[Auditory steady-state response to sinusoidally amplitude-modulated tones. Second report: investigation of response in the sleeping state].

The auditory steady state response (SSR) elicited by a sinusoidally amplitude-modulated (SAM) tone may be an important technique in objective audiometry, which provides frequency-specific information. This response is affected the state of arousal of the subject. An effect of sleep on the amplitude of this response has been observed by a number of previous investigators. Since young children must usually be sedated to carry out evoked potential testing, the effects of sleep on this response are a crucial factor limiting clinical applicability. The basic characteristics of SSR elicited by a 500 Hz SAM tone were studied in 21 adults with normal hearing during sleep. The responses while sleeping were compared with the responses in the awake state. The responses to 2048 stimuli were averaged with the analysis time set at 102.4 ms, and responses were judged by passing the waveforms through a 20-100 Hz digital filter. The same technique was performed in 25 infants (2 months-2 years 9 months) evaluated as having normal hearing. The results obtained are summarized as follows: The amplitude of the response while sleeping was approximately half the amplitude in the awake state. Responses with a modulation frequency of 70Hz, however, showed a persistent decrease in 71.0%, suggesting a trend different from that seen at a modulation frequency of less than 70Hz. The effect of modulation frequency on response amplitude was almost the same for the sleeping state and the awake state. Even during sleep, the response amplitude was maximal at modulation frequencies of 20, 30 and 40Hz, and there was no significant difference between the values at 20Hz -40Hz. The latency of the response was measured by the Diamond method, and the results showed that latency is significantly prolonged while sleeping (p < 0.01). The response threshold at the 500Hz SAM tone (modulation rate 40Hz, modulation depth 90%) was 8.5dBnHL when awake and 13.9dBnHL during sleep. The rise in threshold as a result of sleep was no more than about 5dB. In infants, the response amplitude at a modulation frequency of 20Hz was maximal, and amplitude at the 40Hz was significantly smaller (p < 0.01). Latency measured by the Diamond method in infants was 12.8ms. Thus, a difference from the response in adults was confirmed. Comparisons were made between the responses of infants under 1 year of age and infants over 1 year of age to confirm changes in response according to age.