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Cochlear implant mapping: T and C/M levels, electrical dynamic range, coding strategies, objective measures
The implant's map sets, for each electrode, the softest current that is heard (T) and the loudest current that is still comfortable (C or M), and which frequency band it receives. The external processor splits sound into bands and transmits through the skin, by radio, to the internal implant that stimulates the electrodes in the scala tympani.
Why it matters in the exam
Components: outside — microphone, speech processor and transmitting coil; under the skin — a receiver-stimulator with antenna and magnet, and the electrode array, which the surgeon threads into the scala tympani via the round window or a small cochleostomy. Signal and power cross the skin over a radio-frequency (RF) link, and telemetry is the implant reporting back to the processor. The T level is the softest current the user detects — set with a detection task; the C/M level is the loudest current that is still comfortable, set with loudness scaling. The electrical dynamic range — from T to C/M — is very narrow, roughly 10–20 dB. Mapping starts about two to four weeks after surgery, when the wound has healed and the patient is awake and cooperative.
The coding strategy — ACE, CIS or FSP, for example — is how the processor analyses sound and turns it into pulses. Electrodes follow tonotopy: basal ones receive high frequencies and apical ones low; a user who cannot hear /s/ needs a check of the basal electrodes that carry the highs. If the array was not fully inserted, the apical electrodes are missing and the lows suffer. To get the total stimulation rate, multiply the active channels by each channel's rate: 12 electrodes at 1200 each give 14,400 pulses every second. Facial twitching with one electrode — current spreading to the facial nerve: lower the level or switch the electrode off. Objective measures for infants: the ECAP (the nerve response recorded through the implant's own electrodes) helps the first map, and the eSRT (stapedius reflex threshold to electrical stimulation) relates closely to C/M levels. Impedance testing shows whether each electrode is sound — open or short circuit — but does not set T or C. A user who complains of noise — start with a listening check of the external microphone.
Key points
- External: microphone, processor, coil; internal: receiver-stimulator and array in the scala tympani.
- T — detection; C/M — comfort; a range of roughly 10–20 dB.
- Base highs, apex lows.
- ECAP and eSRT for infants; impedances for integrity.
- Total rate = channels × rate.
Summary table
| Concept | Definition | Use |
|---|---|---|
| T level | The softest current detected | A detection task |
| C/M level | The loudest current still comfortable | Loudness scaling; the eSRT guides it |
| Electrical dynamic range | From T to C/M, roughly 10–20 dB | Far narrower than a hearing ear |
| Frequency allocation | Base — highs; apex — lows | Trouble with /s/ — check basal electrodes |
| Impedance and telemetry | Electrode integrity; implant reports to processor | Do not set T or C |
| ECAP and eSRT | Nerve response through the implant; electrical stapedius reflex | A map for an infant who cannot respond |
| Stimulation rate | Active channels multiplied by per-channel rate | 12 electrodes at 1200 → 14,400 |
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Sources
- Wolfe & Schafer, Programming Cochlear Implants, 2nd edition (2015) — Basic terminology and objective measures
- Hughes, Objective Measures in Cochlear Implants, 2013 — ECAP, ESRT and impedance
- Wilson & Dorman, Hearing Research, 2008 — Cochlear implants: a remarkable past and a brilliant future (coding strategies)
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.