In contrast to simple structures in animal vocal behavior, hierarchical structures such as center-embedded sentences manifest the core computational faculty of human language. Previous artificial grammar learning studies found that the left pars opercularis (LPO) subserves the processing of hierarchical structures. However, it is not clear whether this area is activated by the structural complexity per se or by the increased memory load entailed in processing hierarchical structures. To dissociate the effect of structural complexity from the effect of memory cost, we conducted a functional magnetic resonance imaging study of German sentence processing with a 2-way factorial design tapping structural complexity (with/without hierarchical structure, i.e., center-embedding of clauses) and working memory load (long/short distance between syntactically dependent elements; i.e., subject nouns and their respective verbs). Functional imaging data revealed that the processes for structure and memory operate separately but co-operatively in the left inferior frontal gyrus; activities in the LPO increased as a function of structural complexity, whereas activities in the left inferior frontal sulcus (LIFS) were modulated by the distance over which the syntactic information had to be transferred. Diffusion tensor imaging showed that these 2 regions were interconnected through white matter fibers. Moreover, functional coupling between the 2 regions was found to increase during the processing of complex, hierarchically structured sentences. These results suggest a neuroanatomical segregation of syntax-related aspects represented in the LPO from memory-related aspects reflected in the LIFS, which are, however, highly interconnected functionally and anatomically.DTI ͉ fMRI ͉ hierarchical structure L anguage appears to be a trait specific to humans-at least in its core computational component, that is, grammar. Defining language as a sequence of symbols, Chomsky (1) proposed a hierarchy of grammars as language production mechanisms with increasing generative powers. The lowest-level grammar is finite state grammar (FSG). FSG can be fully specified by transition probabilities between a finite number of states (e.g., words), being not powerful enough to generate structures of natural human languages. Phrase structure grammar (PSG) has more generative power than FSG. A key difference between FSG and PSG is that only PSG can generate the sequence A n B n , where A and B denote symbols and n the number of repetitions. The ability to process the sequence A n B n is crucial for the processing of center-embedded sentences, such as ''The man the boy the dog bit greeted is my friend.'' where subjects (i.e., the man, the boy, and the dog) are A-symbols and the verbs (bit, greeted, and is) are B-symbols. Surprisingly, tests on monkeys (2) and on songbirds (3) showed that whereas songbirds can process A n B n sequences, monkeys cannot. However, even if the birds could correctly discriminate A n B n sequences from A n B m , (4 Ͼ n, m Ͼ0, n m),...

Language comes in utterances in which words are bound together according to a simple rule-based syntactic computation (merge), which creates linguistic hierarchies of potentially infinite length-phrases and sentences. In the current functional magnetic resonance imaging study, we compared prepositional phrases and sentences-both involving merge-to word lists-not involving merge-to explore how this process is implemented in the brain. We found that merge activates the pars opercularis of the left inferior frontal gyrus (IFG; Brodmann Area [BA] 44) and a smaller region in the posterior superior temporal sulcus (pSTS). Within the IFG, sentences engaged a more anterior portion of the area (pars triangularis, BA 45)-compared with phrases-which showed activity peak in BA 44. As prepositional phrases, in contrast to sentences, do not contain verbs, activity in BA 44 may reflect structure-building syntactic processing, while the involvement of BA 45 may reflect the encoding of propositional meaning initiated by the verb. The pSTS appears to work together with the IFG during thematic role assignment not only at the sentential level, but also at the phrasal level. The present results suggest that merge, the process of binding words together into syntactic hierarchies, is primarily supported by BA 44 in the IFG.

The study investigates to what extent the posterior superior temporal cortex is involved in processing complex sentences. Using functional MRI, we show that hierarchically structured sentences activate the superior temporal cortex bilaterally to greater extent than sentences with a linear structure. The activation in the left hemisphere comprises the superior temporal gyrus and sulcus, whereas the activation in the right hemisphere is confined to the superior temporal sulcus. As earlier studies using similar syntactic structures in semantic-free grammars did not show activation in the superior temporal cortex but instead only in the prefrontal cortex, we conclude that the role of the posterior superior temporal cortex is to integrate lexical-semantic and syntactic information during sentence comprehension.

