In eight patients with unilateral pontine and midbrain lesions, brainstem auditory evoked potentials (BAEPs) were studied with ipsilateral (Cz-Ai) and contralateral (Cz-Ac) recordings after monaural stimulation. In all cases, the most prominent abnormality was noted in BAEP generated by stimulating the ear ipsilateral to the lesions. The Cz-Ai and Cz-Ac patterns showed similar abnormalities in five of the patients and dissociated abnormalities in two patients. We conclude that: (1) Lateralization of BAEP is possible in unilateral pontine and midbrain lesions. (2) Monaural stimulation with Cz-Ai and Cz-Ac recordings is essential for lateralization. (3) The BAEP in monaural stimulation is predominantly generated from the auditory structures ipsilateral to the stimulated ear.

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