Brain stem and other responses in electric response audiometry.

For electric response audiometry of young children who require sedation, an indicator is desired that is more reliable than the slow vertex potential. There are four leading candidates: 1) The electrocochleogram (ECochG) is very reliable with a transtympanic electrode on the promontory, but this is a surgical procedure and requires a general anesthetic. 2) The early midbrain responses give similar information with external electrodes, but they are complex and low in voltage. 3) The muscle reflexes (sonomotor responses) are a crude indicator with high and variable thresholds and are not suited to precise audiometry. 4) The "middle" responses, perhaps cortical in origin, are good candidates, but they have not yet been adequately validated in the clinic. For the midbrain responses and particularly for the ECochG, close synchronization of nerve impulses is essential. This requires a compromise with selectivity of frequency. High-tone audiometry by electrocochleography and midbrain responses is satisfactory, but limitations are increasingly severe below 2 kHz, where each sound wave is a separate acoustic stimulus. Low-frequency tones stimulate the basal turn of the cochlea at relatively low sensation levels. This makes assessment of the apical portion of the cochlea very difficult, even with the midbrain "frequency-following response" (FFR).