Sensitivity to Envelope Interaural Time Differences at High Modulation Rates

Abstract

Sensitivity to interaural time differences (ITDs) conveyed in the temporal fine structure of low-frequency tones and the modulated envelopes of high-frequency sounds are considered comparable, particularly for envelopes shaped to transmit similar fidelity of temporal information normally present for low-frequency sounds. Nevertheless, discrimination performance for envelope modulation rates above a few hundred Hertz is reported to be poor—to the point of discrimination thresholds being unattainable—compared with the much higher (>1,000 Hz) limit for low-frequency ITD sensitivity, suggesting the presence of a low-pass filter in the envelope domain. Further, performance for identical modulation rates appears to decline with increasing carrier frequency, supporting the view that the low-pass characteristics observed for envelope ITD processing is carrier-frequency dependent. Here, we assessed listeners’ sensitivity to ITDs conveyed in pure tones and in the modulated envelopes of high-frequency tones. ITD discrimination for the modulated high-frequency tones was measured as a function of both modulation rate and carrier frequency. Some well-trained listeners appear able to discriminate ITDs extremely well, even at modulation rates well beyond 500 Hz, for 4-kHz carriers. For one listener, thresholds were even obtained for a modulation rate of 800 Hz. The highest modulation rate for which thresholds could be obtained declined with increasing carrier frequency for all listeners. At 10 kHz, the highest modulation rate at which thresholds could be obtained was 600 Hz. The upper limit of sensitivity to ITDs conveyed in the envelope of high-frequency modulated sounds appears to be higher than previously considered.

Sensitivity to Envelope Interaural Time Difference: Models of Diverse LSO Neurons

“…Psychoacoustic studies show smaller just-noticeable differences (JNDs) in the ITD TFS of lowfrequency tones (Brughera et al, 2013), compared with larger JNDs in the ITD ENV of modulated high-frequency tones (Bernstein & Trahiotis, 2002Monaghan et al, 2015). These studies demonstrate two additional features specific to ITD ENV .…”

“…1B), where membrane impedance low-pass filters the inputs driven by amplitude-modulated sound, limiting the upper modulation rate for sensitivity to ITD ENV . At high modulation rates of 512 and 800 Hz, modulation output is increased by band-pass filters centered above the carrier frequency (Monaghan et al, 2015). We investigated this 'offfrequency listening' at 512-Hz and 800-Hz modulation, using populations of brisk, HL-quick, fast, and very fast model LSO neurons, stimulated by model ANFs with CF between the carrier frequency and the first upper modulation sideband of each transposed tone.…”

“…For the models, modulation rates were 32, 64, 128, 256, 512, and 800 Hz. In the human studies, modulation rates ranged from 32 to 512 Hz (Bernstein & Trahiotis, 2002), and from 128 to 800 Hz (Monaghan et al, 2015).…”

“…For transposed tones at 4 kHz, human JNDs in ITD ENV are lowest (performance is best) at 128-Hz modulation. At higher modulation-rates, performance worsens (JNDs increase), and eventually the task becomes impossible: all the human listeners were able to perform the task at 256-Hz modulation; 5 of 9 listeners were capable at 512-Hz modulation; 2 of 5 at 600-Hz and 700-Hz; and 1 of 5 at 800-Hz (Bernstein & Trahiotis, 2002;Monaghan et al, 2015). Slow and moderate model LSO neurons generally reflected a competent level in average human performance for modulation from 32 to 128 Hz (Bernstein & Trahiotis, 2002), but then worsened to reflect below-average human performance at 256-Hz modulation.…”

Spectrotemporal weighting of binaural cues: Effects of a diotic interferer on discrimination of dynamic interaural differences

“…For example, studies using Gabor click trains have typically fixed the number of clicks and allowed overall duration to vary with ICI, whereas studies of SAM and transposed tones have typically kept duration constant and allowed the number of modulation periods to vary. Also, Trahiotis (2002, 2009) and Monaghan et al (2015) used 20-ms diotic onset ramps in an attempt to disrupt onset artifacts-e.g., ITD cues conveyed by spectral splatter and/or the onset envelope itselfwhereas other studies did not. Bearing such differences in mind, we argue that comparisons of results across "different" pulsatile stimuli are both sensible and informative.…”