Physiologic identification of eighth nerve subdivisions: Direct recordingswith bipolar and monopolar electrodes

The purpose of this study was to determine stimulation and recording parame ters that maximize differences in evoked responses recorded between the coc hlear nerve and the surrounding tissues. Click-evoked potentials were obtai ned using monopolar and bipolar recording electrodes placed directly on the exposed eighth nerve of anesthetized cats. Responses were compared as stim ulus intensity, electrode location, and bipolar electrode orientation and i nterelectrode spacing were systematically varied. Wave amplitudes increased monotonically with intensity for both monopolar and bipolar configurations , but bipolar configurations exhibited greater selectivity in differentiati ng cochlear from vestibular subdivisions. The optimal stimulus intensity wa s 70 to 80 dB peak sound pressure level (pSPL). Monopolar recordings were o ften confounded by activity originating at remote sites, typically the coch lear nucleus and (for recording sites on the vestibular nerve) the cochlear nerve. Bipolar response amplitudes increased with interelectrode spacing a nd were largest when electrodes were oriented parallel to the long axis of the nerve. Extrapolation of empirical data indicated that amplitudes of bip olar responses would be maximal at an electrode separation of 7.5 mm. Cochl ear nerve conduction velocity, calculated from wave latencies at each of th e two monopolar electrodes, was 11.6 +/- 1.6 m/sec.