Left Anterior Inferior Frontal Gyrus Research Articles

348 Resting state fMRI connectivity in individuals with aphasia and the role of the inferior frontal gyrus

OBJECTIVES/GOALS: This poster will explore lesion-symptom mapping and resting state functional magnetic resonance imaging (rsfMRI) in participants with aphasia. This analysis aims to reveal the behavioral impact of distinct lesions to the left inferior frontal gyrus (LIFG) and the potential for rsfMRI to provide evidence of cortical reorganization. METHODS/STUDY POPULATION: The analyses will be conducted on a small sample of approximately six participants with post-stroke language impairment (i.e., aphasia) recruited as part of an ongoing study. Lesion load in the anterior LIFG (aLIFG), consisting of the orbital (BA 47) and triangular (BA 45) parts, and the posterior LIFG (pLIFG), consisting of the opercular part (BA 44) will be correlated with performance on tasks of semantic and phonological processing. Functional connectivity will be determined between regions of interest (i.e., anterior superior temporal, posterior middle temporal, angular, and supramarginal gyri) in the left and/or right hemispheres based on rsfMRI data. RESULTS/ANTICIPATED RESULTS: Lesion load in the aLIFG is expected to correlate to performance on the semantic task, whereas lesion load in the pLIFG is expected to correlate to performance on the phonological task. For participants whose behavioral performance exceeds expectations based on lesion load (e.g., larger aLIFG lesion load, yet better semantic task performance), rsfMRI connectivity is expected to be greater (e.g., between the posterior middle temporal and angular gyri), suggesting cortical reorganization. DISCUSSION/SIGNIFICANCE: If our hypotheses are supported, our results will corroborate evidence of a functional spectrum (i.e., semantic-phonological) in the LIFG. Future work will compare rsfMRI connectivity to neurotypical controls and sites of connectivity may be used as targets in studies of non-invasive brain stimulation as an adjuvant to behavioral therapy.

Dissociable contributions of frontal and temporal brain regions to basic semantic composition.

Semantic composition is the ability to combine single words to form complex meanings and is an essential component for successful communication. Evidence from neuroimaging studies suggests that semantic composition engages a widely distributed left-hemispheric network, including the anterior temporal lobe, the inferior frontal gyrus and the angular gyrus. To date, the functional relevance of these regions remains unclear. Here, we investigate the impact of lesions to key regions in the semantic network on basic semantic composition. We conducted a multivariate lesion-behaviour mapping study in 36 native German speaking participants with chronic lesions to the language network after left-hemispheric stroke. During the experiment, participants performed a plausibility judgement task on auditorily presented adjective-noun phrases that were either meaningful (‘anxious horse’), anomalous (‘anxious salad’) or had the noun replaced by a pseudoword (‘anxious gufel’), as well as a single-word control condition (‘horse’). We observed that reduced accuracy for anomalous phrases is associated with lesions in left anterior inferior frontal gyrus, whereas increased reaction times for anomalous phrases correlates with lesions in anterior-to-mid temporal lobe. These results indicate that anterior inferior frontal gyrus is relevant for accurate semantic decisions, while anterior-to-mid temporal lobe lesions lead to slowing of the decision for anomalous two-word phrases. These differential effects of lesion location support the notion that anterior inferior frontal gyrus affords executive control for decisions on semantic composition while anterior-to-mid temporal lobe lesions slow the semantic processing of the individual constituents of the phrase.

Differential contributions of left-hemispheric language regions to basic semantic composition

Semantic composition, the ability to combine single words to form complex meanings, is a core feature of human language. Despite growing interest in the basis of semantic composition, the neural correlates and the interaction of regions within this network remain a matter of debate. We designed a well-controlled two-word fMRI paradigm in which phrases only differed along the semantic dimension while keeping syntactic information alike. Healthy participants listened to meaningful (“fresh apple”), anomalous (“awake apple”) and pseudoword phrases (“awake gufel”) while performing an implicit and an explicit semantic task. We identified neural signatures for distinct processes during basic semantic composition. When lexical information is kept constant across conditions and the evaluation of phrasal plausibility is examined (meaningful vs. anomalous phrases), a small set of mostly left-hemispheric semantic regions, including the anterior part of the left angular gyrus, is found active. Conversely, when the load of lexical information—independently of phrasal plausibility—is varied (meaningful or anomalous vs. pseudoword phrases), conceptual combination involves a wide-spread left-hemispheric network comprising executive semantic control regions and general conceptual representation regions. Within this network, the functional coupling between the left anterior inferior frontal gyrus, the bilateral pre-supplementary motor area and the posterior angular gyrus specifically increases for meaningful phrases relative to pseudoword phrases. Stronger effects in the explicit task further suggest task-dependent neural recruitment. Overall, we provide a separation between distinct nodes of the semantic network, whose functional contributions depend on the type of compositional process under analysis.

