distributed sources and exhibit both traveling and standing wave dynamics.