Electrical stimulation activates two different sites within the guinea pig cochlea

This study investigates whether auditory brainstem responses (ABRs) can be used to assess the functioning of electrically stimulated cochleas. Electrically evoked auditory brainstem responses (EABRs) were recorded in guinea pigs with normal hearing and guinea pigs deafened by amikacin, a powerful ototoxic antibiotic, combined with diuretic aminooxyacetic acid (AOAA). Two different types of EABRs were observed in normal animals, depending on the electrical pulse intensity applied to the round window: long-latency brainstem responses were evoked by low stimulation intensities, short-latency brainstem responses by high intensities. The absence of effect of strychnine applied intracochlearly ruled out the possibility of medial efferents being involved in these responses. Conversely, an intracochlear application of tetrodotoxin (TTX), an Na(+)-channel blocker, resulted in the disappearance of both types of responses, attesting that the sites activated by the electrical stimulation were located within the cochlea. In AOAA/ amikacin poisoned cochleas, in which most of the hair cells were missing with apparently normal ganglion neurons, the long-latency brainstem responses evoked by low intensities were completely lacking. These findings suggest that low currents applied to the round window of the guinea pig cochlea primarily activate the hair cells, the neurons being directly excited at higher intensities.