The cortical distribution and interaction of semantic knowledge

Concepts are considered to be the building blocks of human higher-order cognition. Yet theories differ according to how these semantic representations are instantiated within the brain. The amodal characteristics of word meaning imply that this knowledge is stored independent of perceptual experiences. However, mounting evidence suggests that concepts depend upon cortical regions typically ascribed to sensory input. This embodiment of semantic representations through perceptual mechanisms can crucially explain the relationship between the meaning conveyed by words and experience with the associated objects. Across two experiments, this research used functional MRI to examine the role of sensory and prefrontal brain regions while participants verified semantic properties (e.g., sounds loud?; lays eggs?) of word items. The results show that perceptual properties activate the predicted cortical regions associated with vision, audition, taste and smell, and touch. Increased response times for these perceptual decisions were not associated with increased activity in the identified sensory areas but were associated with increased activity in prefrontal brain regions. In contrast, more abstract semantic decisions led to increased activity in the prefrontal cortex but no such increases were seen for the more difficult decisions. These findings indicate that multiple and widely distributed brain regions used to encode perceptual experiences also support semantic knowledge of those sensory experiences. The prefrontal cortex may represent abstract knowledge and control retrieval with increasing semantic demands for decisions further removed from perceptual experiences

Prefrontal Cortical Response to Conflict during Semantic and Phonological Tasks

Debates about the function of the prefrontal cortex are as old as the field of neuropsychology—often dated to Paul Broca’s seminal work. Theories of the functional organization of the prefrontal cortex can be roughly divided into those that describe organization by process and those that describe organization by material. Recent studies of the function of the posterior, left inferior frontal gyrus (pLIFG) have yielded two quite different interpretations: One hypothesis holds that the pLIFG plays a domain-specific role in phonological processing, whereas another hypothesis describes a more general function of the pLIFG in cognitive control. In the current study, we distinguish effects of increasing cognitive control demands from effects of phonological processing. The results support the hypothesized role for the pLIFG in cognitive control, and more task-specific roles for posterior areas in phonology and semantics. Thus, these results suggest an alternative explanation of previously reported phonology-specific effects in the pLIFG

ELUCIDATING THE FUNCTIONAL NEURAL CORRELATES OF EMOTIONAL FACE PROCESSING DEFICITS IN BEHAVIOURAL VARIANT FRONTOTEMPORAL DEMENTIA

Frontotemporal dementia is a devastating neurodegenerative disorder consisting of progressive focal atrophy of the prefrontal and temporal lobes. Emotional facial expression deficits are widely acknowledged in behavioural variant frontotemporal dementia (bvFTD) and are speculated to partially account for patients’ social-cognitive deficits. To our knowledge this is the first study to delineate the functional neuroanatomy of facial expression processing in bvFTD using functional MRI, while controlling for voxel-wise atrophy. The results indicate emotion-specific functional abnormalities in frontotemporal regions in patients with bvFTD. BvFTD patients also demonstrated decreased activity in posterior ventral visual regions, perhaps suggesting reduced input from anterior frontal and limbic regions. Finally, bvFTD was associated with increased activity in the dorsal attentional network, providing some o f the first evidence of a potential compensatory response for functional deficits in frontotemporal regions. Together these findings suggest that functional MRI combined with tasks targeting social-cognitive deficits is a powerful technique to quantify neural systems involved in emotion processing in bvFTD

Down but not out in posterior cingulate cortex : Deactivation yet functional coupling with prefrontal cortex during demanding semantic cognition

