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

The extent o which the 500-Hz component of a steady-state vowel contributed to its phonemic category was measured by estimating the position of the/I/-/t/phoneme boundary along a first formant (F1) continuum. Shifts in the phoneme boundary were calibrated against hifts produced by physical changes inthe level of the 500-Hz component. When the 500-Hz component was presented contralateral tothe rest of the vowel, the phoneme boundary changed by an amount equivalent to a physical reduction i the level of the component ofabout 5 dB (experiment 1). However, giving the 500-Hz com. ponent an interaural time difference (ITD) of 666/zs and the remainder of the vowel an ITD of +666/zs did not reduce the component's contribution to vowel quality (experiment 2). Placing the 500-Hz component of the vowel in a sequence of500-Hz tones ubstantially reduced the contribution f the 500-Hz component; the contribution was further reduced by giving both the tone sequence and the component an ITD of -666 /•s and the rest of the vowel one of +666 ps (experiment 2). When these tone-sequence conditions were presented in the same xperimental block as the ITD conditions without a preceding tone sequence, these latter conditions did then show an effect of grouping by ITD. The results uggest 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. ̧ 1995 Acoustical Society of America.