Emily B. Myers

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

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.

An event-related fMRI investigation of phonological–lexical competition

This study explored the neural correlates of phonological-lexical competition and frequency on word recognition. An event-related fMRI experiment was conducted using an auditory lexical decision task in which word and nonword stimuli varied in terms of neighborhood density (high and low). Word stimuli also varied in terms of frequency (high and low). Behavioral results were similar to those of Luce and Pisoni [Luce, P. A., & Pisoni, D. B. (1998). Recognizing spoken words: The neighborhood activation model. Ear and Hearing, 19, 1-36], with the reaction time data showing a main effect of word frequency and density as well as a significant interaction effect between these two factors. fMRI results revealed an overall greater neural response for high-density compared to low-density words in the left supramarginal gyrus, consistent with the view that there are greater demands on phonological processing under conditions of increased phonological-lexical competition. The comparison between high and low frequency words revealed greater activation for high frequency words in both anterior and posterior left middle temporal gyrus. A significant interaction between density and frequency was found in lateral and medial frontal structures. This frontal activation may reflect the greater computational resources required in integrating frequency and density information in order to access a word. Overall, these findings demonstrate the sensitivity of neural structures to different properties of the lexicon.

Inferior Frontal Regions Underlie the Perception of Phonetic Category Invariance

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. What are the neural mechanisms that underlie this perceptual stability? In this functional magnetic resonance imaging study, a short-interval habituation paradigm was used to investigate neural sensitivity to acoustic changes within and between phonetic categories. A region in the left inferior frontal sulcus showed a pattern of activation consistent with phonetic invariance: insensitivity to acoustic changes within a phonetic category and sensitivity to changes between phonetic categories. Left superior temporal regions, in contrast, showed graded sensitivity to both within- and between-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.

Phonological neighborhood effects in spoken word production: An fmri study

The current study examined the neural systems underlying lexically conditioned phonetic variation in spoken word production. Participants were asked to read aloud singly presented words, which either had a voiced minimal pair (MP) neighbor (e.g., cape) or lacked a minimal pair (NMP) neighbor (e.g., cake). The voiced neighbor never appeared in the stimulus set. Behavioral results showed longer voice-onset time for MP target words, replicating earlier behavioral results [Baese-Berk, M., & Goldrick, M. Mechanisms of interaction in speech production. Language and Cognitive Processes, 24, 527–554, 2009]. fMRI results revealed reduced activation for MP words compared to NMP words in a network including left posterior superior temporal gyrus, the supramarginal gyrus, inferior frontal gyrus, and precentral gyrus. These findings support cascade models of spoken word production and show that neural activation at the lexical level modulates activation in those brain regions involved in lexical selection, phonological planning, and, ultimately, motor plans for production. The facilitatory effects for words with MP neighbors suggest that competition effects reflect the overlap inherent in the phonological representation of the target word and its MP neighbor.

Recruitment of Anterior and Posterior Structures in Lexical-Semantic Processing: An fMRI Study Comparing Implicit and Explicit Tasks

Previous studies examining explicit semantic processing have consistently shown activation of the left inferior frontal gyrus (IFG). In contrast, implicit semantic processing tasks have shown activation in posterior areas including the superior temporal gyrus (STG) and the middle temporal gyrus (MTG) with less consistent activation in the IFG. These results raise the question whether the functional role of the IFG is related to those processes needed to make a semantic decision or to processes involved in the extraction and analysis of meaning. This study examined neural activation patterns during a semantic judgment task requiring overt semantic analysis, and then compared these activation patterns to previously obtained results using the same semantically related and unrelated word pairs in a lexical decision task which required only implicit semantic processing (Rissman, J., Eliassen, J. C., & Blumstein, S. E. (2003). An event-related fMRI investigation of implicit semantic priming. Journal of Cognitive Neuroscience, 15, 1160-1175). The behavioral results demonstrated that the tasks were equivalent in difficulty. fMRI results indicated that the IFG and STG bilaterally showed greater activation for semantically unrelated than related word pairs across the two tasks. Comparison of the two task types across conditions revealed greater activation for the semantic judgment task only in the STG bilaterally and not in the IFG. These results suggest that the pre-frontal cortex is recruited similarly in the service of both the lexical decision and semantic judgment tasks. The increased activation in the STG in the semantic judgment task reflects the greater depth of semantic processing required in this task and indicates that the STG is not simply a passive store of lexical-semantic information but is involved in the active retrieval of this information.

