Auditory brainstem response (ABR): the test that bypasses behavior

What the ABR measures

The auditory brainstem response (ABR), also called the brainstem auditory evoked response (BAER) or brainstem evoked response audiometry (BERA), records the electrical activity generated by the auditory nerve and the lower brainstem in the first 10 milliseconds after a sound is delivered to the ear.

When a click or tone burst is played through an earphone, the cochlea converts it into electrical signals that travel along the eighth cranial nerve (auditory nerve) through several brainstem relay stations before reaching the auditory cortex. Each relay station produces a tiny, characteristic voltage wave. These waves are detected by electrodes on the scalp and averaged over hundreds or thousands of stimulus repetitions to pull the response out of background brain electrical noise.

The five waves and what they represent

The ABR typically shows five positive peaks labeled with Roman numerals I through V, appearing within the first 10 milliseconds after each stimulus.

Wave I originates from the distal auditory nerve, close to the cochlea. Wave II comes from the proximal auditory nerve. Wave III is associated with the cochlear nucleus in the lower brainstem. Wave IV reflects activity in the superior olivary complex. Wave V, the largest and most clinically important peak, is generated in the lateral lemniscus and inferior colliculus.

Clinicians focus heavily on Wave V because it is the most robust and is present even at near-threshold levels. The absolute latency of Wave V (how long after the stimulus it appears) and the interwave intervals between peaks (particularly I-III, III-V, and I-V) are compared to normative values to assess neural conduction along the auditory pathway.

Why the test is valuable: behavior-independent assessment

Standard pure-tone audiometry requires the patient to press a button or raise a hand when they hear a tone. That behavioral response is not always available. Newborns cannot respond voluntarily. Patients with severe cognitive or motor impairments may not be able to complete behavioral testing. In all these situations, the ABR provides an objective, physiological estimate of auditory sensitivity that does not depend on any action from the patient.

The NIDCD-supported newborn hearing screening programs in the United States, and NHS universal neonatal hearing screening in the United Kingdom, both use ABR as one of their two primary screening tools alongside otoacoustic emission testing.

Estimating hearing thresholds with ABR

By presenting stimuli at progressively lower intensity levels and tracking whether Wave V remains detectable, audiologists can estimate the softest sounds the auditory system responds to at each frequency tested. This estimated threshold is called the ABR threshold and, while it is not identical to a behavioral audiogram threshold, it correlates closely enough to guide diagnosis and early intervention decisions in infants.

Frequency-specific ABR can be achieved by using tone bursts rather than broadband clicks. This allows approximate threshold mapping at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz, giving a rough audiogram-like profile without any behavioral input.

ABR for retrocochlear assessment

Beyond threshold estimation, ABR is a key tool for evaluating the auditory nerve and brainstem pathway separately from the cochlea. When someone has asymmetric hearing loss or unilateral tinnitus, the concern is whether a lesion on the eighth nerve, such as a vestibular schwannoma (sometimes called acoustic neuroma), could be responsible.

In a retrocochlear lesion, cochlear function may be relatively intact but the ABR shows prolonged interwave intervals, particularly the I-III and I-V intervals, indicating slowed neural conduction beyond the cochlea. AAO-HNS guidelines for vestibular schwannoma recommend MRI with contrast as the gold-standard imaging, but ABR has historically been used as a screening step because prolonged interwave intervals raise the index of suspicion for a space-occupying lesion.

Auditory neuropathy spectrum disorder

One condition that ABR uniquely helps diagnose is auditory neuropathy spectrum disorder (ANSD). In ANSD, the outer hair cells function normally (producing normal OAEs) while the auditory nerve fails to synchronize its firing reliably to the timing of sound. The result is an ABR that is absent or severely disorganized even though the patient may have some measurable hearing. Standard audiometry and OAE testing alone would not capture this mismatch. ABR is essential for detecting it.

ANSD matters clinically because the management approach differs from standard sensorineural hearing loss. Some children with ANSD benefit substantially from cochlear implants, whereas traditional hearing aids that amplify sound may provide limited benefit. The NIDCD highlights ANSD as a specific condition that universal newborn ABR screening is designed to catch.

How the test is performed

Small electrodes are placed on the forehead and each mastoid (behind the ear) or earlobe. Earphones or insert earphones deliver click or tone-burst stimuli, typically starting at high intensities and working down. The electrodes pick up scalp electrical activity, which the recording system averages over 1,000 to 2,000 sweeps per condition to separate the tiny auditory response from background electroencephalographic (EEG) noise.

Muscle movement degrades the signal, so patients are asked to relax, close their eyes, and ideally sleep or read quietly. Adults typically tolerate this easily. Infants are often tested during natural sleep after a feeding, timed to coincide with a deep sleep stage.

A full diagnostic ABR, including threshold searches at multiple frequencies and retrocochlear analysis, usually takes between 60 and 90 minutes. Newborn screening ABR, which is automated and only needs a pass-fail decision, typically takes 20 to 40 minutes.

Limitations

ABR does not assess central auditory processing above the brainstem. It does not directly measure how speech is understood in noise, how auditory cortex processes language, or whether cognitive factors affect hearing. Those questions require different tests. ABR results are also affected by patient state (alertness, body temperature, certain medications) and equipment quality, so they should always be interpreted by a qualified audiologist or physician with access to the full clinical picture.

If symptoms persist or change, see an audiologist or physician.