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Optimal Contact Spacing

MED EL electrode design philosophy dictates a careful balance between maximum number of electrode contacts and the reduction of channel interaction, a factor which negatively impacts performance. Each electrode array contains the optimal number of contacts for stimulation of nerve fibres leading to best performance.

Several aspects of MED-EL’s Standard and FLEXsoft electrode arrays, as well as the overall system, are unique to the industry. Specifically, three elements of the system have been designed to control channel interaction.

  1. Contact spacing
  2. Signal processing
  3. Channel interaction compensation
     

Contact Spacing
If electrodes are placed too close together, neighbouring electrodes have the potential to trigger the same sets of nerve fibres. Even in the case of sequential stimulation, if neighbouring channels are stimulating the same nerve fibre populations, then the nerve fibres could be stimulated by the wrong channel. If the fibres are in a refractory period when their correct channel fires, a less robust response than desired would occur. Spreading the channels farther apart helps reduce this possibility.

Channel Interaction
Comparison of channel spacing between Cochlear Implant Manufacturers
When adjacent electrodes trigger the same set of nerve fibres, a loss of specificity occurs. This loss causes a blurring of the signal and has shown to be detrimental to speech discrimination. Throckmorton and Collins1 report that channel interactions affect speech recognition negatively. In their study, psychophysical data from implant users revealed pitch reversals, indiscriminable electrodes and forward masking effects.

MED-EL’s standard and FLEXsoft electrode arrays are designed to manage channel interaction. A long electrode array, coupled with 12 channels provides the opportunity to space electrode contacts farther apart. These electrode arrays have channel spacing of 2.4 mm. This large distance reduces the probability of channel interaction. Kwon2 demonstrated a small but significant improvement in consonant recognition with increased channel separation.

Signal Processing
One means of signal processing that is effective in controlling channel interaction is CIS, which stimulates sequentially. Blake Wilson et al3 describe this strategy as trains of balanced biphasic pulses that are delivered to each electrode with temporal offsets that eliminate any overlap across channels. Studies demonstrate a significant improvement in subjects using this strategy.

Channel Interaction Compensation
Recently, MED-EL has developed a coding strategy that stimulates multiple electrodes simultaneously. To overcome potential channel interactions that can arise from parallel stimulation, the MAESTRO system offers channel interaction compensation (CIC).

Channel Interaction Compensation mathematically calculates the spread of electrical current in real-time and automatically reduces the potential for channel interaction when electrodes are fired at the same time. This feature and its benefits are described by Zierhofer et al4 in their paper on frequency discrimination with sequential or simultaneous stimulation. 
 

  1. Throckmorton C. & Collins L, 2002. The effect of channel interactions on speech recognition in cochlear implant subjects: Predictions from an acoustic model. Acoustical Society of America, 112 (1), pp 285-296. 
  2. Kwon B., 2009. Effects of electrode separation between speech and noise signals on consonant identification in cochlear implants. Acoustical Society of America, 126 (6), pp 3258-3267.
  3. Wilson B., et al 1991. Better speech recognition with cochlear implants. NATURE, 352 (18), pp 236-238.
  4. Zierhofer C., Nobbe A., Schliech P. & Nopp P., 2007. Frequency discrimination with sequential or simultaneous stimulation in MED-EL cochlear implants. Acta Oto-Laryngologica, 127, pp 1266-1272.
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