THE EFFECT OF CONTRALATERAL PURE TONES ON THE COMPOUND ACTION POTENTIAL IN HUMANS

Abstract

The compound action potential (CAP) has been suggested in the literature as an alternative to otoacoustic emissions (OAE) for evaluating the efferent auditory system, and is thought to overcome some of the drawbacks associated with OAE. However, very few studies have examined efferent influence on auditory nerve potentials in humans. To help address this need, the present study examines the effects of contralateral pure tones on the CAP onset and offset amplitudes. The general goal of this research is to assess the value of using the CAP as a potential clinical tool for the assessment of efferent auditory function. The CAP was recorded from the tympanic membranes (TM) of 18 normally-hearing young adults (10 males and 8 females) using three different stimuli: broadband clicks, 1 kHz, and 4 kHz tone pips. The signal level was either midway between CAP threshold and saturation, or at the minimum signal level that revealed a reliable CAP. Contralateral tones were presented at levels ranging from 20 to 70 dB HL in 10 dB steps. The frequencies of the contralateral tones were .5, 1, 2, 4, 8 kHz for the click CAP; .5, 1, 2 kHz for the 1 kHz CAP; and 2, 4, 8 kHz for the 4 kHz CAP. Results showed that maximum suppression of 1 kHz CAP onset amplitude was obtained in 7 out of 9 participants by the 1 kHz contralateral pure tone at 40 dB HL (.07 ìV ± .02). The 4 kHz CAP onset amplitude was maximally suppressed in 8 out of 9 participants by the 8 kHz contralateral pure tone at 30 dB HL (.07ìV ± .02). The click CAP offset amplitude was maximally suppressed in 4 out of 8 participants by the 8 kHz contralateral tone presented at 40 dB HL (.17 ìV ± .05). These results along with previous studies suggest that the efferent system is maximally stimulated by moderate signal level tones (i.e. 30 - 40 dB HL), and that the efferent activity is dependent on frequency cues of both the stimulus and suppressor tones. Other factors that might be affecting the efferent influence on CAP such as sound duration, phase, bandwidth, and periodicity need to be further investigated in humans using noninvasive techniques. The long term goal of this research is to lead to the development of more effective clinical tools for investigating the efferent auditory system.