When the deaf listen to music. Pitch perception with cochlear implants

Cochlear implants (CI) are surgically implanted hearing aids that provide auditory sensations to deaf people through direct electrical stimulation of the auditory nerve. Although relatively good speech understanding can be achieved by implanted subjects, pitch perception by CI subjects is about 50 times worse than observed for normal-hearing (NH) persons. Pitch is, however, important for intonation, music, speech understanding in tonal languages, and for separating multiple simultaneous sound sources. The major goal of this work is to improve pitch perception by CI subjects. In CI subjects two fundamental mechanisms are used for pitch perception: place pitch and temporal pitch. Our results show that place pitch is correlated to the sound?s brightness because place pitch sensation is related to the centroid of the excitation pattern along the cochlea. The slopes of the excitation pattern determine place pitch sensitivity. Our results also show that the effectiveness of temporal pitch improves as the modulation depth increases and the different channels are modulated synchronously. We also show that both temporal and place pitch cues, although independent sensations, can be integrated and projected on a ?combined pitch?-axis used for fundamental frequency (F0) discrimination. (The pitch height NH persons experience corresponds (most often) to the F0 of the sound.) The contribution of place pitch cues for F0-discrimination is, however, hindered by the electrical current spread along the cochlea producing shallow slopes of the excitation pattern and by the interference of the pitch cues with the spectral envelope. The temporal pitch mechanism provides the most effective cues for F0-discrimination but it is limited by more central neural processing. Although we show that the effectiveness of either temporal or place pitch cues can be improved for some particular filter banks, we also show that the simultaneous effective presentation of both temporal and place pitch cues for F0-discrimination is not possible with current sound-processing schemes for CI. Therefore, we developed in this thesis a new sound-processing scheme designed to optimize pitch sensation. In this scheme the pitch of the processed sound is explicitly encoded in the electrical stimulation pattern delivered to the subjects. With the new F0mod scheme we found a 3-fold improvement for pitch discrimination with respect to the current sound-processing scheme and that CI subjects recognize more melodies with the F0mod scheme. The investigation of the pitch mechanisms in CI subjects also enabled us to build an acoustic model of CI pitch sensation. In this way NH persons can have an idea of the pitch quality CI subjects? experience.