The Perception of Voice Onset Time: An fMRI Investigation of Phonetic Category Structure

Abstract

This study explored the neural systems underlying the perception of phonetic category structure by investigating the perception of a voice onset time (VOT) continuum in a phonetic categorization task. Stimuli consisted of five synthetic speech stimuli which ranged in VOT from 0 msec ([da]) to 40 msec ([ta]). Results from 12 subjects showed that the neural system is sensitive to VOT differences of 10 msec and that details of phonetic category structure are retained throughout the phonetic processing stream. Both the left inferior frontal gyrus (IFG) and cingulate showed graded activation as a function of category membership with increasing activation as stimuli approached the phonetic category boundary. These results are consistent with the view that the left IFG is involved in phonetic decision processes, with the extent of activation influenced by increased resources devoted to resolving phonetic category membership and/or selecting between competing phonetic categories. Activation patterns in the cingulate suggest that it is sensitive to stimulus difficulty and resolving response conflict. In contrast, activation in the posterior left middle temporal gyrus and the left angular gyrus showed modulation of activation only to the “best fit” of the phonetic category, suggesting that these areas are involved in mapping sound structure to its phonetic representation. The superior temporal gyrus (STG) bilaterally showed weaker sensitivity to the differences in phonetic category structure, providing further evidence that the STG is involved in the early analysis of the sensory properties of speech.

Dissociable effects of phonetic competition and category typicality in a phonetic categorization task: An fMRI investigation

“…Competition and the inferior frontal gyri-Similar to data reported in Blumstein et al (2005), activation emerged in the left inferior frontal gyrus as a function of the degree of competition expected to arise from the contrasting phonetic category. Namely, there was increased activation for the near-boundary stimuli compared to either the exemplar or extreme stimuli.…”

“…Each functional volume acquisition lasted 1200 msec, followed by 20 msec of scanner-added time, plus 780 msec of added silence which was programmed into the acquisition sequence. The result was a clustered design (Blumstein et al, 2005;Edmister, Talavage, Ledden, & Weisskoff, 1999) in which each TR consisted of 1200 msec of scanning, followed by 800 msec of silence, for a total TR of 2 seconds.…”

“…Earlier work has investigated the neural substrates of phonetic category structure in the context of the perception of voicing in stop consonants (Blumstein, Myers, & Rissman, 2005), and the perception of prototypical and non-prototypical vowels (Guenther, NietoCastanon, Ghosh, & Tourville, 2004). However, in both of these studies, the tokens which were poorer exemplars of their phonetic category were those near the phonetic category boundary, and were thus also subject to a greater degree of phonetic competition.…”

A unified account of categorical effects in phonetic perception

“…The NAPP model of Nearey and Hogan (1986) is also focused on identification tasks. Feldman et al (2009) were the first to introduce the idea that discrimination tasks involve inferring a continuous value T , which is a denoised version of the stimulus S. This focus on recovering continuous detail from the speech signal provides an advantage over previous models that have treated only the classification problem, and is consistent with evidence that listeners perceive acoustic detail (Andruski et al, 1994;Blumstein et al, 2005;Joanisse et al, 2007;Pisoni and Tash, 1974;Toscano et al, 2010). The model explains how the perceptual process leads to effects we see in both identification and discrimination, not just categorical effects in identification.…”

“…They showed that stimuli with initial stop consonant VOTs near the category center exhibited a stronger priming effect for semantically-related following stimuli, and that non-central values also elicited longer reaction times. Finally, at the neural level, an fMRI study by Blumstein et al (2005) showed that there are robust neural correlates to subphonemic VOT differences in stimuli. In related work looking at event-related potentials during word categorization in an auditory oddball task, Toscano et al (2010) showed that listeners are sensitive to fine acoustic differences in VOT independent of the categorization.…”