Post-stimulatory effects of direct current stimulation of the cochlea on auditory nerve activity

Glass micro-electrode recordings from the spiral ganglion of the basal turn of the guinea pig cochlea have been obtained before, during and after negative (cathodic) current injection into scala tympani. Electrical stimulation with currents between 100 microA and 900 microA produced a marked increase in firing rate of the afferent neurons for the first 3 min of electrical stimulation. This was followed by a fall in firing rate to rates near or below the pre-stimulatory spontaneous rate if stimulation continued. Continuous electrical stimulation lasting 5 or 10 min reduced neural sensitivity to acoustic stimulation. Although threshold elevation was greatest for sound frequencies near the characteristic frequency of each neuron, thresholds could also be elevated at lower frequencies on the tail of the frequency-threshold tuning curve. After electrical stimulation a fall in the amplitude of the low-frequency microphonic recorded at the round window was also observed, indicating a disruption of the outer hair cell transduction. These effects were highly localized in the basal turn near the site of current injection, and were not associated with any significant structural changes in the organ of Corti, except after stimulation with very high current intensities.