Integration demands modulate effective connectivity in a fronto-temporal network for contextual sentence integration

Previous neuroimaging studies demonstrated that a network of left-hemispheric frontal and temporal brain regions contributes to the integration of contextual information into a sentence. However, it remains unclear how these cortical areas influence and drive each other during contextual integration. The present study used dynamic causal modeling (DCM) to investigate task-related changes in the effective connectivity within this network. We found increased neural activity in left anterior inferior frontal gyrus (aIFG), posterior superior temporal sulcus/middle temporal gyrus (pSTS/MTG) and anterior superior temporal sulcus/MTG (aSTS/MTG) that probably reflected increased integration demands and restructuring attempts during the processing of unexpected or semantically anomalous relative to expected endings. DCM analyses of this network revealed that unexpected endings increased the inhibitory influence of left aSTS/MTG on pSTS/MTG during contextual integration. In contrast, during the processing of semantically anomalous endings, left aIFG increased its inhibitory drive on pSTS/MTG. Probabilistic fiber tracking showed that effective connectivity between these areas is mediated by distinct ventral and dorsal white matter association tracts.Together, these results suggest that increasing integration demands require an inhibition of the left pSTS/MTG, which presumably reflects the inhibition of the dominant expected sentence ending. These results are important for a better understanding of the neural implementation of sentence comprehension on a large-scale network level and might influence future studies of language in post-stroke aphasia after focal lesions.

Multiple neural representations of elementary logical connectives

A defining trait of human cognition is the capacity to form compounds out of simple thoughts. This ability relies on the logical connectives AND, OR and IF. Simple propositions, e.g., ‘There is a fork’ and ‘There is a knife’, can be combined in alternative ways using logical connectives: e.g., ‘There is a fork AND there is a knife’, ‘There is a fork OR there is a knife’, ‘IF there is a fork, there is a knife’. How does the brain represent compounds based on different logical connectives, and how are compounds evaluated in relation to new facts? In the present study, participants had to maintain and evaluate conjunctive (AND), disjunctive (OR) or conditional (IF) compounds while undergoing functional MRI. Our results suggest that, during maintenance, the left posterior inferior frontal gyrus (pIFG, BA44, or Broca's area) represents the surface form of compounds. During evaluation, the left pIFG switches to processing the full logical meaning of compounds, and two additional areas are recruited: the left anterior inferior frontal gyrus (aIFG, BA47) and the left intraparietal sulcus (IPS, BA40). The aIFG shows a pattern of activation similar to pIFG, and compatible with processing the full logical meaning of compounds, whereas activations in IPS differ with alternative interpretations of conditionals: logical vs conjunctive. These results uncover the functions of a basic cortical network underlying human compositional thought, and provide a shared neural foundation for the cognitive science of language and reasoning.

Separate Brain Circuits Support Integrative and Semantic Priming in the Human Language System

Semantic priming is a crucial phenomenon to study the organization of semantic memory. A novel type of priming effect, integrative priming, has been identified behaviorally, whereby a prime word facilitates recognition of a target word when the 2 concepts can be combined to form a unitary representation. We used both functional and anatomical imaging approaches to investigate the neural substrates supporting such integrative priming, and compare them with those in semantic priming. Similar behavioral priming effects for both semantic (Bread-Cake) and integrative conditions (Cherry-Cake) were observed when compared with an unrelated condition. However, a clearly dissociated brain response was observed between these 2 types of priming. The semantic-priming effect was localized to the posterior superior temporal and middle temporal gyrus. In contrast, the integrative-priming effect localized to the left anterior inferior frontal gyrus and left anterior temporal cortices. Furthermore, fiber tractography showed that the integrative-priming regions were connected via uncinate fasciculus fiber bundle forming an integrative circuit, whereas the semantic-priming regions connected to the posterior frontal cortex via separated pathways. The results point to dissociable neural pathways underlying the 2 distinct types of priming, illuminating the neural circuitry organization of semantic representation and integration.

