Chapter 6 — Evoked potentials

ABR, ASSR, CAEP — signal averaging from the EEG floor.

The cochlea’s response to a click (Lesson 5.2) is hundreds of microseconds long and microvolt-scale at the eardrum. The brain’s response to the same click — recorded with scalp electrodes — is a few milliseconds longer, sub-microvolt in amplitude, and buried in an EEG background that is fifty times larger than the response itself. Recovering this signal requires signal averaging: present the same stimulus thousands of times, record the time-locked scalp potential for each presentation, and average. The signal — being time-locked — survives the averaging; the noise — being uncorrelated — falls as $1/\sqrt{N}$. After a few thousand trials the response emerges from the EEG floor with a few millimetres of vertical scale.

This chapter develops three audiologically-relevant evoked potentials:

• The auditory brainstem response (ABR) — five characteristic peaks within 10 ms of a click, generated by successive stages of the brainstem auditory pathway, providing both objective threshold estimation and neurodiagnostic localisation.
• The auditory steady-state response (ASSR) — a frequency-following response to amplitude-modulated tones that gives frequency-specific objective thresholds, increasingly used as the standard pediatric threshold tool.
• The cortical auditory evoked potential (CAEP) — long-latency responses (P1-N1-P2 at 50–250 ms) that probe cortical sound processing and are clinically used in cochlear-implant outcomes and central-auditory evaluation.

The unifying principle across all three is signal averaging — a fundamental measurement technique whose mathematics is worth understanding before the individual responses are interpreted.

Three lessons:

• 6.1 Signal averaging and the EEG noise floor — the √N rule, EEG amplitude statistics, why ABR takes 1500 trials, weighted averaging.
• 6.2 The auditory brainstem response (ABR) — waves I–V, latency-intensity functions, threshold tracking, neurodiagnostic patterns.
• 6.3 ASSR and CAEP: frequency-specific and cortical responses — the steady-state response, multi-frequency parallel testing, P1-N1-P2 cortical markers.