Auditory processing of interaural timing information: New insights

\Differences in the time-of-arrival of sounds at the two ears, or interaura l temporal disparities (ITDs), constitute one of the major binaural cues th at underlie our ability to localize sounds in space. In addition, ITDs cont ribute to our ability to detect and to discriminate sounds, such as speech, in noisy environments. For low-frequency signals, ITDs are conveyed primar ily by "cycle-by-cycle" disparities present in the fine-structure of the wa veform. For high-frequency signals, ITDs are conveyed by disparities within the time-varying amplitude, or envelope, of the waveform. The results of l aboratory studies conducted over the past few decades indicate that ITDs wi thin the envelopes of high-frequency are less potent than those within the fine-structure of low-frequency stimuli. This is true for both measures of sensitivity to changes in ITD and for measures of the extent of the perceiv ed lateral displacement of sounds containing ITDs. Colburn and Esquissaud ( 1976) hypothesized that it is differences in the specific aspects of the wa veform that are coded neurally within each monaural (single ear) channel th at account for the greater potency of ITDs at low frequencies rather than a ny differences in the more central binaural mechanisms that serve these dif ferent frequency regions. In this review, the results of new studies are re ported that employed special high-frequency "transposed" stimuli that were designed to provide the high-frequency channels of the binaural processor w ith envelope-based information that mimics waveform-based information norma lly available only in low-frequency channels. The results demonstrate that these high-frequency transposed stimuli (1) yield sensitivity to ITDs that approaches, or is equivalent to, that obtained with "conventional" low-freq uency stimuli and (2) yield large extents of laterality that are similar to those measured with conventional low-frequency stimuli. These findings sug gest that by providing the high-frequency channels of the binaural processo r with information that mimics that normally available only at low frequenc ies, the potency of ITDs in the two frequency regions can be made to be sim ilar, if not identical. These outcomes provide strong support for Colburn a nd Esquissaud's (1976) hypothesis. The use of high-frequency transposed sti muli, in both behavioral and physiological investigations offers the promis e of new and important insights into the nature of binaural processing. (C) 2001 Wiley-Liss, Inc.