Extents of laterality and binaural interference effects.

Extents of laterality produced by ongoing interaural time delays (ITDs) within high-frequency sinusoidally amplitude-modulated (SAM) target tones were measured in the presence or absence of a second, spectrally remote, diotic, SAM tone. The spectrally remote SAM tones had recently been shown to reduce sensitivity to ITDs conveyed by a 4-kHz SAM tone in a previous experiment employing the same listeners [L. M. Heller and C. Trahiotis, J. Acoust. Soc. Am. 97, 1808-1816 (1995)]. All SAM tones were 100% modulated at 250 Hz and were presented at 77 dB SPL for a duration of 250 ms. The SAM target tone was centered at 4 kHz and the lower-frequency SAM tones were centered at either 500 Hz, 1 kHz, or 2 kHz. The data indicate that spectrally remote, diotic, SAM tones "pull" the lateral position of a 4-kHz SAM tone toward the midline, even when the 4-kHz SAM tone contains an ITD of up to 400 or 600 microseconds. This means that effects of spectrally remote information are not confined to tasks which require that listeners detect, or discriminate between, threshold amounts of ITD. Analyses revealed that changes in lateral position, as measured by an acoustic pointing task, cannot in and of themselves account for the interference effects found in discrimination tasks with similar stimuli. It was found, however, that Buell and Hafter's [J. Acoust. Soc. Am. 90, 1894-1990 (1991)] weighted-combination model, when augmented to include measures of laterality as well as measures of discriminability, could provide reasonably accurate predictions of the amounts of interference obtained in the discrimination task.