We completed 4 experiments exploring the neural systems underlying two acoustic properties—voice-onset time, a temporal acoustic property that distinguishes voiced and voiceless phonetic categories in stop consonants in English, e.g., [t] vs. [d], and formant frequency space, a spectral acoustic property that distinguishes vowel quality, e.g., [i] as in beat vs. [I] as in bit.
The results of a study on voice-onset time (VOT) perception using a phonetic categorization task indicated that VOT is processed primarily by the left hemisphere and that the neural system shows graded activation with details of phonetic category structure retained throughout the phonetic processing stream. In a second study investigating the discrimination of voice-onset time, two distinct hemispheric patterns emerged; left temporal regions appeared to be recruited in initial acoustic-perceptual analysis and right frontal regions appeared to be recruited with increased processing demands.
To examine the neural systems underlying the spectral properties of speech, we investigated whether the right hemisphere (RH) is recruited for the analysis of vowel quality, and whether its involvement is influenced by the acoustic space within which vowels reside, on the one hand, and by vowel duration, on the other. Results showed that the size of the vowel space affected hemispheric processing, with increased left hemisphere activation in frontal and temporal regions for vowels from a narrow vowel space compared to vowels from a wide vowel space. In the second experiment, results indicate strong bilateral activation, with increasing right hemisphere activation as the duration of the stimuli increased suggesting that the LH processes the spectral properties of vowels irrespective of vowel length, but the right hemisphere has a particular advantage at long vowel durations.