Neural Encoding of Amplitude Modulations in the Human Efferent System

Most natural sounds, including speech, exhibit temporal amplitude fluctuations. This information is encoded as amplitude modulations (AM)—essential for auditory and speech perception. The neural representation of AM has been studied at various stages of the ascending auditory system from the auditory nerve to the cortex. In contrast, research on neural coding of AM in the efferent pathway has been extremely limited. The objective of this study was to investigate the encoding of AM signals in the medial olivocochlear system by measuring the modulation transfer functions of the efferent response in humans. A secondary goal was to replicate the controversial findings from the literature that efferent stimulation produces larger effects for the AM elicitor with 100 Hz modulation frequency in comparison with the unmodulated elicitor. The efferent response was quantified by measuring changes in stimulus-frequency otoacoustic emission magnitude due to various modulated and unmodulated elicitors. Unmodulated, broadband noise elicitors yielded either slightly larger or similar efferent responses relative to modulated elicitors depending on the modulation frequency. Efferent responses to the unmodulated and modulated elicitors with 100 Hz modulation frequency were not significantly different. The efferent system encoding of AM sounds—modulation transfer functions—can be modeled with a first-order Butterworth low-pass filter with different cutoff frequencies for ipsilateral and contralateral elicitors. The ipsilateral efferent pathway showed a greater sensitivity to AM information comparted to the contralateral pathway. Efferent modulation transfer functions suggest that the ability of the system to follow AM decreases with increasing modulation frequency and that efferents may not be fully operating on the envelope of the speech.