A new rapid component in the cochlear response to brief electrical efferent stimulation: CM and otoacoustic observations.

The onset and time course of cochlear response changes induced by brief electrical stimulation of medial fibres of the OCB have been studied in detail in guinea pig. Changes in both cochlear microphonic and otoacoustic responses begin within 10 ms of electrical excitation. The sensitive differential technique employed has allowed the use of very short and weak electrical stimulations. The effects of induced levels of activity close to those normally present are reported. After transient electrical excitation (a single 100 microseconds, 20 mu amp shock), changes in the cochlear microphonic response reached a maximum around 20 ms. Thereafter the perturbation decayed initially with a time constant between 30 and 45 ms, then more slowly with a time constant greater than 100 ms. For more intense but still brief excitation the initial response decay was even more rapid often resulting in a bimodal decay pattern - a discrete 'burst' and a 'slow tail'. Otoacoustic data was similar. Only the slow tail component corresponds to previously reported effects and we suggest that it may be associated with overstimulation of the efferent system.