Xiaojian Li

Hemispheric roles in the perception of speech prosody

Speech prosody is processed in neither a single region nor a specific hemisphere, but engages multiple areas comprising a large-scale spatially distributed network in both hemispheres. It remains to be elucidated whether hemispheric lateralization is based on higher-level prosodic representations or lower-level encoding of acoustic cues, or both. A cross-language (Chinese; English) fMRI study was conducted to examine brain activity elicited by selective attention to Chinese intonation (I) and tone (T) presented in three-syllable (I3, T3) and one-syllable (I1, T1) utterance pairs in a speeded response, discrimination paradigm. The Chinese group exhibited greater activity than the English in a left inferior parietal region across tasks (I1, I3, T1, T3). Only the Chinese group exhibited a leftward asymmetry in inferior parietal and posterior superior temporal (I1, I3, T1, T3), anterior temporal (I1, I3, T1, T3), and frontopolar (I1, I3) regions. Both language groups shared a rightward asymmetry in the mid portions of the superior temporal sulcus and middle frontal gyrus irrespective of prosodic unit or temporal interval. Hemispheric laterality effects enable us to distinguish brain activity associated with higher-order prosodic representations in the Chinese group from that associated with lower-level acoustic/auditory processes that are shared among listeners regardless of language experience. Lateralization is influenced by language experience that shapes the internal prosodic representation of an external auditory signal. We propose that speech prosody perception is mediated primarily by the RH, but is left-lateralized to task-dependent regions when language processing is required beyond the auditory analysis of the complex sound.

A Cross-Linguistic fMRI Study of Spectral and Temporal Cues Underlying Phonological Processing

It remains a matter of controversy precisely what kind of neural mechanisms underlie functional asymmetries in speech processing. Whereas some studies support speech-specific circuits, others suggest that lateralization is dictated by relative computational demands of complex auditory signals in the spectral or time domains. To examine how the brain processes linguistically relevant spectral and temporal information, a functional magnetic resonance imaging study was conducted using Thai speech, in which spectral processing associated with lexical tones and temporal processing associated with vowel length can be differentiated. Ten Thai and 10 Chinese subjects were asked to perform discrimination judgments of pitch and timing patterns presented in the same auditory stimuli under two different conditions: speech (Thai) and nonspeech (hums). In the speech condition, tasks required judging Thai tones (T) and vowel length (VL); in the nonspeech condition, homologous pitch contours (P) and duration patterns (D). A remaining task required listening passively to nonspeech hums (L). Only the Thai group showed activation in the left inferior prefrontal cortex in speech minus nonspeech contrasts for spectral (T vs. P) and temporal (VL vs. D) cues. Thai and Chinese groups, however, exhibited similar fronto-parietal activation patterns in nonspeech hums minus passive listening contrasts for spectral (P vs. L) and temporal (D vs. L) cues. It appears that lower level specialization for acoustic cues in the spectral and temporal domains cannot be generalized to abstract higher order levels of phonological processing. Regardless of the neural mechanisms underlying low-level auditory processing, our findings clearly indicate that hemispheric specialization is sensitive to language-specific factors.

A cross-linguistic fMRI study of perception of intonation and emotion in Chinese

Conflicting data from neurobehavioral studies of the perception of intonation (linguistic) and emotion (affective) in spoken language highlight the need to further examine how functional attributes of prosodic stimuli are related to hemispheric differences in processing capacity. Because of similarities in their acoustic profiles, intonation and emotion permit us to assess to what extent hemispheric lateralization of speech prosody depends on functional instead of acoustical properties. To examine how the brain processes linguistic and affective prosody, an fMRI study was conducted using Chinese, a tone language in which both intonation and emotion may be signaled prosodically, in addition to lexical tones. Ten Chinese and 10 English subjects were asked to perform discrimination judgments of intonation (I: statement, question) and emotion (E: happy, angry, sad) presented in semantically neutral Chinese sentences. A baseline task required passive listening to the same speech stimuli (S). In direct between‐group comparisons, the Chinese group showed left‐sided frontoparietal activation for both intonation (I vs. S) and emotion (E vs. S) relative to baseline. When comparing intonation relative to emotion (I vs. E), the Chinese group demonstrated prefrontal activation bilaterally; parietal activation in the left hemisphere only. The reverse comparison (E vs. I), on the other hand, revealed that activation occurred in anterior and posterior prefrontal regions of the right hemisphere only. These findings show that some aspects of perceptual processing of emotion are dissociable from intonation, and, moreover, that they are mediated by the right hemisphere. Hum. Brain Mapping 18:149–157, 2003.

