SPEECH RECOGNITION AS A FUNCTION OF THE NUMBER OF ELECTRODES USED IN THE SPEAK COCHLEAR IMPLANT SPEECH PROCESSOR

Speech recognition was measured in listeners with the Nucleus-22 SPEAK speech processing strategy as a function of the number of electrodes. Speech stimuli were analyzed into 20 frequency bands and processed ac cording to the usual SPEAK processing strategy. In the normal clinical processor each electrode is assigned to represent the output of one F ilter. To create reduced-electrode processors the output of several ad jacent filters were directed to a single electrode, resulting in proce ssors with 1, 2, 4, 7, 10, and 20 electrodes. The overall spectral ban dwidth was preserved, but the number of active electrodes was progress ively reduced. After a 2-day period of adjustment to each processor, s peech recognition performance was measured on medial consonants, vowel s, monosyllabic words, and sentences. Performance with a single electr ode processor was poor in all listeners, and average performance incre ased dramatically on all test materials as the number of electrodes wa s increased from 1 to 4. No differences in average performance were ob served on any test in the 7-, 10-, and 20-electrode conditions. On sen tence and consonant tests there was no difference between average perf ormance with the 4-electrode and 20-electrode processors. This pattern of results suggests that cochlear implant listeners are not able to m ake Full use of the spectral information on all 20 electrodes. Further research is necessary to understand the reasons For this limitation a nd to understand how to increase the amount of spectral information in speech received by implanted listeners.