12Virtual Channels: Current Steering and Current Focusing
With only ~22 physical contacts, can an implant create spectral detail between them? Current steering shares current across adjacent electrode pairs to synthesise virtual channels, while current focusing sharpens the field to reduce channel interaction. Both push at the spatial limits of electric hearing — with mixed clinical payoff.
TCurrent steering and HiRes Fidelity 120
Current steering uses simultaneous stimulation of pairs of adjacent physical electrodes to create virtual channels between contacts, shifting the locus of excitation by varying the relative current to the two electrodes HiRes Fidelity 120 (F120), implemented in Advanced Bionics implants, uses an FFT-based spectral-peak locator over 15 analysis bands to place excitation at up to 120 virtual locations F120 also encodes fine temporal structure for frequencies up to about 1200 Hz plus the amplitude envelope Default Advanced Bionics stimulation rates are about 1800-2900 pps per single electrode.[2007][2009]
CThe Clarion/HiRes platform
Advanced Bionics' Clarion CII / HiRes90K supports up to 16 channels at high rates HiRes envelope extraction uses half-wave rectification with no low-pass filter, unlike traditional CIS full-wave rectification plus a low-pass filter HiRes can run at about 2800 pps with very short pulse widths (~11 microseconds per phase) Higher rate and more channels were associated with improved sentence-in-noise scores in HiRes trials.[2001][2007]
CCurrent focusing and multipolar modes
Current focusing uses multipolar modes such as tripolar or partial tripolar to sharpen the electric field around target fibres and reduce channel interaction Spatial selectivity differs across monopolar, bipolar and tripolar stimulation, with focused modes producing narrower excitation Partial-tripolar 'phantom' stimulation can virtually extend the array toward the apex to encode lower frequencies Combining current focusing with current steering has been explored in a single processing strategy.[2012][2013]
CWhy the payoff is limited
Results have not shown a clear, consistent speech advantage for current-steering strategies Current spread limits the benefit: users can often discriminate virtual channels, but perceived spectral resolution does not consistently improve Multipolar focusing results are mixed, with benefit shown for most recipients in only some studies Literature suggests only about 4-8 effective independent channels even with many electrodes, which caps what virtual channels can add.[2007][2013]
TSteering, focusing, and the limits of virtual channels
Dual-electrode current steering (sharing current between adjacent electrodes, proportion 0.11-0.64) produces 2-9 discriminable intermediate pitches per electrode pair (Donaldson 2005). Despite the 120-channel Fidelity120 label, only a mean of ~63 (range 8-451) spectral channels are actually discriminable across the array, with midarray pairs (~6.0) better than basal (~3.8) (Firszt 2007). Fidelity120 measurably improves spectral resolution: spectral-ripple thresholds rose from 2.31 to 3.42 ripples/octave vs standard HiRes (Drennan 2010). The spectral gain carries a temporal cost: with Fidelity120 all 9 listeners fell to chance on Schroeder-phase discrimination, versus 5/9 above chance with HiRes (Drennan 2010). Adding current FOCUSING on top of steering helps: quadrupolar (focused, sigma=0.75) virtual channels improved cumulative d' by 2.04 over monopolar virtual channels (Landsberger 2009).[2005][2007]
TBy the numbers
How is this apparent contradiction best reconciled?
How does current steering create virtual channels?
What does HiRes Fidelity 120 use to place excitation across its virtual locations?