6 Auditory system

• Deafness:

  • 81 dB or greater hearing threshold (“4FAHL”: avg at 0.5, 1, 2, 4 kHz)

• Disabling hearing impairment

  • 35 dB or greater hearing threshold (avg at 0.5, 1, 2, 4 kHz) in better ear

• Impaired development of speech, language, cognitive skills in children

• Slow progress in school

• Difficulties in obtaining, performing and keeping an occupation

• Social isolation and stigmatization in all ages and both sexes

• Profound social and economic effects in communities and countries.

At first: only one wire (single channel), not possible to understand speech, but ok with lip reading

With an implant, instead, the devices pick up sound and digitize it, convert that digitized sound into electrical signals, and transmit those signals to electrodes embedded in the cochlea. The electrodes electrically stimulate the cochlear nerve, causing it to send signals to the brain.

External:

• one or more microphones that pick up sound from the environment

• a speech processor which selectively filters sound to prioritize audible speech

• a transmitter that sends power and the processed sound signals across the skin to the internal device by radio frequency transmission

Internal:

• a receiver/stimulator, which receives signals from the speech processor and converts them into electric impulses

• an electrode array embedded in the cochlea

amplifier = Verstärker

Temporal coding in nerve fibers limited to <1 kHz

• Feature-Extraction Strategies

• Waveform Strategies

  • compressed analog

  • Continuous Interleaved Sampling (CIS)

  • Advanced Combinational Encoder (ACE)

sound signal is amplified -> filtered -> envelope=fluctuation -> compression -> electrode transmits

one electrode at a time

• Simulate information reduction in CIs

• vocoder can be used to reduce the complexity of audio information -> signal processing

• In general, a rich input signal is replaced by a sparse output signal

• vocoders have become synonymous with cochlear implant (CI)

• Simulate information reduction in CIs for normal-hearing listeners

• Shows information that is transmitted by a cochlear

1. Compressed Analog

2. Continuous Interleaved Sampling (CIS)

3. Advanced Combination Encoder (ACE)

4. Feature Extraction (F0/F1/F2 strategies)

Stimulation Modes:

• Monopolar: wide current spread, low threshold (clinically common).

• Bipolar/Tripolar: more focused stimulation, but higher thresholds.

Put up an electric field -> cell membrane gets charged (channels open for ions)

pulse usually is followed by an action potential

3: not beeing used, too quiet

Pulse Types:

• Biphasic pulses: preferred for charge balance.

• Vary amplitude, phase duration, and inter-phase gap (IPG).

Pulse = Stimulus

The leaky integrate-and-fire model:

1. Simulates auditory nerve response.

2. Includes spike threshold, noise, and refractory period.

3. Captures firing probability, rate adaptation, and latency.

bucket gets filled and bucket has holes

🔸 Rate Adaptation:

• High stimulation rates → adaptation, decreased neural response.

• Limits how fast pulse trains can carry useful information.

action potential

High stimulation rates → adaptation, decreased neural response.

with rate adaptation (if there are too many pulses, AP can not keep up - nerve would eventually stop firing)

Numbers=elektrodes are ranked by pitch by patient

rankings are off because nerver fibers have died, so multiple electrodes stimulate the same instead of different

goal: patients can distinguish the electrodes

Limitation: current spread and # of electrodes

Solution: Use current steering: gain a few “extra pitches” in-between electrodes by steering “center of gravity” of current from one electrode to another

compression

Comfortable level (C-level)

Threshold (T-level)