Temporal masking and The "active process" in normal and hearing-impaired listeners

Physiological studies have shown that contralateral stimulation provides an increment in efferent activity, which can affect cochlear mechanics. In psychoacoustic studies, contralateral stimulation was used in an attempt to activate the efferent system in a series of temporal masking, or “overshoot” experiments. Due to the latency of efferent effects, thresholds are higher for signals presented at the onset of a masker, where the active process of the cochlea is presumably stronger than for signals presented later, when the efferent system has begun to influence the active process. In listeners with sensorineural hearing loss, who do not have an operational efferent feedback loop or an active process in the cochlea due to damaged outer hair cells, no overshoot is observed. In the hearing-impaired listener, the input-output function for cochlear vibration is steeper than when the active process is strong, and a smaller signal is required to achieve the necessary increment.