Page 57 - Spring 2019
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ma ca ms CA us ca ms ca cxs

i.rnportant for recognition. Instead, the depicted strategy,
continuous interleaved sampling (CIS; Wilson et al., 1991), § 5,,
makes no assumptions about how speech is produced or per- 3:3
ceived and si.rnply strives to represent the input in a way that E’ ‘E
will utilize most or all of the perceptual ranges of electrically E
evoked hearing as clearly as possible. In
As shown, the strategy includes multiple channels of sound
processing whose outputs are directed to the different elec- u
trodes in an array of electrodes implanted in the scala tym- Spondee cm SPIN NU-S
pani (ST), one of three fluid-filled chambers along the length
Of the c°chie3 (See X'i'“Y inset in Figilee i— Which 5h°W5 31" Figure 2. Results from initial comparisons of the compressed
eiectmde army in the ST)- The chehiieis dither °iiiY ii‘ the analog (CA) and CIS processing strategies. Green lines, scores
ti’e‘l“ei"CY range for the t’“"d'P“55 ti-ttei'- The chehhei °“t' fior subjects selected for their exceptionally high levels of per-
P“t5 with high Center frequencies f°i’ the fitters are directed fiormance with the CA strategy; blue lines, scores for subjects
to electrodes at the basal end of the cochlea, which is most sglegtedjby their mom [yp,'m[ [eve]; gfpeyfgrmame with that
Sensitive to high'ft'ecl“eiicY 5°““d5 in i"°i'm“i hearing (the strategy. The tests included recognition of two-syllable words
t°ii°t°Pic °i’get"iZ“‘ti°t" mentioned it‘ A S“3P5h°t ‘if the Hie‘ (Spondee); the Central Institute for the Deaf (CID) everyday
t°|'}')v "id the chehhei °“tP“t5 With iowei’ cehtei’ ti'e‘l“ei"cie5 sentences; sentences from the Speech-in-Noise test (SPIN) but
“Te di1'ectedt° eiecti’°de5t°W"\1'dthe Othei’ (3Pic3i) end Ofthe here without the added noise; and theNorthwestern University
cochlea, which in normal hearing is most sensitive to sounds [133 5,"; gfmgm,5},1[a1,,'; words (N1/.6)‘ pmm Wilson and D0,.
at lower frequencies man (2018a), with permission.
The span of the frequencies across the band-pass filters typi-  
cally is from 300 Hz or lower to 6 kHz or higher, and the dis-
tribution of frequencies is logarithmic, like the distribution trodes may be effective in a multichannel context, at least for
of frequency sensitivities along the length of the cochlea in ST implants and the current processing strategies; see Wilson
normal hearing. In each channel, the varying energy in the and Dorman, 2008.) Also, users typically perceive increases
band-pass filter is sensed with an envelope detector, and then in pitch with increases in the rate or frequency of stimula-
the output of the detector is “mapped” onto the narrow dy- tion, or the frequency of modulation for modulated pulse
namic range of electrically evoked hearing (5-20 dB for puls- trains, at each electrode up to about 300 pulses/s or 300 Hz
es vs. 90 dB or more for normal hearing) using a logarithmic but with no increases in pitch with further increases in rate
or power-law transformation. The envelope detector can be or frequency (e.g., Zeng, 2002). For that reason, the cutoff of
as simple as a low-pass filter followed by a rectifier (full wave the low-pass filter in each of the processing channels usually
or half wave) or as complex as the envelope output of a Hil- is set at 200-400 Hz to include most or all of the range over
bert Transform. Both are effective. The compressed envelope which different frequencies in the modulation waveforms
signal from the nonlinear mapping function modulates a can be perceived as different pitches. Fortuitously, the 400-
carrier of balanced biphasic pulses for each of the channels Hz choice also includes the full range of the fundamental
to represent the energy variations in the input. Those modu- frequencies in voiced speech for men, women, and children.
lated pulse trains are directed to the intracochlear electrodes The pulse rate for each channel is the same across channels
as previously described. Implant users are sensitive to both and is usually set at four times the cutoff frequencies (which
place of stimulation in the cochlea or auditory nerve and the also are uniform across channels) to minimize ambiguities in
rate or frequency of stimulation at each place (Simmons et the perception of the envelope (modulation) signals that can
al., 1965). occur at lower rates (Busby et al., 1993; Wilson et al., 1997).
Present-day implants include 12-24 intracochlear electrodes; A further aspect of the processing is to address the effects of
some users can rank all of their electrodes according to pitch, the highly conductive fluid in the ST (the perilymph) and the
and most users can rank at least a substantial subset of the relatively distant placements of the intracochlear electrodes
electrodes when the electrodes are stimulated separately and from their neural targets (generally thought to be the spiral
one at a time. (Note, however, that no more than eight elec- ganglion cells in the cochlea). The high conductivity and the

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