Effect of natural sleep on auditory steady state responses in adult subjects with normal hearing

Auditory 40-Hz steady state response (SSR), auditory brainstem response (ABR), and middle-latency response (MLR) were recorded in 12 healthy adult females with normal hearing while awake and asleep. The responses were recorded with 500-Hz tone pips at 55 dBnHL. Synthesized SSR were made by superimposing the recorded ABR and MLR waveforms (ABR-MLR), and their amplitudes were compared with those of the actually recorded SSR. In the waking state, the ratio of the mean amplitude of recorded SSR to that of synthesized SSR was 0.819, whereas in the sleeping state it decreased to 0.522, a statistically significant difference. The results indicate that the SSR can be predicted from the linear superimposition of ABR and MLR in the waking state, but not in the sleeping state.