PERCEPTUAL SEPARATION OF SIMULTANEOUS VOWELS - WITHIN AND ACROSS-FORMANT GROUPING BY FO

Six experiments explored why the identification of the two members of a pair of diotic, simultaneous, steady-state vowels improves with a di fference in fundamental frequency (DELTAF0). Experiment 1 confirmed ea rlier reports that a DELTAF0 improves identification of 200-ms but not 50-ms duration ''double vowels''; identification improves up to 1 sem itone DELTAF0 and then asymptotes. In such stimuli, all the formants o f a given vowel are excited by the same F0, providing listeners with a potential grouping cue. Subsequent experiments asked whether the impr ovement in identification with DELTAF0 for the longer vowels was due t o listeners using the consistent F0 within each vowel of a pair to gro up formants appropriately. Individual vowels were synthesized with a d ifferent F0 in the region of the first formant peak from in the region of the higher formant peaks. Such vowels were then paired so that the first formant of one vowel bore the same F0 as the higher formants of the other vowel. These across-formant inconsistencies in F0 did not s ubstantially reduce the previous improvement in identification rates w ith increasing DELTAF0's of up to 4 semitones (experiment 2). The subj ects' improvement with increasing DELTAF0 in the inconsistent conditio n was not produced by identifying vowels on the basis of information i n the first-formant or higher-formant regions alone, since stimuli whi ch contained either of these regions in isolation were difficult for s ubjects to identify. In addition, the inconsistent condition did produ ce poorer identification for larger DELTAF0's (experiment 3). The impr ovement in identification with DELTAF0 found for the inconsistent stim uli persisted when the DELTAF0 between vowel pairs was confined to the first formant region (experiment 4) but not when it was confined to t he higher formants (experiment 6). The results replicate at different overall presentation levels (experiment 5). The experiments show that at small DELTAF0's only the first-formant region contributes to improv ements in identification accuracy, whereas with larger DELTAF0's the h igher formant region may also contribute. This difference may be relat ed to other results that demonstrate the superiority of resolved rathe r than unresolved harmonics in coding pitch.