Auditory brainstem response (ABR) has been applied as one of the neurological examinations in patient with brainstem lesions. Clinically, ABR was recorded ipsilaterally to the stimulated ear. However, clicks applied to one ear possibly stimulate the opposit ear and make responses different. In order to discriminate the localization of lesion, it must be done to clarify wether the recorded ABR is the response to binaural or monaural stimulation in a strict sense. Simultaneous bilateral recordings of ABR in cats have been performed using binaural and monaural stimulation. In the case of binaural stimulation, click were delivered to bilateral normal ears and then to one ear after destruction of the opposite ear. In the case of monaural stimulation, clicks were delivered to one ear before and after destruction of the opposite ear. The results were as follows:
1) Bilaterally recorded ABRs in binaural stimulation consisted of waves 1, 2, 3, 4, and 5, and they had the same latencies and amplitudes between them.
2) In the condition of monaural stimulation, difference in latencies and amplitudes were observed between the response ipsilateral and contralateral to the destructed ear: that is, a) Latency of waves 1 and 2 prolonged (contralateral).
b) Reductions in amplitude of waves 1 and 2 (contralateral).
c) Enlargement in amplitude of waves 3, 4, and 5 (contralateral).
3) Waves 1 and 2 seem to be the potentials of ipsilateral cochlear nerve and cochlear nucleus, and waves 3, 4 and 5 to be the contralateral brainstem evoked potentials.
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