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Sentence comprehension is a complex process. Besides identifying the meaning of each word and processing the syntactic structure of a sentence, it requires the computation of thematic information, that is, information about who did what to whom. The present fMRI study investigated the neural basis for thematic reanalysis (reanalysis of the thematic roles initially assigned to noun phrases in a sentence) and its interplay with syntactic reanalysis (reanalysis of the underlying syntactic structure originally constructed for a sentence). Thematic reanalysis recruited a network consisting of Broca's area, that is, the left pars triangularis (LPT), and the left posterior superior temporal gyrus, whereas only LPT showed greater sensitivity to syntactic reanalysis. These data provide direct evidence for a functional neuroanatomical basis for two linguistically motivated reanalysis processes during sentence comprehension.

Various noncanonical sentence constructions are derived from basic sentence structures by a phrase displacement called Movement. The moved phrase (filler) leaves a silent copy at the extracted position (gap) and is reactivated when the hearer/reader passes over the gap. Consequently, memory operations are assumed to occur to establish the filler-gap link. For languages that have a relatively free word order like German, a distinct linguistic operation called Scrambling is proposed. Although Movement and Scrambling are assumed to be different linguistic operations, they both involve memory prone filler-gap processes. To clarify whether filler-gap memory processes in Scrambling and Movement differ neuroanatomically, we designed a functional magnetic resonance imaging study and compared the effect of memory load parameterized by filler-gap distance in the 2 sentence types. Here, we show that processing of the 2 sentence types commonly relies on a left hemispheric network consisting of the inferior frontal gyrus, middle part of the middle temporal gyrus, and intraparietal sulcus. However, we found differences for the 2 sentence types in the linearity of filler-gap distance effect. Thus, the present results suggest that the same neural substrate supports the memory processes of sentences constructed by Movement and Scrambling, although differentially modulated by memory load.

Some neuroimaging studies have reported activation by imitation in the left Brodmann area 44 (BA 44), a part of Broca's area considered to be a neural substrate for speech production. However, in these previous studies the subjects were required to perform the same movements repeatedly so that the experimental stimuli could be viewed as cues when to do the given movements rather than a specification of what to do. Activation in the left BA 44 has also been observed in delayed motor execution tasks. This may confound the activity in BA 44 for imitation tasks used in the former studies. We tested the involvement of bilateral BA 44 and BA 45 in imitation and delayed execution tasks by functional magnetic resonance imaging with cytoarchitectonically defined BA 44 and BA 45 as volumes of interest. Our tasks required the subjects to perform a transformation from visual information (photographs of hand postures or symbolic specification of the postures) into a new hand movement in each trial. A three-way analysis of variance was performed with factors instruction, execution timing and area. The results revealed significant main effect by execution timing and by area, but not by instruction. We conclude that Broca's area does not play a pivotal role in imitation.

We aimed to dissociate two levels of hierarchical structure building in language and mathematics, namely 'first-level' (the build-up of hierarchical structure with externally given elements) and 'second-level' (the build-up of hierarchical structure with internally represented elements produced by first-level processes). Using functional magnetic resonance imaging, we investigated these processes in three domains: sentence comprehension, arithmetic calculation (using Reverse Polish notation, which gives two operands followed by an operator) and a working memory control task. All tasks required the build-up of hierarchical structures at the first-and second-level, resulting in a similar computational hierarchy across language and mathematics, as well as in a working memory control task. Using a novel method that estimates the difference in the integration cost for conditions of different trial durations, we found an anterior-to-posterior functional organization in the prefrontal cortex, according to the level of hierarchy. Common to all domains, the ventral premotor cortex (PMv) supports first-level hierarchy building, while the dorsal pars opercularis (POd) subserves second-level hierarchy building, with lower activation for language compared with the other two tasks. These results suggest that the POd and the PMv support domain-general mechanisms for hierarchical structure building, with the POd being uniquely efficient for language.