Changes in acoustic distortion during long term electrical stimulation of the guinea pig cochlea: a preliminary study

In a preliminary study, acoustic distortion was assessed as a measure of cochlear function in both implanted and contralateral ears of electrically stimulated guinea pigs. Prestimulation distortion levels in implanted ears were below those of unimplanted ears and below those previously reported for guinea pigs. This may have been due to the mechanical or irritant effect of the electrode. In both the ipsilateral and contralateral ears or stimulated animals, distortion levels were depressed during the period of electrical stimulation. In addition, the slope of distortion growth as a function of stimulus level was more gradual in stimulated ears and contralateral ears of the same animals for f2 = 6 kHz and 9 kHz, but was unaffected for f2 = 3 kHz. When the distortion could no longer be recorded in the stimulated ear (4 subjects) or contralateral ear (1 subject) the animal was killed and histological evaluation carried out in both ears. Histological changes were restricted mainly to the efferent terminals in those ears where some distortion was still recordable. In one animal where distortion had become unmeasurable immediately prior to death, the outer hair cells were severely damaged and undergoing lysis. In this preliminary study, intersubject and between-test variability in distortion for implanted (ipsilateral) ears indicated that additional controls are necessary to establish the validity of alterations in acoustic distortion to reflect cochlear pathology.