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.

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.