The Effect of Contralateral Pure Tones on the Compound Action Potential in Humans: Efferent Tuning Curves
Fadi Najem1, John Ferraro1, Mark Chertoff1
1University of Kansas Medical Center, Kansas City, KS.
Background:
The compound action potential (CAP) has been suggested in the literature as an alternative to otoacoustic emissions for evaluating the efferent auditory system. However, very few studies have examined efferent influence on auditory nerve potentials in humans.
Purpose:
This study examines the effects of presenting contralateral pure tones on the ipsilateral CAP onset and offset amplitudes as a potential clinical tool for the assessment of efferent auditory function.
Research Design:
CAPs for 1- and 4-kHz tone pips (TPs) and clicks were recorded from 9, 9, and 8 participants, respectively. Contralateral tones were presented at levels ranging from 20 to 70 dB HL in 10-dB steps. The frequencies of the contralateral tones were 0.5, 1, 2 kHz for the 1-kHz TP CAP; 2, 4, 8 kHz for the 4-kHz TP CAP; and 0.5, 1, 2, 4, 8 kHz for the click CAP.
Data Analysis:
The CAP onset and offset amplitudes in all experimental conditions were analyzed and compared to the CAP amplitude without contralateral stimulation (i.e., baseline).
Results:
Maximum suppression of 1-kHz TP CAP onset amplitude was obtained in seven out of nine participants by the 1-kHz contralateral pure tone at 40 dB HL. The 4-kHz TP CAP onset amplitude was maximally suppressed in eight out of nine participants by the 8-kHz contralateral pure tone at 30 dB HL. The click CAP offset amplitude was maximally suppressed in four out of eight participants by the 8-kHz contralateral tone presented at 40 dB HL. The 1- and 4-kHz TP CAP offset and click CAP onset amplitudes were not affected by contralateral stimulation.
Conclusions:
These results along with the previous studies may suggest that the efferent system is maximally stimulated by moderate signal-level tones (i.e., 30-40 dB HL), and that efferent activity is dependent on frequency cues of both the stimulus and suppressor tones. Other factors that might be affecting efferent influence on the CAP in humans such as sound duration, phase, bandwidth, and periodicity need to be further investigated.
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