Asymmetric temporal envelope encoding: Lateralization with varying envelope shape and spectral mismatch

Bilateral cochlear implants (BiCIs) improve spatial-hearing outcomes relative to a single cochlear implant (CI), but the extent varies across patients. One limiting factor may be interaurally asymmetric spectro-temporal representations of sounds. The present study simulated interaural spectro-temporal asymmetries to explore how they affect binaural processing in listeners with normal hearing. We simulated CI stimulation using high-rate (500 pps) band limited acoustic pulse trains with a 3 mm bandwidth and 4 or 6.5 kHz center frequency. Second-order amplitude modulation (AM) was applied at a 100 Hz modulation rate, 20% or 50% modulation depth, and sinusoidal or sawtooth shape. Intracranial lateralization was used to assess utility of whole-waveform interaural time differences (ITDs). Results demonstrated that lateralization of envelope ITDs was limited by the ear with smaller AM depth. This effect depended upon the sharpness of temporal onsets and the extent to which frequencies overlapped in each ear. These results suggest that interaural asymmetry in spectro-temporal representations may limit binaural outcomes of patients with BiCIs. Two well-established models of binaural processing that assume interaural symmetry and rely on coincidence-detection were used to predict performance. Models were mostly predictive of performance, but made consistent errors, suggesting that models of BiCI stimulation using coincidence-detection should be improved to account for asymmetries.