Cochlear Implants

Introduction

A hearing aid with a cochlear implants translates incoming sound into electrical signals that are directly stimulating the nerves in the cochlea. The process of converting the sound waves into electrical current is done by a sound processor outside of the ear. The steps are the following:

CI signal processing scheme

• Microphone picks up sound
• ADC converts analog signal to digital signal
• Pre-filtering is applied for speech emphasis
• Automatic Gain Control (AGC) is applied
• Signal is split into frequency bands by a filter bank
• Hilbert Envelopes (amplitudes) are calculated per band
• Signal amplitudes per band are mapped to current amplitudes
• Current amplitudes per band are convoluted with spiking patterns
• The resulting signals sent to the electrodes

Automatic Gain Control

Compression

See Dynamic_range_compression on wikipedia for more background info.

The mapping function

The mapping function (from Advance Bionics)

The mapping function translates the Hilbert Envelope Amplitudes for each channel to electrical signal amplitudes.

The function has the following form according the Advanced Bionics:

I = ((M-T)/IDR) * (L - KNEE + IDR + GAIN) + T

where

• I is the electrical output in current units
• M is the most comfortable level in current units (CU)
• T is the threshold level in current units (CU)
• L is the loudness signal (Hilbert Envelope amplitude) in dB SPL
• KNEE is the kneepoint of the compression function in dB SPL
• IDR is the input dynamic range in dB SPL
• GAIN is the channel gain in dB SPL

N.B. Current units (CU) is defined differently by different manufacturers.

It follows from the equation that when L = KNEE - GAIN the electrical output (I) is right at the M-Level. Since the GAIN is set to zero in most cases, the output is at the M-level when L = KNEE.

Terminology of Critical Points in the Mapping Function

In scientific literature, three key levels are recognized: 1. hearing threshold, 2. comfortable level, and 3. pain threshold, each denoted in various ways.

The following abbreviations are commonly used to describe specific levels of electrical stimulation:

• T, THR, or THL: These stand for Threshold Level, or Threshold Hearing Level. This is the lowest level of electrical stimulation that the user can perceive.

• M, MCL, or C: These abbreviations stand for Most Comfortable Level, Maximum Comfortable Level, or Comfortable Level. This level refers to the point at which sounds are comfortably perceived—neither too loud nor too soft.

• USL, MSL, UCL, or LDL: These stand for Upper Stimulus Level, Maximum Stimulation Level, Uncomfortable Loudness Level, or Loudness Discomfort Level. This level indicates the point at which sound becomes painful or uncomfortable for the user.

The terminology can vary depending on the manufacturer. For example, for the comfort level in clinical software:

• Advanced Bionics uses 'M-level',
• Cochlear uses 'C-level',
• Med-El uses 'MCL'.

There can also be some variation in the precise definitions of these terms.

The term "Maximum Comfortable Level" is less commonly used and can sometimes refer to the highest level that is still comfortable, just below the pain threshold. This usage can be confusing and should be avoided where possible.