Causal inference and temporal predictions in audiovisual perception of speech and music.

To form a coherent percept of the environment, the brain must integrate sensory signals emanating from a common source but segregate those from different sources. Temporal regularities are prominent cues for multisensory integration, particularly for speech and music perception. In line with models of predictive coding, we suggest that the brain adapts an internal model to the statistical regularities in its environment. This internal model enables cross-sensory and sensorimotor temporal predictions as a mechanism to arbitrate between integration and segregation of signals from different senses.

Where, when and why brain activation differs for bilinguals and monolinguals during picture naming and reading aloud.

Using functional magnetic resonance imaging, we found that when bilinguals named pictures or read words aloud, in their native or nonnative language, activation was higher relative to monolinguals in 5 left hemisphere regions: dorsal precentral gyrus, pars triangularis, pars opercularis, superior temporal gyrus, and planum temporale. We further demonstrate that these areas are sensitive to increasing demands on speech production in monolinguals. This suggests that the advantage of being bilingual comes at the expense of increased work in brain areas that support monolingual word processing. By comparing the effect of bilingualism across a range of tasks, we argue that activation is higher in bilinguals compared with monolinguals because word retrieval is more demanding; articulation of each word is less rehearsed; and speech output needs careful monitoring to avoid errors when competition for word selection occurs between, as well as within, language.

Auditory-motor expertise alters "speech selectivity" in professional musicians and actors.

Several perisylvian brain regions show preferential activation for spoken language above and beyond other complex sounds. These "speech-selective" effects might be driven by regions' intrinsic biases for processing the acoustical or informational properties of speech. Alternatively, such speech selectivity might emerge through extensive experience in perceiving and producing speech sounds. This functional magnetic resonance imaging (fMRI) study disambiguated such audiomotor expertise from speech selectivity by comparing activation for listening to speech and music in female professional violinists and actors. Audiomotor expertise effects were identified in several right and left superior temporal regions that responded to speech in all participants and music in violinists more than actresses. Regions associated with the acoustic/information content of speech were identified along the entire length of the superior temporal sulci bilaterally where activation was greater for speech than music in all participants. Finally, an effect of performing arts training was identified in bilateral premotor regions commonly activated by finger and mouth movements as well as in right hemisphere "language regions." These results distinguish the seemingly speech-specific neural responses that can be abolished and even reversed by long-term audiomotor experience.

Developmental dyslexia in Chinese and English populations: dissociating the effect of dyslexia from language differences.

Previous neuroimaging studies have suggested that developmental dyslexia has a different neural basis in Chinese and English populations because of known differences in the processing demands of the Chinese and English writing systems. Here, using functional magnetic resonance imaging, we provide the first direct statistically based investigation into how the effect of dyslexia on brain activation is influenced by the Chinese and English writing systems. Brain activation for semantic decisions on written words was compared in English dyslexics, Chinese dyslexics, English normal readers and Chinese normal readers, while controlling for all other experimental parameters. By investigating the effects of dyslexia and language in one study, we show common activation in Chinese and English dyslexics despite different activation in Chinese versus English normal readers. The effect of dyslexia in both languages was observed as less than normal activation in the left angular gyrus and in left middle frontal, posterior temporal and occipitotemporal regions. Differences in Chinese and English normal reading were observed as increased activation for Chinese relative to English in the left inferior frontal sulcus; and increased activation for English relative to Chinese in the left posterior superior temporal sulcus. These cultural differences were not observed in dyslexics who activated both left inferior frontal sulcus and left posterior superior temporal sulcus, consistent with the use of culturally independent strategies when reading is less efficient. By dissociating the effect of dyslexia from differences in Chinese and English normal reading, our results reconcile brain activation results with a substantial body of behavioural studies showing commonalities in the cognitive manifestation of dyslexia in Chinese and English populations. They also demonstrate the influence of cognitive ability and learning environment on a common neural system for reading.

The role of right and left parietal lobes in the conceptual processing of numbers.

