Abstract
Sensory systems adjust to the environment to maintain sensitivity to change. In the auditory system, the medial olivocochlear reflex (MOCR) is a known physiological mechanism capable of such adjustment. The MOCR provides efferent feedback between the brainstem and cochlea, reducing cochlear gain in response to sound. The perceptual effects of the MOCR are not well understood, such as how gain reduction depends on elicitor characteristics in human listeners. Physiological and behavioral data suggest that ipsilateral MOCR tuning is only slightly broader than it is for afferent fibers, and that the fibers feed back to the frequency region of the cochlea that stimulated them. However, some otoacoustic emission (OAE) data suggest that noise is a more effective elicitor than would be consistent with sharp tuning, and that a broad region of the cochlea may be involved in elicitation. If the elicitor is processed in a cochlear channel centered at the signal frequency, the growth of gain reduction with elicitor level would be expected to depend on the frequency content of the elicitor. In the current study, the effects of the frequency content and level of a preceding sound (called a precursor) on signal threshold was examined. The results show that signal threshold increased with increasing precursor level at a shallower slope for a tonal precursor at the signal frequency than for a tonal precursor nearly an octave below the signal frequency. A broadband noise was only slightly more effective than a tone at the signal frequency, with a relatively shallow slope similar to that of the tonal precursor at the signal frequency. Overall, these results suggest that the excitation at the signal cochlear place, regardless of elicitor frequency, determines the magnitude of ipsilateral cochlear gain reduction, and that it increases with elicitor level.
Highlights
An impressive feat that the human auditory system achieves is the ability to hear sounds that range from low to extremely high intensities
Additivity of forward masking predicts that this addition of the precursors would lead to a similar shift in signal threshold, regardless of the masker frequency
There was a larger shift in signal threshold for the off-frequency masker condition (Figure 4)
Summary
An impressive feat that the human auditory system achieves is the ability to hear sounds that range from low to extremely high intensities. Most neurons in the auditory system respond sensitively to changes over a dynamic range of 30–40 dB, yet we are able to hear over a dynamic range of approximately 120 dB (Viemeister, 1988). The MOCR is an efferent pathway between the brainstem and cochlear outer hair cells that is elicited by sound and acts to decrease cochlear gain, with an onset delay of approximately 25 ms (James et al, 2005; Backus and Guinan, 2006) This gain reduction has been well documented physiologically in neural responses (Winslow and Sachs, 1987; Guinan and Gifford, 1988) and basilar membrane responses (Cooper and Guinan, 2003) in animal models, and in otoacoustic emission (OAE) responses (Backus and Guinan, 2006; Lilaonitkul and Guinan, 2009b) in humans. This makes the ipsilateral evoked response of interest and the focus of this paper
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