Question 2 of 7: Cochlear Implant

Question 2: Cochlear Implant (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Functional components and engineering principles. A cochlear implant bypasses a non-functioning organ of Corti and stimulates the surviving auditory (spiral ganglion) nerve fibres directly with electric current. The system has five functional blocks, shown below: an external microphone picks up sound; a speech processor band-splits the signal into a bank of frequency channels (typically 12–22, mimicking the cochlea's own tonotopic frequency-to-place mapping) and computes an amplitude envelope per channel; a transmitter coil, held against the scalp by a magnet, sends the coded signal (and the power to run the implant) across the intact skin by inductive RF coupling; an implanted receiver-stimulator demodulates the RF signal, recovers channel amplitudes and generates biphasic current pulses; and an electrode array, threaded into the scala tympani of the cochlea, delivers each channel's pulses to electrode contacts positioned along the tonotopic axis (base = high frequency, apex = low frequency), directly depolarizing the nearby spiral ganglion fibres.

(ii) Transcutaneous power and signal transmission. Both the coded audio information and the power to operate the implanted receiver-stimulator are sent across the intact skin on a single inductively-coupled RF link (a pair of matched coils, external transmitter and implanted receiver, held in alignment by cooperating magnets) — this avoids any percutaneous connector, which would be a chronic infection risk. The carrier (typically a few MHz) is amplitude- or frequency-modulated to encode the per-channel pulse timing/amplitude data; the implant rectifies and regulates the received RF power to run its own stimulation circuitry, so no implanted battery is needed.

(iii) Effect of more electrode surfaces (contacts). More electrode contacts along the array give finer spatial (tonotopic) sampling of the cochlea's frequency map, which in principle improves spectral resolution and speech perception, particularly in noise and for music/pitch perception. In practice the benefit saturates well before the full array is used independently, because current spread in the conductive perilymph fluid causes adjacent electrodes to activate overlapping populations of spiral ganglion fibres — typically only about 8 effectively independent spectral channels are achievable even with 12–22 physical contacts, so beyond that point additional electrodes mainly add stimulation flexibility and redundancy (for electrode failure) rather than further improving perceived frequency resolution.

(iv) Early implantation vs. waiting. For a child deaf from birth, implantation early (ideally before ~12–18 months of age) is preferred over waiting until the teenage years, because it takes advantage of the critical/sensitive period of auditory-cortex plasticity during which the developing brain is best able to learn to interpret a novel, electrically-coded sound representation and to develop spoken language in step with hearing peers. Waiting until adolescence means the auditory cortex has had a decade or more without any patterned auditory input and has typically been substantially recruited by other sensory modalities (cross-modal reorganization, e.g. for vision), so speech-perception and spoken-language outcomes after late implantation are markedly poorer than after early implantation, even though the electrode array and electrical stimulation are identical in both cases.