A Slow Brain Stem Response for Low-Frequency Audiometry

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Abstract
Proper choices of stimuli and of brain stem electric responses allow us to estimate peripheral auditory thresholds at 500, 1,000, 2,000 and 4,000 Hz with an accuracy of about .+-. 10 dB. With the help of sedation (secobarbital), such audiograms may be obtained from each ear of a child of any age in a single session. Tone pips (filtered clicks) or very brief tone bursts give a clinically adequate frequency selectivity. The rise time must be adjusted to the center frequency. A rise time of 2 periods with a plateau from 0-1 period gives a good compromise between frequency specificity and a synchronous neural discharge. The best threshold indicator for tone pips of 2,00 Hz or higher (or unfiltered clicks) is P6 (Jewett V). At 60 dB nHL [hearing level] its latency is 6.0-7.0 ms (for children of 1 yr or older), but near threshold it is 8.0-9.5 ms. An input pass-band of 140-3,000 Hz is appropriate. The best threshold indicator at 500 or 1,000 Hz is a nearly neglected slower wave with a scalp-negative crest at about 10 ms following a 60-dB click. Latency is 15 ms following a 500-Hz tone pip at 15 dB SL. This wave is called the slow negative (10) or SN10. To see it well a wider input pass-band such as 40-3,000 Hz is needed. SN10 is usually obscured by P6 or by frequency-following response at stimulus levels above 35 dB SL [sound level]. A clinical routine allowing the determination (.+-. 10 dB) of 8 threshold endpoints within about 80 min is detailed. Several precautions and limitations, and the origin of the SN10 wave are discussed.

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