Sorted averaging improves quality of auditory steady-state responses.

Increasing signal-to-noise ratio (SNR) is essential for the recording of auditory evoked potentials with electroencephalography (EEG). Several protocols have been proposed to increase the SNR, starting with an averaging of EEG epochs which decreases noise level. Since artifacts decrease the SNR by increasing the noise level, artifact detection and reduction protocols are other important tools to reduce the noise level. The current study focuses on the sorted averaging protocol where the epochs are sort according to their estimated root-mean-square (RMS) amplitude. Calculating an estimated SNR by averaging of the sorted epochs, this process of averaging can be interrupted at the maximum SNR, i.e., at an optimal number of epochs. In contrast to the often used protocol that weighs every epoch by its inverse average root-mean-square amplitude, sorted averaging is a linear operation, i.e., it does not change signal amplitudes. In this study, the sorted averaging is, for the first time, applied to auditory steady-state responses (ASSR) which are evoked by amplitude modulated tones or trains of transient acoustic stimuli. In contrast to other evoked potentials, the ASSR is analyzed in the frequency domain, using the property of auditory system to retain the modulation frequency (or the repetition rate) of the stimulus. ASSR were recorded in 11 subjects with normal hearing. Results of four artifact processing protocols (1) fixed rejection level, (2) adaptive rejection level, (3) weighted averaging and (4) sorted averaging were compared. The results showed a higher normalized SNR with a sorted averaging protocol than with adaptive rejection level and weighted averaging protocols. An advantage of the sorted averaging protocol is that, compared to a fixed-rejection threshold, the ASSR amplitudes were unchanged when the sorted averaging protocol was used, whereas they were significantly reduced by the weighted averaging protocol. The residual noise was also significantly lower for the sorted averaging protocol than for the weighted averaging and adaptive rejection protocols. Thus, the sorted averaging may be a powerful tool to increase the quality of ASSR.