The effect of binaural coherence on envelope interaural time difference thresholds.

Room reflections alter the envelope of sounds differently at both ears, reducing binaural coherence. Experiment 1 measured interaural time difference (ITD) discrimination thresholds for broadband and transposed speech. A sentence was convolved with binaural room impulse responses for different source‐receiver distances, and the envelopes extracted and multiplied with 4‐kHz tones. Preliminary results indicate that envelope ITD thresholds increased from <100 μs without reverberation to >700 μs at four times the reverberation radius. ITD thresholds for unprocessed speech were more robust to the addition of reverberation, only rising above 350 μs at the same distance. To investigate whether reverberation is detrimental owing to reduction in binaural coherence, stimulus envelopes were created by temporally jittering raised cosine pulses around a 10‐ms separation. Bilaterally independent jittering allowed variation in coherence while minimizing change in other envelope parameters. Preliminary results show ITD thresholds <100 μs for coherent envelopes, increasing to >700 μs for coherences of 0.6, a value consistent with the room simulations above. Envelope coherence strongly affects ITD discrimination, suggesting that ITDs extracted from high‐frequency channels may not provide useful information in many realistic situations. This has implications for bilateral cochlear implant users, as current devices provide ITDs only in envelopes.