The Role of the Left Inferior Frontal Gyrus in Implicit Semantic Competition and Selection: An Event-Related fMRI Study

Recent research suggests that the left inferior frontal gyrus (LIFG) plays a role in selecting semantic information from among competing alternatives. A key question remains as to whether the LIFG is engaged by the selection of semantic information only or by increased semantic competition in and of itself, especially when such competition is implicit in nature. Ambiguous words presented in a lexical context provide a means of examining whether the LIFG is recruited under conditions when contextual cues constrain selection to only the meaning appropriate to the context (e.g., coin-mint-money) or under conditions of increased competition when contextual cues do not allow for the resolution to a particular meaning (e.g., candy-mint-money). In this event-related fMRI study, an implicit task was used in which subjects made lexical (i.e., word/nonword) decisions on the third stimulus of auditorily presented triplets in conditions where the lexical context either promoted resolution toward a particular ambiguous word meaning or enhanced the competition among ambiguous word meanings. LIFG activation was observed when the context allowed for the resolution of competition and hence the selection of one meaning (e.g., coin-mint-money) but failed to emerge when competition between the meanings of an ambiguous word was unresolved by the context (e.g., candy-mint-money). In the latter case, there was a pattern of reduced activation in frontal, temporal and parietal areas. These findings demonstrate that selection or resolution of competition as opposed to increased semantic competition alone engages the LIFG. Moreover, they extend previous work in showing that the LIFG is recruited even in cases where the selection of meaning takes place implicitly.

Word-level information influences phonetic learning in adults and infants

Infants begin to segment words from fluent speech during the same time period that they learn phonetic categories. Segmented words can provide a potentially useful cue for phonetic learning, yet accounts of phonetic category acquisition typically ignore the contexts in which sounds appear. We present two experiments to show that, contrary to the assumption that phonetic learning occurs in isolation, learners are sensitive to the words in which sounds appear and can use this information to constrain their interpretation of phonetic variability. Experiment 1 shows that adults use word-level information in a phonetic category learning task, assigning acoustically similar vowels to different categories more often when those sounds consistently appear in different words. Experiment 2 demonstrates that 8-month-old infants similarly pay attention to word-level information and that this information affects how they treat phonetic contrasts. These findings suggest that phonetic category learning is a rich, interactive process that takes advantage of many different types of cues that are present in the input.

Neural correlates of semantic competition during processing of ambiguous words

The current study investigated the neural correlates that underlie the processing of ambiguous words and the potential effects of semantic competition on that processing. Participants performed speeded lexical decisions on semantically related and unrelated prime–target pairs presented in the auditory modality. The primes were either ambiguous words (e.g., ball) or unambiguous words (e.g., athlete), and targets were either semantically related to the dominant (i.e., most frequent) meaning of the ambiguous prime word (e.g., soccer) or to the subordinate (i.e., less frequent) meaning (e.g., dance). Results showed increased activation in the bilateral inferior frontal gyrus (IFG) for ambiguous-related compared to unambiguous-related stimulus pairs, demonstrating that prefrontal areas are activated even in an implicit task where participants are not required to explicitly analyze the semantic content of the stimuli and to make an overt selection of a particular meaning based on this analysis. Additionally, increased activation was found in the left IFG and the left cingulate gyrus for subordinate meaning compared to dominant meaning conditions, suggesting that additional resources are recruited in order to resolve increased competition demands in accessing the subordinate meaning of an ambiguous word.

Dissociable Effects of Phonetic Competition and Category Typicality in a Phonetic Categorization Task: An fMRI Investigation

The current study used fMRI to explore the extent to which neural activation patterns in the processing of speech are driven by the quality of a speech sound as a member of its phonetic category, that is, its category typicality, or by the competition inherent in resolving the category membership of stimuli which are similar to other possible speech sounds. Subjects performed a phonetic categorization task on synthetic stimuli ranging along a voice-onset time continuum from [da] to [ta]. The stimulus set included sounds at the extreme ends of the voicing continuum which were poor phonetic category exemplars, but which were minimally competitive, stimuli near the phonetic category boundary, which were both poor exemplars of their phonetic category and maximally competitive, and stimuli in the middle of the range which were good exemplars of their phonetic category. Results revealed greater activation in bilateral inferior frontal areas for stimuli with the greatest degree of competition, consistent with the view that these areas are involved in selection between competing alternatives. In contrast, greater activation was observed in bilateral superior temporal gyri for the least prototypical phonetic category exemplars, irrespective of competition, consistent with the view that these areas process the acoustic-phonetic details of speech to resolve a tokens category membership. Taken together, these results implicate separable neural regions in two different aspects of phonetic categorization.

An event-related fMRI investigation of voice-onset time discrimination.

The discrimination of voice-onset time, an acoustic-phonetic cue to voicing in stop consonants, was investigated to explore the neural systems underlying the perception of a rapid temporal speech parameter. Pairs of synthetic stimuli taken from a [da] to [ta] continuum varying in voice-onset time (VOT) were presented for discrimination judgments. Participants exhibited categorical perception, discriminating 15-ms and 30-ms between-category comparisons and failing to discriminate 15-ms within-category comparisons. Contrastive analysis with a tone discrimination task demonstrated left superior temporal gyrus activation in all three VOT conditions with recruitment of additional regions, particularly the right inferior frontal gyrus and middle frontal gyrus for the 15-ms between-category stimuli. Hemispheric differences using anatomically defined regions of interest showed two distinct patterns with anterior regions showing more activation in the right hemisphere relative to the left hemisphere and temporal regions demonstrating greater activation in the left hemisphere relative to the right hemisphere. Activation in the temporal regions appears to reflect initial acoustic-perceptual analysis of VOT. Greater activation in the right hemisphere anterior regions may reflect increased processing demands, suggesting involvement of the right hemisphere when the acoustic distance between the stimuli are reduced and when the discrimination judgment becomes more difficult.