Left middle temporal and inferior frontal regions contribute to speed of lexical decision: A TMS study

Activation of left anterior inferior frontal gyrus (aLIFG) and left middle temporal gyrus (LMTG) has been observed in some functional neuroimaging studies of lexical decision but not others. It is thus unclear whether these two regions are necessary for word recognition. By applying continuous theta-burst transcranial magnetic stimulation (TMS) which temporally suppresses local brain function, we examined whether aLIFG and LMTG play causal roles in word recognition in a visual lexical decision task (LDT). Furthermore, we manipulated stimulus onset asynchrony (SOA) between prime and target to test whether these regions contribute to word recognition differently. In the LDT task, target words were preceded by semantically related primes (Related Condition; RC) or semantically unrelated words (Unrelated Condition; UC), under both short (150ms) and long (600ms) SOA conditions. TMS of aLIFG and LMTG significantly affected the word recognition speed compared to TMS of Vertex. Our results provide evidence that both aLIFG and LMTG contribute to word recognition speed. Furthermore, at short SOA, TMS of aLIFG or LMTG prolonged reaction time (RT). In contrast, at long SOA, there was a significant region by SOA by TMS interaction such that TMS of aLIFG prolonged RT, whereas TMS of LMTG speeded RT. These results suggest that aLIFG and LMTG may play different roles in word recognition.

Perturbation of the left inferior frontal gyrus triggers adaptive plasticity in the right homologous area during speech production.

The role of the right hemisphere in aphasia recovery after left hemisphere damage remains unclear. Increased activation of the right hemisphere has been observed after left hemisphere damage. This may simply reflect a release from transcallosal inhibition that does not contribute to language functions. Alternatively, the right hemisphere may actively contribute to language functions by supporting disrupted processing in the left hemisphere via interhemispheric connections. To test this hypothesis, we applied off-line continuous theta burst stimulation (cTBS) over the left inferior frontal gyrus (IFG) in healthy volunteers, then used functional MRI to investigate acute changes in effective connectivity between the left and right hemispheres during repetition of auditory and visual words and pseudowords. In separate sessions, we applied cTBS over the left anterior IFG (aIFG) or posterior IFG (pIFG) to test the anatomic specificity of the effects of cTBS on speech processing. Compared with cTBS over the aIFG, cTBS over the pIFG suppressed activity in the left pIFG and increased activity in the right pIFG during pseudoword vs. word repetition in both modalities. This effect was associated with a stronger facilitatory drive from the right pIFG to the left pIFG during pseudoword repetition. Critically, response became faster as the influence of the right pIFG on left pIFG increased, indicating that homologous areas in the right hemisphere actively contribute to language function after a focal left hemisphere lesion. Our findings lend further support to the notion that increased activation of homologous right hemisphere areas supports aphasia recovery after left hemisphere damage.

P 132. Modulation of encoding-related networks by intermittent TBS stimulation in healthy aging

