Thresholds for formant frequency discrimination have been established using optimal listening conditions. In normal conversation the ability to discriminate formant frequency is probably substantially degraded. The present study examined formant discrimination under more ordinary listening conditions characterized as tasks under higher levels of stimulus uncertainty with vowels embedded in syllables, phrases and sentences, and with the addition of a sentence identification task. Four vowels synthesized from a female talker were presented in isolation, or in the phonetic context of /bVd/ syllables, three-word phrases or nine-word sentences. In the first experiment, phonetic context was manipulated in a novel experimental protocol. Undesirable training effects were obtained and led to the design of a new protocol for the second experiment to reduce this problem and to manipulate both length of phonetic context and level of difficulty in the simultaneous sentence identification task. Similar results were obtained in both experiments. The effect of longer phonetic context on formant discrimination is compressive such that no difference was found between vowels embedded in the phrase or sentence contexts. The addition of a challenging sentence identification task to the discrimination task did not degrade performance and a stable pattern for formant discrimination emerged. This norm for the resolution of vowel formants under ordinary listening conditions was shown to be a constant of 0.28 Barks.
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