The posterior cingulate cortex (pCC) often deactivates during complex tasks, and at rest is often only weakly correlated with regions that play a general role in the control of cognition. These observations led to the hypothesis that pCC contributes to automatic aspects of memory retrieval and cognition. Recent work, however, has suggested that the pCC may support both automatic and controlled forms of memory processing and may do so by changing its communication with regions that are important in the control of cognition across multiple domains. The current study examined these alternative views by characterising the functional coupling of the pCC in easy semantic decisions (based on strong global associations) and in harder semantic tasks (matching words on the basis of specific non-dominant features). Increasingly difficult semantic decisions led to the expected pattern of deactivation in the pCC; however, psychophysiological interaction analysis revealed that, under these conditions, the pCC exhibited greater connectivity with dorsolateral prefrontal cortex (PFC), relative to both easier semantic decisions and to a period of rest. In a second experiment using different participants, we found that functional coupling at rest between the pCC and the same region of dorsolateral PFC was stronger for participants who were more efficient at semantic tasks when assessed in a subsequent laboratory session. Thus, although overall levels of activity in the pCC are reduced during external tasks, this region may show greater coupling with executive control regions when information is retrieved from memory in a goal-directed manner

Neural Systems for Reading Aloud: A Multiparametric Approach

Reading aloud involves computing the sound of a word from its visual form. This may be accomplished 1) by direct associations between spellings and phonology and 2) by computation from orthography to meaning to phonology. These components have been studied in behavioral experiments examining lexical properties such as word frequency; length in letters or phonemes; spelling–sound consistency; semantic factors such as imageability, measures of orthographic, or phonological complexity; and others. Effects of these lexical properties on specific neural systems, however, are poorly understood, partially because high intercorrelations among lexical factors make it difficult to determine if they have independent effects. We addressed this problem by decorrelating several important lexical properties through careful stimulus selection. Functional magnetic resonance imaging data revealed distributed neural systems for mapping orthography directly to phonology, involving left supramarginal, posterior middle temporal, and fusiform gyri. Distinct from these were areas reflecting semantic processing, including left middle temporal gyrus/inferior-temporal sulcus, bilateral angular gyrus, and precuneus/posterior cingulate. Left inferior frontal regions generally showed increased activation with greater task load, suggesting a more general role in attention, working memory, and executive processes. These data offer the first clear evidence, in a single study, for the separate neural correlates of orthography–phonology mapping and semantic access during reading aloud

Neural overlap of L1 and L2 semantic representations across visual and auditory modalities : a decoding approach/

This study investigated whether brain activity in Dutch-French bilinguals during semantic access to concepts from one language could be used to predict neural activation during access to the same concepts from another language, in different language modalities/tasks. This was tested using multi-voxel pattern analysis (MVPA), within and across language comprehension (word listening and word reading) and production (picture naming). It was possible to identify the picture or word named, read or heard in one language (e.g. maan, meaning moon) based on the brain activity in a distributed bilateral brain network while, respectively, naming, reading or listening to the picture or word in the other language (e.g. lune). The brain regions identified differed across tasks. During picture naming, brain activation in the occipital and temporal regions allowed concepts to be predicted across languages. During word listening and word reading, across-language predictions were observed in the rolandic operculum and several motor-related areas (pre- and postcentral, the cerebellum). In addition, across-language predictions during reading were identified in regions typically associated with semantic processing (left inferior frontal, middle temporal cortex, right cerebellum and precuneus) and visual processing (inferior and middle occipital regions and calcarine sulcus). Furthermore, across modalities and languages, the left lingual gyrus showed semantic overlap across production and word reading. These findings support the idea of at least partially language- and modality-independent semantic neural representations

Asymmetry of the Frontal Aslant Tract is Associated with Lexical Decision

The frontal aslant tract (FAT) is a recently documented white matter tract that connects the inferior and superior frontal gyri with a tendency to be more pronounced in the left hemisphere. This tract has been found to play a role in language functions, particularly verbal fluency. However, it is not entirely clear to what extent FAT asymmetry is related to performance benefits in language-related tasks. In the present study, we aimed to fill this gap by examining the correlations between asymmetric micro- and macro-structural properties of the FAT and performance on verbal fluency and lexical decision tasks. The results showed no correlation between the FAT and verbal fluency; however, lexical decision was correlated with FAT laterality. Specifically, greater left lateralization in both micro- and macro-structural properties was related to faster lexical decision response times. The results were not due merely to motor or decision-making processes, as responses in a simple discrimination task showed no correlation with laterality. These data are the first to suggest a role for the FAT in mediating processes underlying lexical decision