Clarifying the role of the midbrain in identification of changes in the acoustic environment

Abstract

Background. In some cases of impaired perception of auditory information, patients are diagnosed with preservation of the inner ear, direct conduction through brainstem structures, and functions of the cerebral cortex. The neurophysiological basis of such disorders remains unclear. In addition, no reliable method has been developed to answer conclusively the question at what level of the auditory system the functional disorders emerge. Such situation may lead to diagnostic errors and, as a result, to a delayed and significantly more expensive treatment.

Aim. To identify scalp analogues of the midbrain activity associated with changes in the acoustic environment, which could be used for noninvasive assessment of the midbrain functional safety. The beginning and the end a simple tone sound were taken as a change in the acoustic environment.

Methods. The study analyzed and compared the midbrain activity recorded with deep electrodes in 6 patients during intraoperative monitoring (IOM). The midbrain activity was recorded with scalp electrodes in 17 healthy volunteers. Evoked potential (EP) responses to simple tones produced with a modified oddball technique were analyzed.

Results. Peaks reflecting the audio signal conduct were clearly distinguished on the patients' EPs. Complexes of short-latency peaks associated with the beginning and the end of the sound were identified, as well as the long-latency peak E that is recorded after the end of the tone. Two complexes of peaks associated with the beginning and the end of the sound were recorded on the EPs of healthy volunteers. These complexes can serve as an indirect confirmation of the correct assessment of changes in the acoustic environment by the midbrain. There were no analogues of the deep peak E on the scalp EPs.

Conclusions. The complexes of peaks detected on deep EPs reflect the midbrain response to the beginning and the end of the sound, as well as the division of the sound stream into separate units. The complexes of peaks detected from the scalp in response to the beginning and the end of the sound can serve as an indirect confirmation of the correct assessment of changes in the auditory environment in the midbrain. These complexes can be recommended as an additional method for studying the auditory perception, which allows highly reliable determination of the functional safety of midbrain structures.