Evoked Potential: Audition

Compound Action Potentials and Compound Synaptic Potential

The neural activity underlying AEPs, typically evoked by clicks or short tone pips, consists either of compound action potentials, that is, composed of firings occurring at the same time, in nerve tracts that make up the auditory brain stem response (ABR), or of simultaneously occurring postsynaptic potentials in cortical cell dendrites that make up the cortically generated evoked potentials. Action potentials are of much shorter duration (about 1 ms) than synaptic potentials (about 15 ms) and because the extracellularly recorded activity is biphasic, action potentials can cancel each other for small time differences whereas postsynaptic potentials typically are much less sensitive to this. In both cases, the amplitude of the AEP components is proportional to the number of synchronously activated nerve fibers (in case of the ABR) or number of synchronously activated synapses on cortical pyramidal cell dendrites. AEPs can only be detected if the generating elements (nerve fibers, dendrites) are spatially aligned and currents therein flow in the same direction.

The Auditory Brain Stem Response

The ABR is a short latency subset of the AEPs. It is a multi-peaked response that alternates between vertex positive and negative voltages starting with a positive peak termed wave I, followed with 1-ms interval by further positive peaks II and III, and then by an often double-peaked wave IV/V. Waves I and II originate from the extracranial and intracranial part of the auditory nerve, respectively, wave III from the first auditory nucleus in the brain stem, and wave IV/V from the tract of the lateral lemniscus on its way to the auditory midbrain. The ABR threshold matches that of behavioral measures. The amplitude at the scalp is small (about 0.1 mV). The latency difference between waves I and V represents the brain stem conduction time, including synaptic delays and nerve conduction. [Page 398]This latency difference is a sensitive indicator of auditory brain stem maturation in the first year of life and of the presence of tumors pressing on the auditory nerve or brain stem, diseases such as multiple sclerosis that slow nerve conduction, and auditory neuropathy. Immaturity and the listed disorders all increase the brain stem conduction time or make waves beyond wave I undetectable or, as in auditory neuropathy, do not show any wave in the ABR. The ABR is unaffected by sleep and light sedation or anesthesia and is therefore of prime interest in newborn hearing screening.

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