Spatial Distribution of Brainstem Auditory Evoked Potentials and Their Alterations in Lesions of the VIIIth Nerve and Brainstem1

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Abstract
Brainstem auditory evoked potentials (brainstem AEPs) were simultaneously recorded from 13 scalp and earlobe electrodes from normal subjects employing a noncephalic reference. The scalp distributions of the individual components (waves I-V) were presented as isopotential maps with the use of a topographic computer display system. Binaural clicks produced symmetrically distributed brainstem AEPs over the scalp. With monaural stimulation, the topography of the responses differed in locus of maximum amplitude for each of the components, suggesting that different generators are involved in the production of these components (for example, wave V is of maximal amplitude with the shortest peak latency over the contralateral frontal area). Wave I was the only component that reversed its polarity according to electrode locations. Other waves were positive over the scalp and earlobes in confonnity with the concept that they are volume conducted, far field potentials. Brainstem AEPs in subjects with lesions in the Vlllth nerve and brainstem have different distributions from those of normal subjects, that is, reversal of polarities of the components after wave I at the ipsilateral earlobe and generalized reduction of their amplitudes over the scalp and contralateral earlobe. Thus an accentuation as well as an attenuation should be carefully evaluated in the clinical assessment of brainstem AEP changes associated with brainstem lesions, for brainstem AEPs are commonly recorded from the vertex referenced to the ipsilateral earlobe. These alterations in the observed field distributions, including polarity reversals of brainstem AEPs, seem to reflect changes in the spatial properties of the generators associated with brainstern lesions, such as a reduction in the magnitude of currents with a possible deviation of the dipole axes assuming that the generator for a given component is approximated by an equivalent dipole layer source. Brainstem auditory evoked potentials (brainstem AEPs) that can be recorded from the scalp of humans have been considered far field reflections of the potentials generated within the brainstem auditory pathways. In contrast to the long-latency AEPs, it was suggested that the position of the scalp electrode is not critical in determining the waveforms of brainstern AEPs because of the large distance of the electrode from the supposed generators. The concept of far field thus defined by Jewett and Williston (1971) has led many workers to record the potentials only from a single electrode at the vertex with the earlobe or mastoid ipsilateral to stimulation as a reference in clinical applications. In our laboratory, however, simultaneous bilateral recordings with C3 to Al and C4 to A2 configurations have been employed. Brainstem AEPs obtained froin both sides were similar in morphology in normal subjects except for wave I, and in lesins of the VIIIth nerve and brainstem considerable asymmetries were recorded. These asymmetries of the brainstem AEPs were correlated with the site of the lesions (Hashimoto et al., 1978; Hashimoto et al., 1979a). Apart from the clinical implications of the asymmetric brainstem AEPs, we think that such profound differences seen at the different locations on the scalp are seemingly inconsistent with the definition of the volume conducted far field potentials. To our knowledge, there have been few studies involving detailed mapping of the distribution of the potentials in humans (Picton et al., 1974; Streletz et al., 1977; Martin and Moore, 1977). In mapping studies, however, amplitudes and latencies of the brainstem AEP components were measured on separate recordings from various locations on the scalp, presenting the obvious problem of run-to-run variability. The main objectives of the present study were (1) to map the scalp distribution of each component of brainstem AEPs in normal subjects and patients with VIIIth nerve and brainstem lesions on the basis of simultaneous recordings from multiple electrodes and (2) to relate the altered distributions to the lesions involving various levels of the brainstem. The scalp distributions of the components were presented as isopotential maps with the use of a topographic computer display system (Veno and Matsuoka, 1976).

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