Clinico-experimental studies on auditory evoked middle latency response (AEMLR) with specific reference to generation and auditory dominancy.

The present study was carried out on animals and humans in order to clarify auditory dominancy or lateralization and the contribution site of auditory evoked middle latency responses (AEMLRs). Normal AEMLRs on guinea pigs and humans were quite similar and consisted of two negative and two positive peaks between 8 and 50 msec following the start of an auditory stimulus. In normal human subjects, the component Pa (peak to baseline measurement) or Na-Pa (trough to peak measurement) was significantly greater in the temporal area contralateral to the stimulated ear than in the ipsilateral temporal area. In guinea pigs, however, AEMLRs were attenuated only by stimulation contralateral to the side, on which a lesion was made by unilateral aspiration of the lemniscus or the inferior colliculus. This auditory lateralization and contralateral dominancy were also verified by the direct cortical recording of AEMLRs in humans and also by analysis of auditory evoked brain mapping. In clinical studies, AEMLRs have been obtained even in a premature infant born after a 44-week pregnancy. In patients with well localized lesions of the brain stem, all components of the AEMLR to stimulation contralateral to the lesion side were affected. However, component Pa of the response to stimulation contralateral to the lesion was mainly abolished in patients with unilateral thalamic or temporal lesions. The following results were obtained: 1) component Pa (or Na-Pa) of AEMLR has significant lateralization, 2) the generation site of Pa may be in the subcortical thalamic projection of the contralateral lobe, 3) component Po (or No-Po) is a true neurogenic response but is frequently enhanced by the post-auricular reflex, 4) the contralateral inferior colliculus is very important for the generation of Po, 5) the auditory system (hearing) may have contralateral AEMLR dominancy.