Differences in the temporal course of interaural time difference sensitivity between acoustic and electric hearing in amplitude modulated stimuli

Abstract

Previous studies have shown that normal-hearing (NH) listeners' spatial perception of non-stationary interaural time differences (ITDs) is dominated by the carrier ITD during rising amplitude segments. Here, ITD sensitivity throughout the amplitude-modulation cycle in NH listeners and bilateral cochlear implant (CI) subjects is compared, the latter by means of direct stimulation of a single electrode pair. The data indicate that, while NH listeners are most sensitive to ITDs applied toward the beginning of a modulation cycle at 600 Hz, NH listeners at 200 Hz and especially bilateral CI subjects at 200 pulses per second (pps) are more sensitive to ITDs applied to the modulation maximum. This has implications for spatial-hearing in complex environments: NH listeners' dominant 600-Hz ITD information from the rising amplitude segments comprises direct sound information. The 200-pps low rate required to get ITD sensitivity in CI users results in a higher weight of pulses later in the modulation cycle where the source ITDs are more likely corrupted by reflections. This indirectly indicates that even if future binaural CI processors are able to provide perceptually exploitable ITD information, CI users will likely not get the full benefit from such pulse-based ITD cues in reverberant and other complex environments.

Auditory brainstem models: adapting cochlear nuclei improve spatial encoding by the medial superior olive in reverberation

“…Our hemispheric-difference model, therefore, suggests that a slower, more integrative MSO 590 neuron may assist lateralisation in lower-frequency channels, in particular by extending the 591 extraction of ITD information towards peak energy in the AM cycle, where human ITD 592 detection is best when comparing at 200 Hz across the AM cycle (Hu et al, 2017). 593 Additionally, our model qualitatively matched the behavioural data by demonstrating a higher 594…”

“…Spike counts in the model MSO 530 neuron (Fig. 4A, lower three rows) matched the patterns of significance in human ITD-531 detection for AM stimuli at 200 Hz (Hu et al, 2017): spike counts were slightly higher but not 532 significantly for best-ITD during peak vs. rising energy, and spike counts were significantly 533 different (higher) only for best-ITD at peak energy vs. falling energy (P = 0.0079 for modulation 534 at 8 Hz; P = 0.0014 for modulation at 20 Hz number of instances when the AM stimulus containing a (right-leading) +150 μs ITD in its 566 rising-energy phase was correctly lateralised [d' > 1 in any bin was considered a correct 567 lateralisation of a signal from the 'right' (Fig. 6 pink vertical bands)].…”

“…Two stimuli were presented to the model: the first, a SAM tone with non-zero ITDs inserted at 336 different phases of the AM cycle, as used behaviourally to determine how carrier frequency 337 affects ITD sensitivity over the time course of an AM cycle(Hu et al, 2017); the second, a 338 natural speech stimulus with early reflections, has been proposed as a simple simulation of a 339 reverberant environment where correct lateralisation is only possible if ITD cues in the onset 340 waveform are processed preferentially. 341SAM tones with non-zero ITDs inserted at different phases of AM cycle342 This stimulus was generated according to the Hu et al (2017) study that first used it.…”

Localization of broadband sounds carrying interaural time differences: Effects of frequency, reference location, and interaural coherence

“…Even though these aspects were not considered here, the presented methods may be extended to measure the (relative) weights across the different cues as well as over the time course of a reverberant signal with time-(and frequency-) varying IC. Such research would complement other relevant studies that applied amplitude modulated stimuli to determine the temporal weighting of interaural cues (e.g., Dietz et al, 2013;Stecker and Bibee, 2014;Hu et al, 2017).…”

General neural mechanisms can account for rising slope preference in localization of ambiguous sounds

“…The perceived IPD was also shown to increase with modulation frequency. Later on, Hu et al (2017) conducted a similar experiment and uncovered that IPD extraction in the rising slope is conditional to the value of the carrier frequency. For 200 Hz stimuli, the subjects mostly reported IPDs located at the peak of the envelope.…”