Activation of the Left Planum Temporale in Pitch Processing Is Shaped by Language Experience

Implicit, abstract knowledge acquired through language experience can alter cortical processing of complex auditory signals. To isolate prelexical processing of linguistic tones (i.e., pitch variations that convey part of word meaning), a novel design was used in which hybrid stimuli were created by superimposing Thai tones onto Chinese syllables (tonal chimeras) and Chinese tones onto the same syllables (Chinese words). Native speakers of tone languages (Chinese, Thai) underwent fMRI scans as they judged tones from both stimulus sets. In a comparison of native vs. non‐native tones, overlapping activity was identified in the left planum temporale (PT). In this area a double dissociation between language experience and neural representation of pitch occurred such that stronger activity was elicited in response to native as compared to non‐native tones. This finding suggests that cortical processing of pitch information can be shaped by language experience and, moreover, that lateralized PT activation can be driven by top‐down cognitive processing. Hum Brain Mapp, 2005.

Neural Correlates of Segmental and Tonal Information in Speech Perception

The Chinese language provides an optimal window for investigating both segmental and suprasegmental units. The aim of this cross‐linguistic fMRI study is to elucidate neural mechanisms involved in extraction of Chinese consonants, rhymes, and tones from syllable pairs that are distinguished by only one phonetic feature (minimal) vs. those that are distinguished by two or more phonetic features (non‐minimal). Triplets of Chinese monosyllables were constructed for three tasks comparing consonants, rhymes, and tones. Each triplet consisted of two target syllables with an intervening distracter. Ten Chinese and English subjects were asked to selectively attend to targeted sub‐syllabic components and make same‐different judgments. Direct between‐group comparisons in both minimal and non‐minimal pairs reveal increased activation for the Chinese group in predominantly left‐sided frontal, parietal, and temporal regions. Within‐group comparisons of non‐minimal and minimal pairs show that frontal and parietal activity varies for each sub‐syllabic component. In the frontal lobe, the Chinese group shows bilateral activation of the anterior middle frontal gyrus (MFG) for rhymes and tones only. Within‐group comparisons of consonants, rhymes, and tones show that rhymes induce greater activation in the left posterior MFG for the Chinese group when compared to consonants and tones in non‐minimal pairs. These findings collectively support the notion of a widely distributed cortical network underlying different aspects of phonological processing. This neural network is sensitive to the phonological structure of a listeners native language. Hum. Brain Mapping 20:185–200, 2003.

Neural circuitry underlying sentence-level linguistic prosody

This study investigates the neural substrates underlying the perception of two sentence-level prosodic phenomena in Mandarin Chinese: contrastive stress (initial vs. final emphasis position) and intonation (declarative vs. interrogative modality). In an fMRI experiment, Chinese and English listeners were asked to selectively attend to either stress or intonation in paired 3-word sentences, and make speeded-response discrimination judgments. Between-group comparisons revealed that the Chinese group exhibited significantly greater activity in the left supramarginal gyrus and posterior middle temporal gyrus relative to the English group for both tasks. These same two regions showed a leftward asymmetry in the stress task for the Chinese group only. For both language groups, rightward asymmetries were observed in the middle portion of the middle frontal gyrus across tasks. All task effects involved greater activity for the stress task as compared to intonation. A left-sided task effect was observed in the posterior middle temporal gyrus for the Chinese group only. Both language groups exhibited a task effect bilaterally in the intraparietal sulcus. These findings support the emerging view that speech prosody perception involves a dynamic interplay among widely distributed regions not only within a single hemisphere but also between the two hemispheres. This model of speech prosody processing emphasizes the role of right hemisphere regions for complex-sound analysis, whereas task-dependent regions in the left hemisphere predominate when language processing is required.

