STEADY-STATE VISUAL EVOKED-RESPONSES IN HIGH AND LOW ALPHA SUBJECTS

Reactivity to photic stimulation was studied in 16 subjects who were d ivided into 2 groups based on the relative amounts of spontaneous EEG alpha produced during a 4 min eyes-closed baseline condition. During t he experimental session the subjects were presented with thirteen 1 mi n periods of sinusoidally modulated light stimulation at frequencies r anging from 2 Hz below their spontaneous peak alpha frequency (SPAF) t o 2 Hz above in 0.33 Hz intervals. Using FFTs, steady-state visual evo ked responses (VERs) were extracted from each subject's EEG for each c ondition. VER magnitude for high alpha subjects varied with the proxim ity of the stimulus frequency to the SPAF, the VER for low alpha subje cts did not. Conversely, low alpha subjects showed a similar effect in side-band alpha activity (once the VER had been extracted) whereas hi gh alpha subjects did not. The results are explained in terms of a pos sible difference in the coupling Strength between thalamic and cortica l alpha sources.