Comparison of speech recognition with different speech coding strategies (SPEAK, CIS, and ACE) and their relationship to telemetric measures of compound action potentials in the nucleus CI 24M cochlear implant system.

Abstract

Speech understanding and subjective preference for three different speech coding strategies (spectral peak coding [SPEAK], continuous interleaved sampling [CIS], and advanced combination encoders [ACE]) were investigated in 11 post-lingually deaf adult subjects, using the Nucleus CI 24M cochlear implant system. Subjects were randomly assigned to two groups in a balanced crossover study design. The first group was initially fitted with SPEAK and the second group with CIS. The remaining strategies were tested sequentially over 8 to 10 weeks with systematic variations of number of channels and rate of stimulation. Following a further interval of 3 months, during which subjects were allowed to listen with their preferred strategy, they were tested again with all three strategies. Compound action potentials (CAPs) were recorded using neural response telemetry. Input/output functions in relation to increasing stimulus levels and inter-stimulus intervals between masker and probe were established to assess the physiological status of the cochlear nerve. Objective results and subjective rating showed significant differences in favour of the ACE strategy. Ten of the 11 subjects preferred the ACE strategy at the end of the study. The estimate of the refractory period based on the inter-stimulus interval correlated significantly with the overall performance with all three strategies, but CAP measures could not be related to individual preference of strategy or differences in performance between strategies. Based on these results, the ACE strategy can be recommended as an initial choice specifically for the Nucleus CI 24M cochlear implant system. Nevertheless, access to the other strategies may help to increase performance in individual patients.