The evaluation demonstrates that the adaptive noise reduction algorithm BEAM in the Nucleus Freedom CI-system may significantly increase the speech perception by cochlear implantees in noisy listening conditions. This is the first monolateral (adaptive) noise reduction strategy actually implemented in a mainstream commercial CI.

Music perception and appraisal is very poor in cochlear implant (CI) subjects partly because (musical) pitch is inadequately transmitted by the current clinically used sound processors. A new sound processing scheme (F0mod) was designed to optimize pitch perception, and its performance for music and pitch perception was compared in four different experiments to that of the current clinically used sound processing scheme (ACE) in six Nucleus CI24 subjects. In the F0mod scheme, slowly varying channel envelopes are explicitly modulated sinusoidally at the fundamental frequency (F0) of the input signal, with 100% modulation depth and in phase across channels to maximize temporal envelope pitch cues. The results of the four experiments show that: (1) F0 discrimination of single-formant stimuli was not significantly different for the two schemes, (2) F0 discrimination of musical notes of five instruments was three times better with the F0mod scheme for F0 up to 250 Hz, (3) melody recognition of familiar Flemish songs (with all rhythm cues removed) was improved with the F0mod scheme, and (4) estimates of musical pitch intervals, obtained in a musically trained CI subject, matched more closely the presented intervals with the F0mod scheme. These results indicate that explicit F0 modulation of the channel envelopes improves music perception in CI subjects.

The effect of the filter bank on fundamental frequency (F0) discrimination was examined in four Nucleus CI24 cochlear implant subjects for synthetic stylized vowel-like stimuli. The four tested filter banks differed in cutoff frequencies, amount of overlap between filters, and shape of the filters. To assess the effects of temporal pitch cues on F0 discrimination, temporal fluctuations were removed above 10 Hz in one condition and above 200 Hz in another. Results indicate that F0 discrimination based upon place pitch cues is possible, but just-noticeable differences exceed 1 octave or more depending on the filter bank used. Increasing the frequency resolution in the F0 range improves the F0 discrimination based upon place pitch cues. The results of F0 discrimination based upon place pitch agree with a model that compares the centroids of the electrical excitation pattern. The addition of temporal fluctuations up to 200 Hz significantly improves F0 discrimination. Just-noticeable differences using both place and temporal pitch cues range from 6% to 60%. Filter banks that do not resolve the higher harmonics provided the best temporal pitch cues, because temporal pitch cues are clearest when the fluctuation on all channels is at F0 and preferably in phase.

Although in a number of experiments noise-band vocoders have been shown to provide acoustic models for speech perception in cochlear implants (CI), the present study assesses in four experiments whether and under what limitations noise-band vocoders can be used as an acoustic model for pitch perception in CI. The first two experiments examine the effect of spectral smearing on simulated electrode discrimination and fundamental frequency (FO) discrimination. The third experiment assesses the effect of spectral mismatch in an FO-discrimination task with two different vocoders. The fourth experiment investigates the effect of amplitude compression on modulation rate discrimination. For each experiment, the results obtained from normal-hearing subjects presented with vocoded stimuli are compared to results obtained directly from CI recipients. The results show that place pitch sensitivity drops with increased spectral smearing and that place pitch cues for multi-channel stimuli can adequately be mimicked when the discriminability of adjacent channels is adjusted by varying the spectral slopes to match that of CI subjects. The results also indicate that temporal pitch sensitivity is limited for noise-band carriers with low center frequencies and that the absence of a compression function in the vocoder might alter the saliency of the temporal pitch cues.