The Atlas · Chapter 2

Anatomy & Generators

The map from wave to generator anchors every ABR interpretation: five waves, five stations along an ascending pathway.

ABR waves are far-field potentials — the scalp electrode is not on the generator, only somewhere on the conductive volume of the head. What appears at the scalp is the synchronous discharge of many neurons firing in unison at one place in the pathway[5]. Wave-to-generator correspondences below are well established but not absolute; a single wave can receive contributions from multiple structures[2].

the ABR waveform (time after the click)123456msIIIIIIIVV…is a volley climbing the auditory pathwaycochleaIdistal VIII nerveIIproximal VIII nerveIIIcochlear nucleusIVsuperior oliveVlateral lemniscus / ICwave III · cochlear nucleus
Each ABR wave is the far-field signature of a synchronous volley reaching one station of the ascending auditory pathway — so the trace, read left to right, is the volley climbing from the eighth nerve (waves I–II) through the cochlear nucleus (III) and superior olive (IV) to the lateral lemniscus / inferior colliculus (V). Generators are well established but not absolute; a wave can receive contributions from more than one site. Simplified educational model.
CochleaWave IWave IIProximal CN VIII (intracranial)Wave IIICochlear nucleusWave IVSuperior olivary complexWave VLateral lemniscus → ICWave → generator
Schematic mapping of ABR waves to their primary brainstem generators.

Wave I — distal eighth nerve

Wave I is the compound action potential of the spiral-ganglion terminals at the cochlea[6]. It is the only ABR wave generated entirely peripheral to the brainstem. Its preservation in the face of later-wave abnormalities is the single most informative piece of localising information in retrocochlear pathology. Typical adult latency at 80 dB nHL click is 1.6 ms.

Wave II — proximal eighth nerve

Wave II reflects activity in the intracranial portion of the eighth nerve, between exit from the internal auditory canal and entry into the brainstem. Anatomically interesting, clinically a minor player.

Wave III — cochlear nucleus (lower pons)

Wave III arises near the cochlear nucleus in the lower pons. It is the first central wave. Lesions between distal eighth nerve and lower pons prolong the I–III interpeak interval — the canonical localisation for a retrocochlear lesion proximal to the cochlea but distal to the brainstem proper.

Wave IV — superior olivary complex

Wave IV is generated bilaterally by the superior olivary complex, the first major site of binaural integration. It commonly fuses with wave V into a single IV–V complex on routine traces.

Wave V — lateral lemniscus / inferior colliculus

Wave V is generated near the inferior colliculus along the lateral lemniscus[3]. Typical adult latency at 80 dB nHL click is 5.6 ms. Because of its scalp prominence and robustness as intensity drops, wave V is the workhorse landmark for threshold estimation.

The clinical interpeak latencies

Three interpeak latencies anchor interpretation. IPL I–III (~2.2 ms) brackets the eighth nerve and lower brainstem. IPL III–V (~1.8 ms) brackets the lateral lemniscus. IPL I–V (~4.0 ms) is the sum: the central conduction time[7].