Discrimination of interaural time differences from envelopes is severely degraded in the presence of reverberation J. J. M. Monaghan, K. Krumbholz and B. U. Seeber, MRC Institute of Hearing Research, Nottingham, UK Room reflections alter the envelopes of sounds differently at each ear, reducing binaural coherence, modulation depth, and the gradient of envelope peaks. Experiment 1 measured discrimination thresholds of six normal hearing participants for interaural time differences (ITD) carried in reverberant speech waveforms and also for ITDs applied only to the broadband envelopes of the stimuli. A sentence was convolved with binaural room impulse responses for different source- receiver distances; the envelopes extracted and multiplied with 4 kHz tones. Broadband envelope ITD thresholds increased from <250 µs without reverberation, to ~600 µs in the least extreme reverberation condition. When the source-receiver distance was equal to the reverberation radius, thresholds were above the ecological limit of ITD discrimination (~700 µs). ITD thresholds for the full waveform speech were more robust to the addition of reverberation, only rising above 200 µs at the same distance. In additional experiments, the individual effects of the ways in which reverberation changes envelopes were in investigated. The effect of the reduction in binaural coherence was studied using stimulus envelopes created by applying temporal jitter to raised cosine pulses. Bilaterally independent jittering allowed variation of coherence while minimising change in other envelope parameters. ITD thresholds were ~200 µs for coherent envelopes, increasing linearly as a function of coherence to >700 µs for coherences of 0.7, a value consistent with that found in real rooms. Modulation depth was varied parametrically for pulses of two different gradients. The results suggest that modulation depth and pulse onset gradient affect ITD threshold via independent processes with thresholds decreasing linearly as modulation depth is reduced until performance reaches a floor determined by the gradient of the pulse alone. Varying the gradient of the pulse onsets revealed a linear relationship between ITD thresholds and duration of pulse onset. Reduction in envelope coherence, onset gradient and modulation depth all strongly affect 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. Acknowledgements This work was funded by the Intramural Programme of the MRC