An fMRI study of grammatical morpheme processing associated with nouns and verbs in Chinese

This study examined whether the degree of complexity of a grammatical component in a language would impact on its representation in the brain through identifying the neural correlates of grammatical morpheme processing associated with nouns and verbs in Chinese. In particular, the processing of Chinese nominal classifiers and verbal aspect markers were investigated in a sentence completion task and a grammaticality judgment task to look for converging evidence. The Chinese language constitutes a special case because it has no inflectional morphology per se and a larger classifier than aspect marker inventory, contrary to the pattern of greater verbal than nominal paradigmatic complexity in most European languages. The functional imaging results showed BA47 and left supplementary motor area and superior medial frontal gyrus more strongly activated for classifier processing, and the left posterior middle temporal gyrus more responsive to aspect marker processing. We attributed the activation in the left prefrontal cortex to greater processing complexity during classifier selection, analogous to the accounts put forth for European languages, and the left posterior middle temporal gyrus to more demanding verb semantic processing. The overall findings significantly contribute to cross-linguistic observations of neural substrates underlying processing of grammatical morphemes from an analytic and a classifier language, and thereby deepen our understanding of neurobiology of human language.published_or_final_versio

Heterogeneity of the Left Temporal Lobe in Semantic Representation and Control: Priming Multiple versus Single Meanings of Ambiguous Words

Semantic judgments involve both representations of meaning plus executive mechanisms that guide knowledge retrieval in a task-appropriate way. These 2 components of semantic cognition—representation and control—are commonly linked to left temporal and prefrontal cortex, respectively. This simple proposal, however, remains contentious because in most functional neuroimaging studies to date, the number of concepts being activated and the involvement of executive processes during retrieval are confounded. Using functional magnetic resonance imaging, we examined a task in which semantic representation and control demands were dissociable. Words with multiple meanings like “bank” served as targets in a double-prime paradigm, in which multiple meaning activation and maximal executive demands loaded onto different priming conditions. Anterior inferior temporal gyrus (ITG) was sensitive to the number of meanings that were retrieved, suggesting a role for this region in semantic representation, while posterior middle temporal gyrus (pMTG) and inferior frontal cortex showed greater activation in conditions that maximized executive demands. These results support a functional dissociation between left ITG and pMTG, consistent with a revised neural organization in which left prefrontal and posterior temporal areas work together to underpin aspects of semantic control

Semantic processing of Chinese in left inferior prefrontal cortex studied with reversible words

This study utilized fast event-related fMRI with reversible words to examine the role of left inferior prefrontal cortex (PFC) in semantic processing of Chinese. As a special linguistic phenomenon in Chinese, a reversible word is a two-character word (AB) that, when read from right to left (BA), opposite to the normal left to right reading direction, is also a real word. The two words, AB and BA, can have very different meanings. Fourteen native Chinese saw a reversible word (BA) and were asked to read it backward silently to obtain the meaning of AB, defined as the target meaning. They then saw two test words and decided which of the two was semantically related to the target meaning. Activity in a subregion of BA47 was found to be modulated by the extent to which irrelevant semantic activation of the distractor word BA interfered with semantic retrieval of the target word AB. This finding demonstrated the involvement of the left inferior PFC in the control processes of semantic retrieval in Chinese. In addition, comparing conditions using reversible with that using nonreversible words, we found evidence suggesting a semantic/phonological functional subdivision in left inferior PFC, consistent with that in English.This study utilized fast event-related fMRI with reversible words to examine the role of left inferior prefrontal cortex (PFC) in semantic processing of Chinese. As a special linguistic phenomenon in Chinese, a reversible word is a two-character word (AB) that, when read from right to left (BA), opposite to the normal left to right reading direction, is also a real word. The two words, AB and BA, can have very different meanings. Fourteen native Chinese saw a reversible word (BA) and were asked to read it backward silently to obtain the meaning of AB, defined as the target meaning. They then saw two test words and decided which of the two was semantically related to the target meaning. Activity in a subregion of BA47 was found to be modulated by the extent to which irrelevant semantic activation of the distractor word BA interfered with semantic retrieval of the target word AB. This finding demonstrated the involvement of the left inferior PFC in the control processes of semantic retrieval in Chinese. In addition, comparing conditions using reversible with that using nonreversible words, we found evidence suggesting a semantic/phonological functional subdivision in left inferior PFC, consistent with that in English. (C) 2004 Elsevier Inc. All rights reserved

Controlling memory and resolving interference : prefrontal contributions to flexible behavior

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2005.Includes bibliographical references.Flow does the brain meet shifting task demands? The experiments and formal theoretical framework presented in this dissertation characterize the cognitive and neural processes by which flexible performance is enabled during task switching. Chapter 1 reviews major findings and controversies in the task switching literature, highlighting (1) evidence that behavioral switch costs may derive from proactive interference due to the facilitated retrieval of irrelevant competitors from long-term memory and (2) the consistent finding of activation in ventrolateral prefrontal cortex (VLPFC) during task switching. These observations motivate the hypothesis that left VLPFC may resolve proactive interference arising from long-term memory during a task switch. Chapters 2 and 3 describe three fMRI experiments conducted in experimental contexts independent of task switching that directly link left mid-VLPFC (Brodmann's Area 45; inferior frontal gyrus pars triangularis) to a post-retrieval selection process that resolves proactive interference from irrelevant representations retrieved from long-term memory. Chapter 4 introduces a computational model that derives its task switch cost from interference due to performance-dependent changes in its associative structure, and that resolves this interference through a control process that biases retrieved conceptual representations. Critically, a conflict signal, derived from retrieved conceptual representations in the model, is shown to be characteristic of the pattern of response in left mid-VLPFC during an fMRI experiment that manipulates preparation and interference in task switching. Furthermore, this pattern dissociates left mid-VLPFC from other regions active during a task switch.(cont.) These data strongly support the hypothesis that task switch costs derive from proactive interference due to facilitated retrieval of irrelevant representations and left mid-VLPFC serves to overcome this proactive interference. Chapter 5 provides further details of the model, demonstrates its power to explain a number of common task switching phenomena, and explores its relationship with three other prominent formal models of task switching. The experiments and associated theory presented in this thesis provide evidence that instances of flexible behavior, like task switching, may be understood as acts of memory, and are enabled by prefrontal cortex mechanisms that control memory to overcome interference.by David Badre.Ph.D