The Atlas · Chapter 3

Recording Technique

The cortical response is large and forgiving in some ways and unforgiving in others. The montage is simple and few sweeps are needed — but the patient must be awake, and the stimulus must be chosen to answer the clinical question.

Electrodes and montage

Because the N1–P2 complex is largest at the top of the head, the active electrode sits at the vertex (Cz), referenced to the mastoids or linked earlobes, with a forehead ground. A single recording channel is enough for routine threshold work [18].

A typical single-channel montage. The N1–P2 complex is largest at the vertex, so the active electrode sits at Cz, referenced to the mastoids or linked earlobes, with a forehead ground. The large size of the cortical response means a simple montage and relatively few sweeps suffice — unlike the much smaller ABR. Schematic — not to scale.

The response is measured in microvolts, so it tolerates a simpler montage and far fewer stimulus repetitions than the sub-microvolt ABR. The limiting factor is rarely the response — it is the background EEG noise sitting on top of it.

Stimuli: tones and speech tokens

Two stimulus families dominate. Tone bursts at the audiometric frequencies give frequency-specific threshold estimates, much as in behavioural audiometry. The stimulus needs a long enough rise time and a slow enough presentation rate to give a clean onset response; rate and rise time both shape the size of the recorded response [6].

Speech tokens — short natural sounds such as /m/, /g/ and /t/ — sample the low, mid and high frequency regions of speech. A cortical response to each token shows whether that part of the speech spectrum is audible, which is the basis of aided testing in infants [13]. Speech stimuli also bring the test closer to the everyday listening question that matters to patients [10].

Arousal state and free-field testing

The single biggest practical constraint is state. The late cortical response is generated in the cortex and weakens with drowsiness and sleep, so the patient must be awake, relaxed and quietly alert — reading or watching a silent, captioned video works well [5]. This is the mirror image of infant ABR, which is recorded during sleep.

For aided assessment the stimulus is delivered in the free field through a loudspeaker rather than an insert earphone, so the patient can be tested wearing their hearing aids or cochlear implant. The recording then reflects what the device actually delivers to the cortex.

Averaging and artefact

As with any evoked potential, the stimulus is repeated and the EEG epochs are averaged: the time-locked response adds up while random EEG averages toward zero. Because the cortical response is large, far fewer sweeps are needed than for the ABR. Epochs contaminated by blinks, movement or muscle activity are rejected, and the practical goal is to drive the leftover residual noise low enough to see the response clearly [11].

The technique in one line. Vertex-referenced single channel, tone bursts or speech tokens, an awake patient, free field when testing aided — then average enough sweeps to push residual noise below the response.