Auditory Evoked Potentials and Electrophysiology

Auditory evoked potentials are small electrical responses of the auditory nerve and central pathway to sound, recorded from surface electrodes and extracted from background activity by averaging many repetitions. The most clinically used is the auditory brainstem response, a series of waves generated within the first milliseconds after a stimulus that allows hearing to be estimated and the auditory pathway to be tested objectively, without any behavioural response.

Definition

Auditory evoked potentials are electrical potentials elicited by acoustic stimulation and recorded from the scalp or ear, time-locked to the stimulus and averaged to reveal the responses of the auditory nerve and central auditory pathway.

Scope

This entry covers the principle of signal averaging, the main response classes by latency (electrocochleography, the auditory brainstem response, and later cortical and steady-state responses), and how these measures are used for objective threshold estimation, newborn screening, and site-of-lesion testing. It is a reference description of the methods, not clinical interpretation for individuals.

Core questions

• What electrical response does the auditory nerve and brainstem produce to sound?
• How does signal averaging extract a tiny response from larger background activity?
• How are evoked potentials used to estimate threshold when behaviour is unavailable?
• How do response patterns help localize dysfunction along the pathway?

Key concepts

• Signal averaging and time-locking
• Electrocochleography
• Auditory brainstem response (ABR)
• Wave I-V latencies and morphology
• Auditory steady-state response (ASSR)
• Cortical auditory evoked potentials
• Objective threshold estimation
• Site-of-lesion testing

Mechanisms

An acoustic stimulus, such as a click or tone burst, is presented many times while electrodes record scalp activity; because the genuine response is time-locked to each stimulus and the background is not, averaging across repetitions reinforces the response and cancels random activity, revealing potentials measured in fractions of a microvolt (Jewett & Williston 1971). Responses are grouped by latency: electrocochleography captures the earliest cochlear and nerve potentials, the auditory brainstem response comprises a characteristic sequence of waves over the first several milliseconds generated along the nerve and brainstem, and longer-latency middle and cortical responses follow. Lowering the stimulus level until the response disappears estimates threshold objectively, while the latencies and morphology of the waves give information about where along the pathway a problem lies. The auditory steady-state response uses modulated tones and automated detection to estimate frequency-specific thresholds. Comparing an absent or abnormal evoked response with present otoacoustic emissions is the basis for identifying disorders of neural transmission with preserved cochlear function (Starr et al. 1996).

Clinical relevance

Auditory evoked potentials provide objective estimates of hearing and information about the integrity of the auditory pathway when behavioural testing is not possible or needs corroboration, which is why the automated auditory brainstem response is, with otoacoustic emissions, a foundation of newborn hearing screening. They are interpreted as part of a battery alongside behavioural and other objective measures. This entry describes how the responses are recorded and what they reflect; it is not a basis for individual diagnosis or treatment.

Epidemiology

Automated auditory brainstem response testing is one of the two physiological measures used in universal newborn hearing screening programmes worldwide, alongside otoacoustic emissions, and is the preferred screen for infants at higher risk of neural hearing disorders (JCIH 2007).

History

Scalp-recorded far-field responses from the human auditory brainstem were described by Jewett and Williston in 1971, building on the development of signal-averaging computers, and the auditory brainstem response rapidly became a clinical tool for objective threshold estimation and site-of-lesion testing. Later work characterized auditory neuropathy through the dissociation of absent neural responses from preserved otoacoustic emissions (Starr et al. 1996), and automated brainstem-response screening became integral to early hearing detection programmes (JCIH 2007).

Key figures

• Don Jewett
• Hallowell Davis
• Terence Picton
• Arnold Starr

Seminal works

• jewett-williston-1971

Frequently asked questions

What is the auditory brainstem response?

It is a series of small electrical waves generated by the auditory nerve and brainstem within a few milliseconds of a sound, recorded from scalp electrodes and used to estimate hearing and test the pathway objectively.

Why is averaging necessary?

The response is far smaller than ongoing brain and muscle activity, so the stimulus is repeated many times and the recordings averaged; the time-locked response adds up while unrelated activity cancels out.

How can evoked potentials test hearing in someone who cannot respond?

Because the response is generated by the nervous system and recorded electrically, no voluntary reaction is needed, so thresholds can be estimated in newborns and other listeners who cannot give behavioural responses.