One family of theories regarding vowel perception suggests onset and offset formant-frequencies are important for identification and that the shape of the transitions themselves are not otherwise perceptually important. The present study determined just-noticeable-differences in deviations from linear formant trajectories. Diphthong-like stimuli were manipulated by inserting a point of inflection into the otherwise linear transition. Several parameters were manipulated including vowel duration, location of the inflection point in time, and fundamental frequency. Data from the first experiment indicate that listeners are largely insensitive to deviations from linearity of formant trajectory but that large enough deviations could eventually be detected. The size of these deviations seems dependent on the range of onset-offset formant frequencies. However, a second experiment in which only the first half of stimuli was presented thereby affecting the frequency range of the stimuli, gave different results. Results from these experiments along with several hypotheses are presented.

The present study investigated the hypothesis that later and/or early learners of English as a second language may exhibit an exaggerated or restricted degree of change in their production performance between clear and conversational speech styles for certain acoustic cues. Monolingual English talkers (MO), early Spanish-English bilinguals (EB) and late Spanish-English bilinguals (LB) were recorded using both clear and conversational speaking styles. The stimuli consisted of six target vowels /i, I, e, E, ae/ and /a/, embedded in /bVd/ context. All recorded target-word stimuli were isolated into words. Vowel duration was computed, and fundamental frequency (F0), and formant frequency values (F1-F4) were measured at 20%, 50%, and 80% of the vowel duration. Data from the MO and EB talkers indicates that these two groups are very similar in that they emphasize duration differences in clear speech, have similar spacing of vowels (static & dynamic properties), and have similar frequency changes in clear speech. Data from the LB talkers indicates that this group failed to emphasize differences in clear speech, particularly duration differences. In addition, the high-mid front vowels (/i, I, e/ and /E/) were found to be very poorly separated in the F1-F2 space for the LB talkers. In support of the hypothesis, the data showed that LB talkers exhibited a restricted degree of change in their production performance between clear and conversational speech styles for duration, as compared to monolingual talkers. Data analyzed for the EB talkers do not reveal systematic reductions in the degree of change in their production performance between clear and conversational speech styles, as compared to monolingual talkers.

Examined was whether the mismatch negativity (MMN) varied predictably in response to changes in the direction of frequency-modulated tone glides and equivalent second formant transitions in vowels (e.g., /ɪ/ as in “bit” and /e/ as in “bate”). A novel stimulus presentation paradigm was designed to distinguish the MMN from the N1 component. 10 normal-hearing adults with no neurological diseases were recruited and presented stimuli via insert earphones while they watched a silent, subtitled movie. Event-related potentials (ERPs) were recorded from 128 scalp electrodes. The MMN was successfully distinguished from the N1, marking participants’ ability to discriminate vowel stimuli only. A significant early P2 component, which decreased in size with successive stimulus presentations, was also elicited for vowels only and is believed to reflect formant encoding. Discrepancies between vowel and tone results are discussed along with clinical implications and contributions to the fields of ERP and vowel research.