The binaural processing of double vowels: Experimental measurements and computational models

Abstract

The degree to which the binaural processing of a ‘‘target’’ vowel is affected by the simultaneous presentation of a ‘‘distractor’’ vowel (of different ITD to the target) was studied using three pairs of synthetic vowels. The stimuli were an ‘‘er’’ (of 100-Hz F0) paired together with either an ‘‘ai,’’ ‘‘er,’’ or ‘‘oo’’ (all of 125-Hz F0). In Experiment 1 ITD thresholds for the target vowel were measured. The results showed that thresholds were larger in double-vowel pairs than in control, target-alone situations. Moreover, the size of the effect was dependent both on whichever vowel of the pair was the target and on the relative level of the two vowels. Computational modeling of the binaural correlograms of the stimuli showed that these results do not require any explicit segregation of frequency channels for the two vowels. In Experiment 2 the perceived lateralization of the target vowel (when given an ITD of 0 ms) was measured. The results showed that this lateralization was ‘‘pulled’’ slightly toward the distractor vowel. Further computational modeling showed that some segregation of the frequency channels in the binaural correlogram, perhaps based on the pitch information in the corresponding autocorrelogram, is required to account for this result.