6. Cochlear implants

acoustics -> periphery -> neuronal processing -> perception

• fluid-filled, spiral structure

• basilar membrane moves in response to sound frequency

• hair cells on the membrane detect movement

• convert mechanical energy into electrical signals

• lead to a loss of compression and hence dynamic range

• result in “deaf regions” in cohlea, frequency ranges that cannot be heard

• 81 db or greater hearing threshold

• 146.000

• 35 dB or greater hearing impairment

• 250 million

3.1 billion USD

• inner ear

• provides good speech understanding in most cases, even on telephone

• Voltage

• puts 50 V batter in his ear canal and remarked a boom in his head

An early cochlear implant with one electrode that electrically stimulated the auditory nerve, providing basic sound perception for profoundly deaf individuals—developed by Dr. William House and 3M.

Michaelson 1971 and House 1976

• electrical repsponses in nerve of cat similar in frequency and amplitude to sound

• effects of electrode placement, polarity, rate and intensity of stimulus

• feature extraction

• waveform

• prototype 1977

• multi channel 1984

The loudness (signal strength) of the sound wave

1. AGC (automatic gain control): normalized loudness to keep input stable

2. Filter banks to split the sound into freq bands

3. In each Band extract envelope (slow amplitude changes over time)

4. apply compression -> fits envelope into listener´s dynamic range

5. pulse mapping -> convert envelope into electrical pulses

6. send pulses to electrodes along the cochlea

• adjust thresholds: It maps soft sounds (just above the acoustic threshold) to the user’s electrical threshold (T-level) — the minimumstimulation the user can perceive.

• set maximum: It maps loud sounds (like shouting) to the user's C-level — the maximum comfortable level of stimulation.This avoids overstimulation or pain.

• follow loundess growth: The compression function follows a nonlinear (usually logarithmic) curve that mimics natural loudness growth.

• A vocoder (voice encoder) is a system that analyzes speech by extracting amplitude envelopes from multiple frequency bands.

• It replaces fine temporal structure with a carrier (noise or sine wave) and resynthesizes the signal.

• Used to simulate cochlear implant processing for normal-hearing listeners.

• 1. filtering 2. evelope extraction 3. carrier modulation 4. recombination

noise:

• band-limited white noise

• rough, robotiv, degraded

• pitch cues mostly lost

sinusoidal:

• pure sine wave per frequency band

• preserves some pitch cues

• clean, tonal, musical

• monopolar electrical stimulation:

  • one active electrode inside cochlea, one reference electrode outside cochlea

  • low threshold and less focused but large current spead

• bipolar stimulation:

  • Two electrodes inside the cochlea: one active, one return

  • More focused stimulation but Requires more power, narrower spread

• Tripolar stimulation:

  • One active electrode, flanked by two return electrodes

  • Very focused, best spatial resolution but High energy demand, used mostly in research

• These are waveforms of electrical stimulation used to safely and effectively activate neurons

• monophasic pulse:

  • single one directional current

  • not charged balanced -> risk of tissue damage

• biphasic pulse:

  • two oppostie phases with IPG (interphase gap can improve neural responsivness)

  • charge balanced, standard cochlear

• Strong initial phase, followed by long, weak opposite phase

  • Still charge-balanced

  • Designed for efficient neuron activation

• Higher intensity = more current → stronger depolarization

• Longer duration = more charge transfer → greater chance of reaching threshold

• In symmetric biphasic pulses, the first (usually cathodic) phase is the main driver of neural firing

• If strong or long enough, it triggers a spike before the opposite phase cancels it out

• leaky integrate-and-fire

• model can reproduce firing probability for single monophasic pulses

• dependence on pulse current amplitude & pulse duration

A simplified mathematical model of a neuron that:

• Integrates incoming current to raise the membrane potential

• Leaks charge over time (like a leaky capacitor)

• Fires a spike when a threshold is reached, then resets

1. Phase-locked responses occur when neurons fire at a consistent phase of a repeated stimulus (e.g. a tone or pulse).

   • This is strongest for low-frequency or low-rate stimuli (e.g. <1000 Hz).

   • Important for encoding temporal details like pitch or rhythm.

2. Rate adaptation refers to the decrease in firing probability over time during ongoing stimulation (e.g. pulse trains).

   • Caused by refractory periods, synaptic fatigue, or ionic adaptation.

   • Leads to fewer spikes per stimulus at high pulse rates.

3. When stimulation rate increases:

   • Phase-locking becomes less precise

   • Neurons may skip pulses or fire inconsistently

   • Temporal coding degrades, especially above ~1000 Hz

4. This explains why:

   • Auditory neurons respond better to slow, sparse input

   • Speech perception is clearer at moderate rates

   • Cochlear implants balance rate to optimize clarity without causing adaptation

Neural Response Telemetry

Measure the nerve`s response to electric stimulation by recording the potential on the CI electrodes

• Pulse rate discrimination = the ability to distinguish between different rates of repeated electrical pulses (e.g. 200 Hz vs. 400 Hz).

• It measures the auditory system’s temporal resolution – how precisely it can follow changes in timing.

• Spectral Resolution refers to the ability to distinguish between different frequencies (or pitches) in sound. In cochlear implants, this is related to how finely the system can discriminate between different frequencies through the electrodes.

• Place Pitch refers to the perception of pitch based on the position of the stimulating electrode within the cochlea. Different positions of the electrode evoke different pitch perceptions.

Key Points:

• Electrode Place: Different electrode locations in cochlea implants give rise to different perceived pitches.

• Pitch Matching: There are often multiple electrodes evoking the same pitch, which challenges the idea of a direct, tonotopic mapping of frequencies.

• Complexity: Place and rate of stimulation together affect pitch perception, not just electrode position alone.

Current spread and #electrodes