MIDDLE LATENCY RESPONSES TO ELECTRICAL-STIMULATION OF THE AUDITORY-NERVE IN UNANESTHETIZED GUINEA-PIGS

Middle latency responses (MLR) to sinusoidal and pulsatile electrical stimulation (ES) of the cochlea and to acoustical stimulation (AS) wer e evaluated in awake guinea pigs with chronically implanted electrodes . The ear, which was later electrically stimulated, was deafened by lo cal intracochlear application of gentamicin, the opposite ear was left intact. Waveforms and P1-P2 interpeak intervals of the electrically e voked MLR (ES-MLR) were similar to those evoked by acoustical stimulat ion of the intact ear (AS-MLR) and the latencies of the ES-MLR were sh orter by about 1-3 ms. Thresholds of ES-MLR in the frequency range 0.5 -32 kHz increased with increasing ES frequency (slope 3.2 dB/octave), thresholds were 3.5-9.5 dB lower for intracochlear than for extracochl ear ES. Dynamic ranges for ES-MLR varied between 6-20 dB. MLR amplitud e-intensity functions for ES were steeper (slope 2-12 muV/dB) than tho se for AS (slope 0.2-2 muV/dB). Maximal ES-MLR amplitudes exceeded usu ally 1.5-3 times the amplitudes of the acoustically evoked MLR. Both t ypes of stimulations evoked larger MLR amplitudes to contralateral sti mulation than to ipsilateral stimulation (average ratio = 4.1 +/- 2.2 for AS and 3.3 +/- 2.2 for ES). Because of the relatively long latency and therefore insensitivity to electrical artifact, the ES-MLR can be used for the evaluation of different strategies of the