Question Memory dysfunction is one of the main cognitive hallmarks of aging. Repetitive Transcranial Magnetic Stimulation induces brain plasticity changes that may underlie its capacity to modulate associative memory in elders (Sole-Padulles et al., 2006). Memory recall also depends on the manner in which stimuli are encoded (Innocenti et al., 2010) (i.e. the depth or shallowness of the processing) according to level of processing (LOP) theory (Craik and Lockhart, 1972). Our aim was to investigate the impact of a single session of excitatory intermittent theta burst stimulation (iTBS) on behavioral outcomes and the expression of brain networks in healthy elders, related to an episodic memory task with two types of processing. Materials and methods After an initial MRI scanner and neuropsychological testing 21 healthy elder subjects (71.6yo.) were randomly assigned to Real ( n =11) or Sham iTBS group. Subjects underwent a baseline functional MRI session (Siemens 3T) followed by Real or Sham iTBS (MagVenture, MagPro x100 Opt; 80% AMT; 600pulses). iTBS was neuronavigated (Nexstim-NBS-eXimia) over the Left Inferior Frontal Gyrus (LIFG) (group coordinates from metanalysis (Kim, 2011)). Immediately after iTBS, subjects realized a second equivalent MRI session. The memory task consisted in 8 24″ block-design paradigm were subjects were instructed to try to remember words either using deep (i.e. semantic) or shallow (i.e. perceptual) encoding. Repeated stimuli were also presented for control purposes. After both MRI sessions participants performed a recognition memory test. Results In a three way ANOVA, only main effects of LOP ( p For fMRI results, we found a significant interaction in Encoding Deep>Repeated Deep in Left and Right occipital cortex and in Encoding Deep>Encoding Shallow contrast in the anterior LIFG (Fig. 1) and in Left visual cortex indicating that Real iTBS compared to Sham stimulation resulted in increased BOLD signal but only associated to deep encoding processes (positive slope differences). Furthermore, these areas corresponded to those related to deep memory encoding in our baseline analyses. Conclusion While failing to observe a behavioral effect, iTBS specifically modulated the expression of a brain network only when subjects engaged a cognitive process related to encoding at Deeper LOP. Within the general framework of the state-dependency postulates (Silvanto et al., 2008), we suggest that our results provide further first evidence, in a study combining neuroimaging and a behavioral paradigm in elder participants, of the importance of considering the interaction between the objective brain stimulation parameters with the functional state of the neuronal populations, to account for the brain responses to non-invasive brain stimulation studies in cognitive studies in aging population.

Neural correlates of experimentally induced flow experiences

Flow refers to a positive, activity-associated, subjective experience under conditions of a perceived fit between skills and task demands. Using functional magnetic resonance perfusion imaging, we investigated the neural correlates of flow in a sample of 27 human subjects. Experimentally, in the flow condition participants worked on mental arithmetic tasks at challenging task difficulty which was automatically and continuously adjusted to individuals' skill level. Experimental settings of “boredom” and “overload” served as comparison conditions. The experience of flow was associated with relative increases in neural activity in the left anterior inferior frontal gyrus (IFG) and the left putamen. Relative decreases in neural activity were observed in the medial prefrontal cortex (MPFC) and the amygdala (AMY). Subjective ratings of the flow experience were significantly associated with changes in neural activity in the IFG, AMY, and, with trend towards significance, in the MPFC. We conclude that neural activity changes in these brain regions reflect psychological processes that map on the characteristic features of flow: coding of increased outcome probability (putamen), deeper sense of cognitive control (IFG), decreased self-referential processing (MPFC), and decreased negative arousal (AMY).

Cortical Dynamics of Semantic Processing during Sentence Comprehension: Evidence from Event-Related Optical Signals

Using the event-related optical signal (EROS) technique, this study investigated the dynamics of semantic brain activation during sentence comprehension. Participants read sentences constituent-by-constituent and made a semantic judgment at the end of each sentence. The EROSs were recorded simultaneously with ERPs and time-locked to expected or unexpected sentence-final target words. The unexpected words evoked a larger N400 and a late positivity than the expected ones. Critically, the EROS results revealed activations first in the left posterior middle temporal gyrus (LpMTG) between 128 and 192 ms, then in the left anterior inferior frontal gyrus (LaIFG), the left middle frontal gyrus (LMFG), and the LpMTG in the N400 time window, and finally in the left posterior inferior frontal gyrus (LpIFG) between 832 and 864 ms. Also, expected words elicited greater activation than unexpected words in the left anterior temporal lobe (LATL) between 192 and 256 ms. These results suggest that the early lexical-semantic retrieval reflected by the LpMTG activation is followed by two different semantic integration processes: a relatively rapid and transient integration in the LATL and a relatively slow but enduring integration in the LaIFG/LMFG and the LpMTG. The late activation in the LpIFG, however, may reflect cognitive control.

Please Get to the Point! A Cortical Correlate of Linguistic Informativeness

The production of informative messages is an effortful endeavor that relies on the interaction between microlinguistic (i.e., lexical and grammatical) and macrolinguistic (i.e., pragmatic and discourse) levels of processing. Although the neural correlates of microlinguistic processing have been extensively studied, investigation of the ability to organize the macrolinguistic aspects of message production is scanty. In this article, we show that repetitive TMS of the dorsal portion of the anterior left, but not right, inferior frontal gyrus reduces the levels of lexical informativeness and global coherence of narratives produced by healthy individuals. Interestingly, levels of productivity and microlinguistic processing were unaffected by the stimulation. These results suggest that the dorsal aspect of the anterior left inferior frontal gyrus is an epicenter of a wider neural network subserving the selection of contextually appropriate semantic representations.

