Combination of binaural information across frequency bands

Perceptual grouping is required when localizing a complex sound in an environment where other sounds are also present. The auditory system must combine the appropriate partials arising from one source while segregating them from competing partials. Two acoustic regularities that might allow for grouping components from a single source are a harmonic relation among the partials and a commonality of their spatial positions. The utility of these cues was examined in a lateralization task in which listeners were asked to detect which of three intervals contained a signal, defined as a delay of the sound presented to one ear. Interaural delay is a predominant cue used in spatial hearing. Discrimination was measured for sounds composed of either a single frequency (Experiment I) or pairs of frequencies (Experiment II). A signal detection analysis used to predict the effects of combining information from the two frequencies found summation in the second experiment to be optimal. This was true regardless of the harmonic relation of the complex. The third experiment used complex signals where only one member of the pair (the target) could contain an interaural delay, while the other (the distractor) was constant in all of the intervals. For inharmonic complexes, performance for the target-distractor combinations was equivalent to that found for targets presented alone. This was not the case for harmonic target-distractor combinations, where performance was relatively poor. A signal detection analysis here showed that decisions were based on a combination of the target and non-informative distractor. Thus the subjects seemed unable to ignore the harmonically related distractor. The fourth experiment placed the two cues for grouping in opposition. Again, performance was measured with the target-distractor pairs, except that the distractor now had an interaural delay that could be quite different from the target in all three intervals. The question was whether this dissimilarity between target and distractor would cause a listener not to combine the two. This was not the result, with performance remaining relatively poor for the harmonically related cases. In sum this study shows that for the grouping of complex acoustic signals in space, harmonic structure is much more important than commonality of spatial position.