Effects of contralateral presentation and of interaural time differences in segregating a harmonic from a vowel

The extent to which the 500-Hz component of a steady-state vowel contributed to its phonemic category was measured by estimating the position of the /I/–/ε/ phoneme boundary along a first formant (F1) continuum. Shifts in the phoneme boundary were calibrated against shifts produced by physical changes in the level of the 500-Hz component. When the 500-Hz component was presented contralateral to the rest of the vowel, the phoneme boundary changed by an amount equivalent to a physical reduction in the level of the component of about 5 dB (experiment 1). However, giving the 500-Hz component an interaural time difference (ITD) of −666 μs and the remainder of the vowel an ITD of +666 μs did not reduce the component’s contribution to vowel quality (experiment 2). Placing the 500-Hz component of the vowel in a sequence of 500-Hz tones substantially reduced the contribution of the 500-Hz component; the contribution was further reduced by giving both the tone sequence and the component an ITD of −666 μs and the rest of the vowel one of +666 μs (experiment 2). When these tone-sequence conditions were presented in the same experimental block as the ITD conditions without a preceding tone sequence, these latter conditions did then show an effect of grouping by ITD. The results suggest that listeners can perceptually segregate sound on the basis of different ITDs, but that this segregation is substantially enhanced if the direction has previously been cued.