Enhancing the temporal fine structure with the temporal limits encoder for cochlear implants: Effects on pitch ranking

Abstract

Pitch perception remains a challenge for cochlear-implant (CI) users. Temporal periodicity, as the main cue for pitch, could not be finely encoded by current CI strategies that transmit envelopes only and discard the temporal fine structure (TFS). Moreover, temporal electric pitch percept saturates at a few hundred hertz, which is controversial and has been studied using various psychophysical and physiological methods. The temporal-limits-encoder (TLE) was proposed to enhance TFS representation by frequency-downshifting the high-frequency band-limited signals to a low-frequency temporal-pitch-limits range of CIs. In this work, eleven Nucleus CI subjects were recruited in a pitch ranking task using a CCiMobile processor. The original stimuli were five-harmonics complex tones. Fundamental-frequency difference limens (F0DLs) at two reference F0s 250 and 313 Hz (i.e., center and upper cross-over frequencies of the most apical electrode) were tested using three TLE versions with different modulator lower limits 100, 200, and 300 Hz respectively and a standard advanced-combinational-encoder (ACE) strategy. Results showed that TLE had a comparable performance with ACE at 250-Hz reference F0, but could outperform ACE at 313-Hz reference F0. These indicate that the TLE strategy has the potential to improve pitch ranking in CIs, which implies an enhancement of TFS representation with TLE.