Electrophysiologic auditory tests.

Auditory stimulation elicits a complex series of electric signals in the ear and nervous system which can be used for hearing assessment, audiologic and neurologic diagnosis, intraoperative monitoring, and neurophysiologic research. The earliest components comprise the electrocochleogram. The cochlear microphonic arises from receptor potentials from cochlear hair cells, and is used to assess hair cell function in patients with auditory neuropathy. The summating potential becomes larger in Ménière's disease. The eighth-nerve compound action potential is useful for objective audiometry. Brainstem auditory evoked potentials, a series of components generated in the eighth nerve and the brainstem auditory pathways, can be used for diagnostic assessment and intraoperative monitoring of the ears and of the auditory pathways up through the mesencephalon. They are relatively easy to record, highly consistent in normal subjects, and little unaffected by surgical anesthesia. Middle-latency and long-latency auditory evoked potentials are generated in multiple areas of cerebral cortex that are activated by auditory stimulation. Anesthetic effects limit their utility for intraoperative monitoring, and substantial intersubject variability limits their utility as a diagnostic test in individual patients, but they are important research tools for the study of memory processes and the way in which the brain analyzes auditory stimuli.