Cochlear implants Flashcards
The block diagram represents …
a contemporary CI system, consisting of an external unit, a transcutaneous radio-frequency (RF) transmission unit, an internal unit, and a fitting unit (Zeng et al., 2015).
external unit
” The external unit is often called a sound processor.
“ On the external unit is a microphone, which captures acoustic signals and transduces the input into an electrical signal.
digital signal processor
” Next, the signal is received by the digital signal processor (DSP).
o The DSP controls signal flow from this point through RF transmission.
o The use of a DSP in a front-end design ensures that the device accurately captures the acoustic environment.
amplifier
The electrical signal is now sent through an amplifier to improve the signal-to-noise ratio during transmission and to boosts the high-frequency components of the signal. From the amplifier, the DSP will convert the electrical signal into a digital signal and classify the stimulus according to intensity, frequency, and time domains. The basis for sound processing strategies in a cochlear implant is an extraction of the envelope information.
battery
” The external unit contains a battery attached to the sound processor unit (DSP and power amplifier).
o The internal CI does not have a battery.
o All power for the internal device moves across the skin from the transmitter coil.
o As such, the burden of power for the entire device rests upon what power is received by the sound processor.
RF coil
” After the signal is analyzed by the DSP of the external unit, it is converted into an electrical code that represents key features at the level of the auditory nerve, and the electrical signal is sent along the cable to the RF coil for transmission.
“ The coil converts the signal from electrical to electromagnetic signals and transmits to the receiving coil (which includes an internal antenna and magnet) via radio waves.
o The electromagnetic pull allows the energy to move across the skin.
application-specific-integrated-circuit (ASIC)
” First, it derives power from the RF signal, which serves as the power supply for the internal device.
“ The ASIC also decodes information from the RF signal and subsequently delivers electrical stimulation to electrodes.
“ The ASIC decoder, which receives information from the internal antenna, makes decisions about what to do with the incoming signal.
transmission of pulses
The signal is converted into a code for electrical pulses based on characteristics of the input signal and a set of rules, also known as a coding strategy. The stimulator then allows electrical pulses to be sent along the electrode lead to the stimulating intra-cochlear electrode contacts at different points in the cochlea, depending on frequency information. Multi-channel CIs take advantage of the natural tonotopic organization of the cochlea by delivering high-frequency information to electrodes located at the basal end of the cochlea and low-frequency signals are sent to more apical locations. Due to the localized excitation of the cochlear nerve fibers, place pitch information is available to the listener for speech understanding. Last, the electrical stimulation returns to an extra-cochlear electrode contact which serves as the ground/reference electrode. Different from acoustic stimulation, an auditory nerve is directly activated by membrane potential changes (depolarization) in electric stimulation. No mechanical tuning is present in electric stimulation, but rather the excitation pattern is determined by the electric field distribution, the cochlear electrical impedance, and the excitability of the nerve tissues (van den Honert and Stypulkowski, 1987; Abbas and Brown, 1988).
back-telemetry
” The ASIC also measures feedback signals from the electrodes through the back-telemetry component of the internal device and transmits these measurements back to the sound processor or the clinical fitting system (Zeng et al., 2015).
o This process allows for close, accurate monitoring and measurement of electrode impedance, electrical field distribution, and nerve activities (Zeng, 2004).
o The impedance monitoring can detect both open and shorted electrodes, while electrical field distribution and nerve activities allow objective measures of electrode interaction and nerve survival.
o These measures are becoming increasingly important for the objective fitting of CIs, particularly in children who cannot provide reliable subjective responses (Brown, 2003).
fitting unit
” The fitting unit allows each implant user to be individually fitted or mapped to ensure safe and effective electric stimulation.
o The fitting unit is not worn by the user but used only by clinicians to adjust connection and stimulation parameters for optimal performance (Zeng et al., 2015).
“ Both the choice of different speech processing strategies and the number of adjustable parameters has increased significantly with the modern devices, allowing an individual user to store multiple maps in the speech processor for listening to different sounds in different environments such as quiet versus noise and speech versus music (Zeng, 2004).
