Binaural hearing measured with the temporal limits encoder using a vocoder simulation of cochlear implants

Abstract

Cochlear implants (CIs) convert sound to electrical stimulation by extracting the envelope in each frequency band while discarding the temporal fine structure (TFS). This processing removes the fine structure interaural time differences (ITDs), which are an important cue for locating sounds on the horizontal plane in normal-hearing (NH) listeners, but are unavailable to CI users. A temporal limits encoder (TLE) strategy was previously proposed to enhance TFS in CIs, and our previous studies via tone-carrier vocoder simulation have shown improved unilateral speech-in-noise understanding and pitch perception. Here, binaural benefits of TLE were assessed, by measuring the binaural intelligibility level difference (BILD), using a 22-channel tone-carrier vocoder in NH listeners. TLE was compared to continuous interleaved sampling (CIS), a common CI strategy. Speech reception thresholds (SRTs) were measured for diotic target speech (male), and diotically-colocated or dichotically-separated (applying AE625 ms delay between ears) competitors (male or female). Compared to CIS, TLE showed significantly larger BILDs for different genders, indicating that TLEsimulation listeners were able to benefit from both pitch and spatial cues. However, SRTs for vocoded conditions were much higher than non-vocoded listening, likely due to a lack of familiarity with vocoded speech listening.

Pitch Perception With the Temporal Limits Encoder for Cochlear Implants

“…For TLE, a "modulator" is computed using a frequency down-shifting process (Fig. 1(c)) to amplitude modulate the electrical pulses [30], [31], [33].…”

“…The TLE strategy has previously been tested in some pitch-related listening tests in normal-hearing (NH) listeners with offline vocoder simulations. The advantage of the TLE strategy over standard envelope-based strategies was observed in tasks including pure tone discrimination [29], binaural intelligibility level difference [31], Mandarin speech-in-noise reception, and Mandarin tone recognition [30]. However, the benefit of the TLE strategy for providing pitch perception benefits for CI users remains unclear because electric hearing is vastly different to acoustic hearing.…”