Neuropsychological and functional imaging studies have associated the conceptual processing of numbers with bilateral parietal regions (including intraparietal sulcus). However, the processes driving these effects remain unclear because both left and right posterior parietal regions are activated by many other conceptual, perceptual, attention, and response-selection processes. To dissociate parietal activation that is number-selective from parietal activation related to other stimulus or response-selection processes, we used fMRI to compare numbers and object names during exactly the same conceptual and perceptual tasks while factoring out activations correlating with response times. We found that right parietal activation was higher for conceptual decisions on numbers relative to the same tasks on object names, even when response time effects were fully factored out. In contrast, left parietal activation for numbers was equally involved in conceptual processing of object names. We suggest that left parietal activation for numbers reflects a range of processes, including the retrieval of learnt facts that are also involved in conceptual decisions on object names. In contrast, number selectivity in right parietal cortex reflects processes that are more involved in conceptual decisions on numbers than object names. Our results generate a new set of hypotheses that have implications for the design of future behavioral and functional imaging studies of patients with left and right parietal damage.

Left insula activation: a marker for language attainment in bilinguals.

Several lines of evidence suggest the importance of phonological working memory (PWM) in language acquisition. We investigated the neural correlates of PWM in young adults who were under compelling social pressure to be bilingual. Equal bilinguals had high proficiency in English and Chinese as measured by a standardized examination, whereas unequal bilinguals were proficient in English but not Chinese. Both groups were matched on several measures of nonverbal intelligence and working memory. In-scanner behavioral results did not show between-group differences. Of the regions showing load-dependent increments in activation, the left insula showed greater activation in equal bilinguals. Unequal bilinguals showed greater task-related deactivation in the anterior medial frontal region and greater anterior cingulate activation. Although unequal bilinguals kept apace with equal bilinguals in the simple PWM task, the differential cortical activations suggest that more optimal engagement of PWM in the latter may correlate with better second-language attainment.

Reproducibility of the word frequency effect: comparison of signal change and voxel counting.

We determined the reproducibility of both the direction and the effect size of the word frequency effect (WFE) as it relates to associative semantic judgments. Sixteen volunteers were scanned twice. At the group level of analysis, signal change and voxel counting could both reproducibly detect the existence of a WFE. However, signal change data showed less intersession variation, particularly in the left inferior frontal gyrus. The effect size of WFE was well reproduced only with signal change measurements. In consideration of the signal change data, statistical threshold did not have a major effect on the detection or determination of the effect size. In general, while the direction of the WFE was reasonably reproducible at the individual level, the effect size was far less well reproduced. These findings suggest that with existing techniques, fMRI may be used to track changes in brain activation stemming from improvement in language proficiency at the group level but not at the individual level.

Common and segregated neuronal networks for different languages revealed using functional magnetic resonance adaptation.

The effect of word repetition within and across languages was studied in English-Chinese bilinguals who read rapidly presented word pairs in a block design and an event-related fMRI study. Relatively less increase in MR signal was observed when the second word in a pair was identical in meaning to the first. This occurred in the English-only and mixed-languages conditions. Repetition-induced reductions in BOLD signal change were found in the left lateral prefrontal and lateral temporal regions in both types of conditions in the block experiment, suggesting that processing in these regions is sensitive to semantic features present in words and characters, and that part of the semantic neuronal networks serving English and Chinese is shared. In addition, these regions showed greater absolute signal change in the mixed-languages trials relative to the English-only trials. These findings were mostly replicated in an event-related experiment. Together, the experiments suggest that while the networks for Chinese and English word processing have shared components, there are also components that may be language specific.

Frequency of concrete words modulates prefrontal activation during semantic judgments.

The effect of word frequency on semantic processing was characterized by studying two groups of right-handed participants using fMRI. Stimuli were presented in blocks of either high frequency or low frequency word triplets where a sample word appeared above a pair of test words. One group (n = 8) made semantic judgments by selecting the word from the test pair that was more closely associated with the sample. Stimulus triplets were designed such that relatedness between sample and "correct" items was obvious. The other group (n = 8) read the words silently without making any semantic decision and pressed a button on completing the reading of each triplet. Semantic judgments while no less accurate, were associated with greater left prefrontal BOLD signal change when they involved low frequency words, whereas there was no reliable effect of word frequency in the reading condition. These findings suggest that retrieval effort modulates left prefrontal activity when deliberate access to semantics is required.