Neural basis of first and second language processing of sentence-level linguistic prosody

A fundamental question in multilingualism is whether the neural substrates are shared or segregated for the two or more languages spoken by polyglots. This study employs functional MRI to investigate the neural substrates underlying the perception of two sentence‐level prosodic phenomena that occur in both Mandarin Chinese (L1) and English (L2): sentence focus (sentence‐initial vs. ‐final position of contrastive stress) and sentence type (declarative vs. interrogative modality). Late‐onset, medium proficiency Chinese‐English bilinguals were asked to selectively attend to either sentence focus or sentence type in paired three‐word sentences in both L1 and L2 and make speeded‐response discrimination judgments. L1 and L2 elicited highly overlapping activations in frontal, temporal, and parietal lobes. Furthermore, region of interest analyses revealed that for both languages the sentence focus task elicited a leftward asymmetry in the supramarginal gyrus; both tasks elicited a rightward asymmetry in the mid‐portion of the middle frontal gyrus. A direct comparison between L1 and L2 did not show any difference in brain activation in the sentence type task. In the sentence focus task, however, greater activation for L2 than L1 occurred in the bilateral anterior insula and superior frontal sulcus. The sentence focus task also elicited a leftward asymmetry in the posterior middle temporal gyrus for L1 only. Differential activation patterns are attributed primarily to disparities between L1 and L2 in the phonetic manifestation of sentence focus. Such phonetic divergences lead to increased computational demands for processing L2. These findings support the view that L1 and L2 are mediated by a unitary neural system despite late age of acquisition, although additional neural resources may be required in task‐specific circumstances for unequal bilinguals. Hum. Brain Mapp, 2007.

Hemispheric asymmetries in phonological processing of tones versus segmental units

The aim of this functional magnetic resonance imaging study is to identify neuroanatomical substrates underlying phonological processing of segmental (consonant, rhyme) and suprasegmental (tone) units. An auditory verbal recognition paradigm was used in which native speakers of Mandarin Chinese were required to match a phonological unit that occurs in a list of three syllables to the corresponding unit of a following probe. The results show that hemispheric asymmetries arise depending on the type of phonological unit. In direct contrasts between phonological units, tones, relative to consonants and rhymes, yield increased activation in frontoparietal areas of the right hemisphere. This finding indicates that the cortical circuitry subserving lexical tones differs from that of consonants or rhymes.

The phonological similarity effect in immediate recall: Positions of shared phonemes

Earlier literature proposes two ways phonological similarity could harm immediate recall: (1) It could increase the degradation of the representations of items in memory, or (2) it could decrease the probability that a degraded representation is correctly reconstructed. A multinomial processing tree model for each hypothesis was used to analyze an immediate recall experiment. Both gave a good account of the data, but, of the two, results favor the hypothesis that the effect of phonological similarity is to impair reconstruction of degraded representations. A second issue is whether positions of repeated phonemes in phonologically similar items matter. We found that mere repetition of phonemes produced a phonological similarity effect. Repeated phonemes in the same positions appeared to produce a greater effect. A final finding is that when reading rate was preequated, phonological similarity affected memory span by changing the time taken to recall a list of span length.

Neural network for encoding immediate memory in phonological processing

The aim of this fMRI study was to identify neuroanatomical substrates of immediate memory underlying phonological processing. To distinguish encoding of immediate memory from rehearsal, participants were required to match tones from the first and last positions of a three-syllable list to their following probes in an immediate-recognition paradigm. The first position task included intervening distractors between the target and probe. Increased activations were found in the left inferior frontal gyrus, right lateral cerebellum, and medial frontal gyrus for the target tone in first position. This network mediates articulatory encoding in immediate-response, and articulatory rehearsal in delayed-response paradigms. These findings support a working memory model in which rehearsal is optional, while encoding is an obligatory component of the phonological loop.