Temporal masking and the ''active process'' in normal and hearing-impaired listeners

Current models of auditory perception are based primarily on information from the afferent auditory fibers as they transmit information from the cochlea to higher levels of the nervous system. Yet there is ample evidence that efferent fibers descending from higher auditory centers exist and can potentially affect information that is transmitted to the brain. Physiological studies have shown that contralateral stimulation provides an increment in efferent activity, which can affect cochlear mechanics. In our psychoacoustic studies, contralateral stimulation was used in an attempt to activate the efferent system in a series of temporal masking, or ''overshoot'' experiments. In nearly all subjects with normal hearing the contralateral stimulus reduced the masked threshold of a probe presented at the onset of a gated masker, whereas in hearing-impaired subjects no such effect was observed. These results are suggestive of an ''automatic gain control'' mechanism in the normal auditory system that operates via the efferent system. This feedback loop appears to be affected by sensorineural hearing loss.