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Number of channels
BackgroundHow many independent channels are needed to achieve high levels of speech understanding? It is difficult to answer this question using implant patients because of all the confounding factors (e.g., number of surviving ganglion cells) that may affect their performance. For example, if a patient obtains poor auditory performance using 4 channels of stimulation, we do not know if it is because of the small number of channels or because there are not enough surviving ganglion cells near the stimulating electrodes.
Acoustic simulations were used by Dr. Loizou and Dr. Michael Dorman (from Arizona State University) to unconfound the effect of surviving ganglion cells, etc., and therefore determine how many independent channels are needed to achieve high auditory performance, assuming that that all other factors are held equal.
The acoustic simulations mimic the front-end processing of the implant processor and represent speech as a sum of sinusoids with time-varying amplitudes and fixed frequencies. The amplitudes of the sine-waves are computed by filtering the signal into L logarithmic frequency bands (or channels) and detecting the envelopes of the filtered waveforms by full-wave rectification and low-pass filtering (with 400 Hz cutoff frequency).
Listening demos(To listen to the simulations, just click on the sentences below in the second column, and wait a few seconds until a small window pops up.)
# of Channels Processed speech Original speech 1 1-channel simulation Original sentence 2 2-channel simulation Original sentence 4 4-channel simulation Original sentence 6 6-channel simulation Original sentence 8 8-channel simulation Original sentence
How many channels?How many independent channels are needed to achieve high levels of speech understanding? The results, with normal-hearing listeners, are given in Figure 1. As it can be seen, the number of channels needed to reach asymptotic performance depended on the test material. For the most difficult test, i.e., vowel recognition, eight channels were needed, while for the least difficult test, i.e., sentence recognition, five channels were needed. These results suggest that high levels of speech understanding could be obtained with 5-8 independent channels of stimulation, at least for adults.
For children, we found that the answer to the above question is different. Children need more channels than adults to understand speech. Click here to find out more about our studies with normal-hearing children and children with implants.
P. Loizou, M. Dorman and Z. Tu (1999). "On the number of channels needed to understand speech," Journal of Acoustical Society of America 106(4), 2097-2103. ( download a copy)
M. Dorman, P. Loizou and D. Rainey (1997). "Speech intelligibility as a function of the number of channels of stimulation for signal processors using sine-wave and noise-band outputs," Journal of Acoustical Society of America, 102(4), 2403-2411.
Simulating the effect of electrode insertion depth
Electrode arrays are inserted only partially into the cochlea, typically 22-30 mm, depending on the state of the cochlea. The fact that the electrode array is not fully inserted into the cochlea creates a frequency mismatch between the analysis frequency and the stimulating frequency.
Acoustic simulations were used by Dr. Loizou and Dr. Michael Dorman (from Arizona State University) to determine the effect of electrode insertion depth on speech understanding for a 5-channel cochlear prosthesis. Different insertion depths were simulated ranging from 22 mm to 25 mm insertion. Greenwood's frequency-to-place equation was used to determine the sinewave output frequencies which simulated different electrode depths. For example, to simulate the 22 mm insertion into the cochlea with 4 mm electrode spacing, sinewaves were generated with output frequencies 831, 1566, 2844, 5056 and 8924 Hz. The corresponding sinewave amplitudes were computed using analysis filters with center frequencies 418, 748, 1339, 2396, and 4287 Hz respectively.
Insertion depth (mm) Processed speech Original speech 22 22 mm simulation Original sentence 23 23 mm simulation Original sentence 24 24 mm simulation Original sentence 25 25 mm simulation Original sentence
Insertion depth simulations
M. Dorman, P. Loizou and D. Rainey (1997). "Simulating the effect of cochlear-implant electrode insertion depth on speech understanding," Journal of Acoustical Society of America, 102(5), 2993-2996.