The Atlas · Chapter 1

Generators & Neuroanatomy

The cortical auditory evoked potential is the last and largest link in a chain of responses that runs from the cochlea to the cortex. Knowing where it is generated explains both what it can tell us — that sound reached the cortex — and what it cannot.

One pathway, many responses

A single sound sets off electrical activity all the way up the ascending auditory pathway — and we can record it from the scalp at every level, separated by whenit arrives. Within the first 10 milliseconds we see the auditory brainstem response (ABR) from the eighth nerve and brainstem nuclei. Between roughly 10 and 50 ms the middle latency response appears, with contributions from the thalamus and primary cortex. From about 50 ms onward comes the slow, late cortical auditory evoked potential— the P1–N1–P2 complex [3].

CochleaAuditory nerveCochlear nucleusSuperior oliveLateral lemniscusInferior colliculusMedial geniculate (thalamus)Auditory cortex (Heschl's gyrus)Late: P1–N1–P2Middle latencyEarly: ABR
The same ascending pathway generates every auditory evoked response — but at different levels and different times. The brainstem stations produce the early ABR within 10 ms; the thalamus adds the middle latency response; and the auditory cortex generates the late P1–N1–P2 complex tens to hundreds of milliseconds after the sound. A cortical response therefore certifies that sound has reached and been processed by the cortex. Schematic — not to scale.

Each response interrogates a different stretch of the same pathway. The ABR asks whether the nerve and brainstem can fire in tight synchrony; the cortical response asks a higher-level question — whether sound has been detected and processed at the cortex. That difference is the whole clinical value of the late response.

Where the late response is generated

The obligatory cortical response is not generated at a single point. Its largest component, N1, has several overlapping generators in and around the supratemporal plane of the auditory cortex, together with contributions that give it its characteristic vertex-maximum scalp distribution [4]. P1 and P2 reflect activity in primary and secondary auditory cortical areas and their thalamocortical input. Because the sources sit in auditory cortex, the response is largest at the top of the head — the reason the recording electrode goes at the vertex.

The decisive clinical fact follows directly from this anatomy: a present cortical response means the signal travelled the entire pathway and engaged the cortex. The slow vertex response to sound was described in the earliest days of human electroencephalography[1], and its dependence on stimulus level — the property that makes it useful for audiometry — was quantified soon after [2].

Obligatory versus cognitive responses

The cortical responses split into two families. The obligatory (or exogenous) responses — P1, N1 and P2 — depend on the physical stimulus and appear whether or not the listener attends to the sound. This is exactly what makes them usable in the clinic: a child or a medicolegal claimant cannot suppress them at will, and they can be recorded passively while the patient sits quietly [5].

The cognitive (endogenous) responses — the P300 and the mismatch negativity— depend on attention, memory and the detection of an oddball among standard sounds. They index processing rather than audibility, are far more variable, and are largely research tools. Throughout this atlas, “the cortical response” means the obligatory P1–N1–P2 complex unless stated otherwise.

A cortex that must be awake

One more consequence of generating the response in the cortex: it depends on arousal. The ABR is generated low in the pathway and survives sleep and sedation, which is why infants are often tested asleep. The late cortical response is the opposite — N1 in particular shrinks and degrades in drowsiness and sleep[5]. Cortical audiometry therefore requires an awake, relaxed but alert patient, a constraint that shapes the whole recording technique.

The pathway in summary.Cochlea → auditory nerve → brainstem (ABR) → thalamus (middle latency) → auditory cortex (late P1–N1–P2). The cortical response is generated in auditory cortex, is largest at the vertex, appears only when the listener is awake, and certifies detection at the cortex — but says nothing about how the sound is understood.