Effects of dynamic range and amplitude mapping on phoneme recognition in nucleus-22 cochlear implant users

Objective: To determine the consequences for phoneme recognition of errors in setting threshold and loudness levels in cochlear implant listeners usin g a 4-channel continuous interleaved sampling (CIS) speech processor. Design: Three Nucleus-22 cochlear implant listeners, who normally used the SPEAK speech processing strategy participated in this study. An experimenta l 4-channel CIS speech processor was implemented in each listener as follow s. Speech signals were band-pass filtered into four broad frequency bands a nd the temporal envelope of the signal in each band was extracted by half-w ave rectification and low-pass filtering. A power function was used to conv ert the extracted acoustic amplitudes to electric currents. The electric cu rrents were dependent on the exponent of the mapping power function and the electrode dynamic range, which was determined by the minimum and maximum s timulation levels. In the baseline condition, the minimum and maximum stimu lation levels were defined as the psychophysically measured threshold level (T-level) and maximum comfortable level (C-level). In the experimental con ditions, the maximum stimulation levels were fixed at the C-level and the d ynamic range (in dB) was changed by varying the minimum stimulation levels on all electrodes. This manipulation simulates the effect of an erroneous m easurement of the T-level. Phoneme recognition was obtained as the dynamic range of electrodes was changed from 1 dB to 20 dB and as the exponent of t he power-law amplitude mapping function was changed from 0.1 to 0.4. Results: For each mapping condition, the electric dynamic range had a signi ficant, but weak effect on vowel and consonant recognition. For a strong co mpression (p = 0.1), best vowel and consonant scores were obtained with a l arge dynamic range (12 dB). When the exponent of the mapping function was c hanged to 0.2 and 0.4, the dynamic range producing the highest scores decre ased to 6 dB and 3 dB, respectively. Conclusions: Phoneme recognition with a 4-channel CIS strategy was only mil dly affected by large changes in both electric threshold and loudness mappi ng. Errors in threshold by a factor of 2 (6 dB) and in the loudness mapping exponent by a factor of 2 were required to produce a significant decrease in performance. In these extreme conditions, the effect of the electric dyn amic range on phoneme recognition could be due to two independent factors: abnormal loudness growth and a reduction in the number of discriminable int ensity steps. The decrease in performance caused by a reduced electric dyna mic range can be compensated by a more expansive power-law mapping function , as long as the number of discriminable intensity steps is moderately larg e (e.g., >8).