Enhancing sensitivity to interaural delays at high frequencies by using "transposed stimuli".

It is well-known that thresholds for ongoing interaural temporal disparities (ITDs) at high frequencies are larger than threshold ITDs obtained at low frequencies. These differences could reflect true differences in the binaural mechanisms that mediate performance. Alternatively, as suggested by Colburn and Esquissaud [J. Acoust. Soc. Am. Suppl. 1 59, S23 (1976)], they could reflect differences in the peripheral processing of the stimuli. In order to investigate this issue, threshold ITDs were measured using three types of stimuli: (1) low-frequency pure tones; (2) 100% sinusoidally amplitude-modulated (SAM) high-frequency tones, and (3) special, "transposed" high-frequency stimuli whose envelopes were designed to provide the high-frequency channels with information similar to that available in low-frequency channels. The data and their interpretation can be characterized by two general statements. First, threshold ITDs obtained with the transposed stimuli were generally smaller than those obtained with SAM tones and, at modulation frequencies of 128 and 64 Hz, were equal to or smaller than threshold ITDs obtained with their low-frequency pure-tone counterparts. Second, quantitative analyses revealed that the data could be well accounted for via a model based on normalized interaural correlations computed subsequent to known stages of peripheral auditory processing augmented by low-pass filtering of the envelopes within the high-frequency channels of each ear. The data and the results of the quantitative analyses appear to be consistent with the general ideas comprising Colburn and Esquissaud's hypothesis.