Experimental analyses of the source of ABR wave II

Recent reports based on direct recording from various locations along the auditory nerve in humans during neurosurgical procedures have suggested that the auditory nerve-brainstem response (ABR) wave II is generated by the same neurons which generate wave I. In order to analyze this possibility using a different approach, ABR was recorded in ten rats in response to several click intensities and click repetition rates. These studies were also repeated in ten human volunteers. The amplitudes and latencies of ABR waves I, II and III were analyzed in order to determine if wave I and II behaved in a parallel fashion with changes in stimulus intensity and latency, as would be expected from the physiological "all or none law" if both waves were generated by the same axons. Several types of analyses indicate that the amplitudes of ABR waves I and II do not grow in amplitude in a parallel fashion with increases in stimulus intensity and decreases in stimulus rate. This is evidence either for independent sources of waves I and II or for composite sources of wave II both from the auditory nerve and the cochlear nuclei.