Discrimination of interaural temporal disparities conveyed by high-frequency sinusoidally amplitude-modulated tones and high-frequency transposed tones: effects of spectrally flanking noises

Abstract

Dreyer and Oxenham [(2008). "Effects of level and background noise on interaural time difference discrimination for transposed stimuli," J. Acoust. Soc. Am. 123, EL1-EL7] reported that spectrally flanking noise increased threshold interaural temporal disparities (ITDs) conveyed by high-frequency transposed tones but rendered them indiscriminable when they were conveyed by high-frequency sinusoidally amplitude-modulated (SAM) tones. This study extends those observations and evaluates the role of "off-frequency listening." Threshold ITDs were measured using 4-kHz-centered transposed or SAM tonal "targets." In "baseline" conditions, targets were presented without spectrally flanking noise. Additionally, targets were presented along with continuous diotic broadband Gaussian noise spectrally "notched" between 3.6 and 4.4 kHz. In another condition, only the high-pass segment of the notched noise was continuously present. In the final condition, only the low-pass segment was continuously present. Results indicate that (1) relative to baseline, adding notched noise resulted in similar relative increases of threshold ITDs for both SAM and transposed targets; (2) the presence of the high-pass segment of the notched noise resulted in greater relative increases in threshold ITDs over those obtained in baseline conditions for SAM tones as compared to transposed tones; (3) comparisons among all of the data were consistent with the interpretation that both on-frequency and off-frequency processing of envelope-based ITDs can be disrupted by the presence of a notched noise.

Figure 1

Threshold ITDs measured as a function of the overall level of the target. Left and right panels depict average threshold ITDs obtained from the two more-sensitive and the two less-sensitive listeners, respectively. Panels in the top and bottom rows depict data obtained with the 4-kHz-centered transposed and SAM targets, respectively. The rate of modulation was 128 Hz. Closed symbols represent the data obtained in the baseline condition; open symbols represent the data obtained when the targets were surrounded by a 3.6–4.4 kHz notched noise. The error bars represent ±1 standard error of the mean. The small squares in the left panel represent average threshold ITDs transcribed from Dreyer and Oxenham (2008).

Figure 2

Each row displays data obtained from a pair of listeners (BT∕RS or KM∕RC). The bars within each plot represent, for each listener, threshold ITDs normalized against the mean threshold ITD obtained in the respective SAM (left-hand panels) or transposed (right-hand panels) target baseline conditions. The data were collapsed across all three levels of presentation of the targets (55, 75, and 85 dB SPL). Error bars represent one standard error of the mean.