Objective: To investigate the modulation of amplitude and phase precision of the auditory steady-state response (SSR) to 20 Hz stimulation in two conditions varying in the level of activation.
Methods: Click stimuli (20 Hz) were applied while subjects were sitting upright silently reading a book of interest (high activation level) and while subjects were sitting in a reclined position with eyes closed and the lights turned off (low activation level). Sixty-one channel EEG data was wavelet transformed, the amplitude and phase precision measures extracted and decomposed by the multi-subject non-negative multi-way factorization (NMWF).
Results: The NMWF decomposition of amplitude and phase precision measures resulted in the observation of two distinct components: a component at the frequency of stimulation--20 Hz SSR and a component emerging at 40 Hz--20 Hz SSR-related 40 Hz activity. Modulation by the activation level was observed only for 20 Hz SSR-related 40 Hz activity as increased amplitude and phase precision during low activation level. No such effects were observed for 20 Hz SSR.
Conclusion: The discrete components of the 20 Hz SSR are distinguished through modulation of activation level, 20 Hz SSR-related 40 Hz being higher in low activation state.
Significance: The biological modulation of 20 Hz SSR-related 40 Hz activity by the level of activation points to a physiological nature of this activity beyond a mere periodic effect in relation to the 20 Hz activity.