To determine the relationship between skull surface brainstem auditory evoked potentials (BAEPs) and near-field potentials, short latency auditory-evoked potentials were directly recorded from the brainstem surface in humans and cats using monopolar and bipolar recording techniques. However, monopolar recordings did not sufficiently define BAEP generator sources. Detection results of phase-reversal bipolar recordings in cats demonstrated the importance of two-electrode positions, fiber directions, and impulse directions. In contrast, responses recorded from bipolar electrodes on the contralateral caudal pons demonstrated that only the component associated with time at P3 revealed a phase reversal. These results suggested that waves I, II, and III in humans corresponded to the same components in cats. In addition, wave III generated the structures in the contralateral pons, which was likely the contralateral superior olive to the stimulated ear. Further studies combining other experimental techniques are needed to determine the exact BAEP generators.
The present study determined the generation sites of brainstem auditory evoked potentials (BAEPs) and the effects of space-occupying lesions. The subjects were assigned to six groups: midbrain tumor, pontine intramedullary tumor, IVth ventricle tumor, C-P angle tumor, clival tumor, and cervico-medullary space-occupying lesions according to location and effect of brainstem compression. In the midbrain tumor group, most pineal tumors did not have an effect on BAEPs, because they stayed in the rostral half of the midbrain. In the midbrain tumor, which primarily involved the midbrain tegmentum, including the quadrigeminal plate, only wave V on BAEPs was severely attenuated from left and right stimulation. An additional tumor in the unilateral side of the ponto-midbrain junction exhibited attenuation of wave IV to ipsilateral stimulation, and of wave V to contralateral stimulation, from the lesion side. Results of midbrain tumors suggested that the generator(s) of wave V were located in the lower midbrain, and generators and/or pathways of waves IV and V were present on the opposite side of the brainstem to the stimulated ear. Effects on BAEPs in cases with pontine intramedullary tumors and IVth ventricle tumors the long tracts were similar, i.e., wave III was easily affected even if symptoms remained mild. In contrast, in the C-P angle tumor group, when the T/BS ratio reached 80%, the wave V or wave IV/V complex on the BAEP contralateral stimulation to the lesion side was lost. These suggested that auditory tracts generating waves IV and V were located more laterally in the lower pons than wave III. Four kinds of tumors—IVth ventricle tumor, cerebellar tumor, C-P angle tumor, and cervico-medullary tumor—exhibited similar unilateral BAEP abnormalities, i.e., wave II was absent or severely attenuated, and the succeeding components were delayed. According to MRI and neurological findings, tumor lesions in the former two cases of the above-mentioned four cases did not affect the VIIIth nerve, but rather compressed the cochlear nucleus. Those results strongly suggested that wave II was generated by cochlear nucleus. In summary, results from the present study showed the following: 1) wave V was generated in the caudal region of the midbrain tegmentum; 2) generators and/or pathways of waves IV and V were on opposite sides at the rostral pons level; 3) generators and/or pathways of waves IV and V were more lateral in the caudal half of the pons than wave III; and 4) wave II was generated in and around the cochlear nucleus.