A chronic implant for recording of cochlear potentials in primates.

A new technique for the continuous recording of peripheral bioelectrical activity in the auditory system of primates is described. Because of basic differences in the anatomy of the temporal bone, the approach to the round window of the cochlea is more difficult in most primates than in lower animals. A relatively simple surgical approach, which made possible the placement of an electrode into the perilymph of the inner ear via the well-demarcated horizontal semicircular canal was therefore developed and is described in detail. The bared tip of a Teflon-coated wire was cemented into the canal opening with carboxylate cement, and the wire attached to a permanent electrical connector on the skull. Cochlear microphonic and action potentials of 50 to 100 GV amplitude were thus recorded on a continuing basis at the same time that behavioral studies of primate auditory acuity were conducted. In recent years, the body of knowledge concerning the sense of hearing in subhuman primates has grown significantly. These data have been obtained through anatomical studies, behavioral psychophysical experiments, and the recording, on an acute basis, of the bioelectrical potentials arising within the cochlea. Behavioral methods, while time-consuming and costly, have proven rewarding; they have permitted accurate assessment of primate auditory acuity in the waking state and have provided reliable serial measures of auditory function over long periods of time. Through the use of such techniques, the effects of overstimulation and drug ototoxicity are being examined in animal models closely akin to human beings (Stebbins, '70). The value of bioelectrical potential measurements in the study of the physiology of the sensory cells of the cochlea is wellestablished (Wever, '66; Lawrence, Wolsk and Burton, '59). Cochlear potentials, recorded from the round window by acute methods, have provided valuable information on the sensitivity of the primates' peripheral transducing mechanism (Wever, Vernon and Lawrence, '58), as well as data of phylogenetic significance (PeterAM. J. PHYS. ANTHROP., 38: 351-356. son, Wruble and Ponzoli, '68). The round window exposure, however, destroys the normal anatomical integrity of the middle ear cavity of the primate; and this factor must be taken into account when these data are examined. Furthermore, although chronic experiments of short duration have been performed with round window electrodes in cats (Marsh and Worden, '69), it is virtually impossible to maintain an electrode in a constant relationship to the primate round window membrane for any length of time after the initial placement. For these reasons, the present experiments were carried out to develop a long-term implant whose installation would preserve normal middle ear physiology. The bony horizontal semicircular canal is a prominent anatomical landmark in clinical mastoid surgery. It was utilized by Lempert ('38) for the production of an artificial fenestra as a pathway for sound transmission to the cochlea in cases of otosclerosis. This canal is rather easily exposed through a mastoid approach, which does not disturb the middle ear cavity. The present study demonstrates the feasibility 1 This work was supported in part by NIH Program Project grant NS 05785-08 and NINDS Special Fellowship 5Fl l NS2425-02.