The problem of mapping differing sensory stimuli onto a common category is fundamental to human cognition. Listeners perceive stable phonetic categories despite many sources of acoustic variability. At issue is identifying those neural mechanisms underlying this perceptual stability. A short-interval habituation fMRI paradigm was used to investigate neural sensitivity to within and between phonetic category acoustic changes. A region in the left inferior frontal sulcus showed an invariant pattern of activation: insensitivity to acoustic changes within a phonetic category in the context of sensitivity to changes between phonetic categories. Left superior temporal regions, in contrast, showed graded sensitivity to both within-and between-phonetic category changes. These results suggest that perceptual insensitivity to changes within a phonetic category may arise from decision-related mechanisms in the left prefrontal cortex and add to a growing body of literature suggesting that the inferior prefrontal cortex plays a domain-general role in computing category representations.

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.

The lexical effect is a phenomenon whereby lexical information influences the perception of the phonetic category boundary for stimuli from word-nonword continua. At issue is whether this effect is due to "top-down" influence of upper levels of processing on perceptual processing, or instead is due to decision-stage processes. In this study, brain activity was monitored using functional magnetic resonance imaging as subjects performed a phonetic categorization task on items taken from 2 continua in which one end of the continuum was a real word and the other was not (gift-kift and giss-kiss). If the lexical effect has a perceptual basis, modulation of activation should be seen as a function of the lexical effect in areas such as the superior temporal gyri (STG) which have previously been implicated in perceptual processing. In contrast, if the effect is purely due to decision-related factors, such modulation would be expected only in areas which have been linked to executive processes, such as frontal and midline structures. Modulation of activation as a function of the lexically biased shift in phonetic category boundary was observed in the STG bilaterally as well as in frontal and midline structures. This activation pattern suggests that the lexical effect has at minimum a perceptual component, in addition to an executive decision-related component. These results challenge the view that lexical effects on phonetic boundary placement are due solely to postperceptual, decision-stage processes, and support those models of language processing which allow for higher-level lexical information to directly influence the perception of incoming speech.