Biomedical Engineering Reference
In-Depth Information
FIGURE 6-37
Effect of stimulation
pulse rates on fine
temporal modulation
of the syllable /ti/ for
a single band-pass
channel centered at
3.2 kHz.
CIS design parameters:
The CIS strategy can be configured in a number of ways to
maximize the patient's ability to understand speech. These include filter spacing and
envelope cutoff frequencies, the shape of the compression function and stimulation rate
(number of pulses delivered by each channel every second).
Stimulation Rate:
The rate at which current pulses are delivered to each electrode can be
as low as 250 pps or as high as 5,000 pps in some devices. It is reasonable to expect that
higher pulse rates would result in superior performance of the cochlear implant because
high pulse rate stimulation better represents fine temporal modulations present in the
signal. This effect is shown in Figure 6-37. However, test results are inconsistent, with
some finding a small improvement and some finding no improvement at all at the higher
rate. It is possible that these ambiguous results are due to differences in the experimental
procedures used or to the actual hardware and algorithms used to implement the CIS
strategy.
For example, the Nucleus device uses an FFT to generate each of the envelopes, and
because its frame rate is limited frames are repeated if the pulse rate is very high. In that
case, no new information is provided with the increase. Results using the Med-El/CIS-Link
tests showed that pulse rates above 2100 pps resulted in improved speech and consonant
identification compared to the 800 pps base rate (Møller, 2006).
Current commercial implant processors operate at stimulation rates between 800 pps
and 2,500 pps with research being undertaken at 5,000 pps. This higher rate aims to restore
some randomness to the firing rate of neurons, which is the more natural state.
Compression function:
Compression of the envelope is essential as it transforms the large
dynamic range of the acoustic signal into the small dynamic range required by electrical
stimulation. The dynamic range is defined as the range in amplitudes between threshold
(barely audible) and uncomfortable loudness (extremely loud). In conversational speech
