Greenberg, DL; (2014) Sensitivity to interaural timing differences within the envelopes of acoustic waveforms. Doctoral thesis , UCL (University College London).

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Abstract

Interaural-timing-differences (ITDs) are a cue for sound-source localisation and can be conveyed in the temporal-fine-structure (TFS) of low-frequency tones or in the envelope of high-frequency, amplitude-modulated sounds such as sinusoidally amplitude-modulated (SAM) and transposed-tones. Sensitivity to these cues has been measured in human psychophysical experiments and has revealed that the tranposed-tone elicits just-noticeable-differences (JNDs) in ITDs that are equivalent to those of low-frequency pure-tones when the modulation frequency is below 512-Hz. At modulation frequencies above 512-Hz performance rapidly declines for the transposed-tone while sensitivity to ITDs in pure-tones is robust until around 1200-Hz. Furthermore, transposed-tones elicit JNDs smaller than SAM tones. In the present study, ITD JNDs are assessed psychophysically for pure-tones and transposed-tones using off-midline reference locations. The results demonstrate that frequency, whether the ITD is conveyed in the TFS or the envelope, and location, all have a significant effect on human ITD JNDs and suggest that a difference exists in how ITDs are coded neuronally when conveyed by either high- or low-frequency sounds. ITD-sensitive neurons located within several brainstem nuclei display a high degree of phase-locking to both the TFS of low-frequency pure-tones and the envelopes of SAM and transposed-tones. Echoing the psychophysical findings, phase-locking to the waveform envelope at low modulation frequencies is equivalent to that of low-frequency pure-tones, while declining at high rates of modulation to a lesser degree for tranposed-tones than SAM tones. In order to assess factors critical to the localisation of high-frequency sounds a series of electrophysiology experiments were conducted. Recordings were made from single neurons within the inferior colliculus of the guinea pig in response to ITDs conveyed by 18 unique envelope shapes to evaluate how the envelope segments; Pause, Attack, Sustain and Decay each effect ITD JNDs. Amplitude-modulations with envelope shapes comprising relatively long Pause but short Attack durations have been found to elicit the greatest ITD discrimination of high-frequency sounds.

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