Effects of Contralateral Noise on Cortical Auditory Evoked Potential Latencies and Amplitudes.

Purpose: There is evidence from past animal work that the neural signal-tonoise ratio (SNR) is modulated through the action of the medial olivocochlear reflex (MOCR). This is commonly referred to as unmasking. However, evidence of unmasking in humans is limited, perhaps due to the traditional approach of measuring the MOCR using otoacoustic emissions--a preneural metric. The amplitudes and latencies of the late latency response (LLR) are sensitive to changes in SNR and may provide a means to noninvasively evaluate MOCR unmasking at the neural level. The purpose of this study was to investigate MOCR-mediated enhancement of ipsilateral noise in humans using the LLR. Method: Fifty normal-hearing adults were recruited. The LLR was measured for a 60 dB SPL, 1-kHz tone in both ipsilateral quiet and ipsilateral noise, with and without presentation of contralateral noise. For the ipsilateral noise conditions, the noise was presented at three different levels to achieve SNRs of +5 dB, +15 dB, and +25 dB. The contralateral noise was always 60 dB SPL white noise. LLR latencies (P1, N1, and P2) and interpeak amplitudes (P1-N1 and N1-P2) were measured for all conditions. In addition, otoacoustic emissions (OAEs) for a 1-kHz tone burst were measured in ipsilateral quiet both with and without contralateral noise. The same contralateral noise was used for both OAEs and LLRs. Results: For the ipsilateral noise conditions, SNR had a significant effect on LLR latencies and interpeak amplitudes: Latencies decreased, and amplitudes increased as SNR improved. The presentation of contralateral noise had a significant effect on P1 and N1 latencies, both of which decreased. LLR interpeak amplitudes significantly increased upon the presentation of contralateral noise. For the ipsilateral quiet condition, there were no significant effects of contralateral noise on LLR metrics. Though OAE magnitudes were significantly reduced upon presentation of contralateral noise, consistent significant relationships between OAE magnitude changes and changes in the LLR metrics were not found. Conclusion: Findings suggest that the presentation of contralateral noise enhances the neural response to an ipsilateral noise, potentially through MOC efferent feedback. [ABSTRACT FROM AUTHOR]