Continuous Interleaved Sampling (CIS) - General
” With Continuous Interleaved Sampling (CIS), the acoustic signal is digitally filtered and sent through a bank of bandpass filters.
o In a typical CIS implementation, the number of bandpass filters is identical to the number of electrodes in the internal device being used (Zeng, 2004).
o Each electrode is stimulated sequentially during each stimulation cycle.
o The amplitude of stimulation depends on the energy present in each band, such that greater energy correlates to greater amplitude.
o In its basic form, CIS is available in devices from Advanced Bionics, Cochlear Americas, and Med-El.
“ The continuous interleaved sampler CIS stimulation strategy is one of the most successful speech processing strategies for electrical stimulation of the auditory nerve (Fu & Shannon, 2000)
CIS Med-El
” CIS+ is a version of CIS that utilizes expanded frequency bands from Med-El.
o High Definition CIS (HDCIS) is another version of CIS from Med-El, which in addition the use of sequential stimulation for each stimulation cycle uses sequential stimulation between neighboring stimulation sites to elicit a percept that falls between the percept elicited by the two electrodes.
o The use of a wider frequency range for Med-El’s CIS+ and HDCIS allows for the creation of virtual channels to improve spectral resolution.
o Additionally, Med-El’s Fine Structure Processing (FSP) is a CIS-type strategy with the additional modulation of the timing of pulse bursts in the low-frequency channels to provide improved temporal cues and provides information regarding intermediate pitches by using overlapping bandpass filters.
“ Theoretically, FSP should improve speech recognition, sound quality, recognition of vocal pitch, music appreciation, and music recognition.
“ Specifically, it is designed to improve perception of high-freq phonemes which are identified based on place cues (/s/, /t/, and /f/) and provide access to fine temporal structure by stimulating the low-frequency nerve fibers at the same rate as the low-frequency signal.
“ FSP-4 includes fine structure processing up to 1000 Hz with paired stimulation on some channels.
CIS AB
” Advanced Bionics’ Hi Resolution (HIRES) strategies are a version of CIS that utilizes 16 channels, stimulates at a higher maximum rate, and has higher cutoff frequencies for low-pass filters than traditional CIS.
o In Advanced Bionics’ HIRES120, current steering is incorporated to attempt to provide an increase in the number of perceptual channels to 120.
o Multiple Pulsatile Sampler (MPS) is another strategy from Advanced Bionics that is rooted in CIS.
“ MPS is a partially simultaneous strategy that stimulates two or more non-adjacent electrodes at the same time, allowing for a doubling of stimulation rates and improved temporal resolution.
NofM strategies - general
” Unlike CIS strategies, which stimulate all active electrodes during each stimulation cycle, NofM strategies evaluate the acoustic energy present in each “m” channel and then stimulated is administered to only the “n” channels with the highest amplitude inputs.
o The number of filters (“m”) can be greater than the number of stimulating electrodes (“n”), in which the envelopes from 6 to 8 subbands with the highest energy are used to stimulate the corresponding electrodes (Zeng, 2004).
o The “n” is typically referred to as maxima and remains constant for each stimulation cycle.
o Reducing the number of active electrodes results in faster stimulation rates, minimized channel interactions and masking, increased battery life.
o Occasionally, desired information lies in channels with lower amplitudes and is therefore not transmitted.
n-0f-m cochlear
” The n-of-m strategy, also known as the peak picking strategy, is currently used in the Advanced Combination Encoder (ACE) and Spectral Peak (SPEAK) processing strategies from Cochlear Americas.
o In these strategies, the envelope signals for the different channels are scanned prior to each frame of stimulation and then the highest amplitudes are identified and stimulus is delivered to those electrodes.
o SPEAK typically uses up to 8 maxima and electrodes are stimulated at a rate of 250 pps.
o ACE utilizes a faster stimulation rate of 1000 pps and is capable of using more maxima then SPEAK.
o For both SPEAK and ACE, the number of maxima may change during stimulation cycles depending on the energy of the incoming signal.
