The audiometric utility of brain stem responses to low-frequency sounds

The human scalp-recorded vertex-positive brain stem response to a click or high-frequency tone pip is an excellent audiometric indicator. Its latency of 6-9 ms is practically independent of the polarity of the stimulus but is inversely related to intensity. With a 4,800- or 2,400-Hz tone pip (filtered click) its threshold of detectability is usually at or below 10 dB SL. With a 500-Hz tone pip, with rise and fall of 2-3 ms, the response at 30 dBSL is low in amplitude, rounded in wave form, and has a latency of about 10 ms. When the polarity of the stimulus is reversed, the latency shifts by 1 ms. At and above 40 dB, this late response is obsured by a larger and earlier response. High-pass (1,500 Hz) masking noise does not affect the low-level response but the earlier high-level response is reduced in amplitude and delayed by about 1 ms. The large early response seems to be initiated by stimulation of the basal turn of the cochlea by the low-frequency transient. The frequency-following response (FFR) to a 500-Hz tone burst with a 2-ms rise time has a threshold at about 40 dB SL. Its relatively short latency is appropriate to the basal turn. A later low-amplitude apically generated response can sometimes be detected, either at a lower stimulus level or in the presence of high-pass masking noise. The usual FFR often has complex wave forms and some individuals show only an onset response, even at 70 dB SL. It is almost impossible to edentify with certainty the first individual waves of FFR as they relate to the individual waves of the tone burst and as they change amplitude with intensity. The audiometric usefulness of the high-threshold responses to 500 Hz that are initiated in the basal turn is doubtful. The low-threshold responses initiated in the apical turn are so difficult to identify with certainty that they are not likely to be of clinical value unless high-pass masking noise can be used to clarify them.