Lateralization of sinusoidally amplitude-modulated tones: Effects of spectral locus and temporal variation

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Abstract
Listeners are sensitive to interaural temporal disparities (ITD) of low-frequency (i.e., < 1600 Hz) stimuli. Listeners are also sensitive to ITD within the envelope of high-frequency, complex stimuli. Because these studies, for the most part, employed discrimination tasks, few data exist concerning the extent of laterality produced by ITD as a function of the spectral locus of the stimulus. An acoustic pointing task in which listeners varied the interaural intensity difference of a 500-Hz narrow-band noise (the pointer) so that it matched the intracranial position of a 2nd, experimenter-controlled stimulus (the target) was used. Targets were sinusoidally amplitude-modulated tones centered on 500 Hz, 1, 2, 3 or 4 kHz and modulated at rates ranging from 50-800 Hz. Targets were presented with either the entire waveform delayed or with only the envelope delayed. For low-frequency targets, lateralization is influenced by ITD in the envelope but is dominated by ITD in the fine structure. For high-frequency targets, envelope-based delays produce displacements of the acoustic images which are affected greatly by the rate of modulation; rather large extents of laterality could be produced with high rates of modulation; these data are consistent with those obtained previously in discrimination experiments. For low rates of modulation (e.g., 100 Hz), delays of the entire waveform (both envelope and fine structure) produce much greater displacements of the acoustic image for low-frequency than for high-frequency targets (where fine-structure-based cues are not utilizable). There appear to be no consistent relations among extent of laterality, rate of modulation and the frequency of the carrier within and across listeners.

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