Steady-state visual evoked responses in high and low alpha subjects

Reactivity to photic stimulation was studied in 16 subjects who were divided into 2 groups based on the relative amounts of spontaneous EEG alpha produced during a 4 min eyes-closed baseline condition. During the experimental session the subjects were presented with thirteen 1 min periods of sinusoidally modulated light stimulation at frequencies ranging from 2 Hz below their spontaneous peak alpha frequency (SPAF) to 2 Hz above in 0.33 Hz intervals. Using FFTs, steady-state visual evoked responses (VERs) were extracted from each subject's EEG for each condition. VER magnitude for high alpha subjects varied with the proximity of the stimulus frequency to the SPAF, the VER for low alpha subjects did not. Conversely, low alpha subjects showed a similar effect in side-band alpha activity (once the VER had been extracted) whereas high alpha subjects did not. The results are explained in terms of a possible difference in the coupling strength between thalamic and cortical alpha sources.