Behavioral and fMRI evidence that cognitive ability modulates the effect of semantic context on speech intelligibility

Text cues facilitate the perception of spoken sentences to which they are semantically related (Zekveld, Rudner, et al., 2011). In this study, semantically related and unrelated cues preceding sentences evoked more activation in middle temporal gyrus (MTG) and inferior frontal gyrus (IFG) than nonword cues, regardless of acoustic quality (speech in noise or speech in quiet). Larger verbal working memory (WM) capacity (reading span) was associated with greater intelligibility benefit obtained from related cues, with less speech-related activation in the left superior temporal gyrus and left anterior IFG, and with more activation in right medial frontal cortex for related versus unrelated cues. Better ability to comprehend masked text was associated with greater ability to disregard unrelated cues, and with more activation in left angular gyrus (AG). We conclude that individual differences in cognitive abilities are related to activation in a speech-sensitive network including left MTG, IFG and AG during cued speech perception.

Left Inferior Frontal Activations Differentially Modulated by Scrambling in Ditransitive Sentences

In order to clarify the relationship among grammatical knowledge, processing components, and neural substrates in sentence comprehension, we used functional magnetic resonance imaging to investigate how brain activation is affected by two types of scrambling (short scrambling and middle scrambling) in ditransitive sentences in Japanese. Short scrambling and middle scrambling enhanced activation in the anterior and posterior left inferior frontal gyrus respectively. This finding accords with the view that the anterior left inferior frontal gyrus is involved in the automatic processing that establishes a dependency relation between a verb and its arguments, and the posterior left inferior frontal gyrus supports this kind of processing through its role in verbal working memory. This result is more congruent with a process-based approach to neural bases for sentence processing, which searches for neurological correlates of psycholinguistically defined processing components, than with a grammar-based approach, which probes neural networks with the assumption that major grammatical operations are neurologically individuated.

The anterior left inferior frontal gyrus contributes to semantic unification

Semantic unification, the process by which small blocks of semantic information are combined into a coherent utterance, has been studied with various types of tasks. However, whether the brain activations reported in these studies are attributed to semantic unification per se or to other task-induced concomitant processes still remains unclear. The neural basis for semantic unification in sentence comprehension was examined using event-related potentials (ERP) and functional Magnetic Resonance Imaging (fMRI). The semantic unification load was manipulated by varying the goodness of fit between a critical word and its preceding context (in high cloze, low cloze and violation sentences). The sentences were presented in a serial visual presentation mode. The participants were asked to perform one of three tasks: semantic congruency judgment (SEM), silent reading for comprehension (READ), or font size judgment (FONT), in separate sessions. The ERP results showed a similar N400 amplitude modulation by the semantic unification load across all of the three tasks. The brain activations associated with the semantic unification load were found in the anterior left inferior frontal gyrus (aLIFG) in the FONT task and in a widespread set of regions in the other two tasks. These results suggest that the aLIFG activation reflects a semantic unification, which is different from other brain activations that may reflect task-specific strategic processing.

Neural systems for guilt from actions affecting self versus others

Guilt is a core emotion governing social behavior by promoting compliance with social norms or self-imposed standards. The goal of this study was to contrast guilty responses to actions that affect self versus others, since actions with social consequences are hypothesized to yield greater guilty feelings due to adopting the perspective and subjective emotional experience of others. Sixteen participants were presented with brief hypothetical scenarios in which the participant's actions resulted in harmful consequences to self (guilt-self) or to others (guilt-other) during functional MRI. Participants felt more intense guilt for guilt-other than guilt-self and guilt-neutral scenarios. Guilt scenarios revealed distinct regions of activity correlated with intensity of guilt, social consequences of actions, and the interaction of guilt by social consequence. Guilt intensity was associated with activation of the dorsomedial PFC, superior frontal gyrus, supramarginal gyrus, and anterior inferior frontal gyrus. Guilt accompanied by social consequences was associated with greater activation than without social consequences in the ventromedial and dorsomedial PFC, precuneus, posterior cingulate, and posterior superior temporal sulcus. Finally, the interaction analysis highlighted select regions that were more strongly correlated with guilt intensity as a function of social consequence, including the left anterior inferior frontal gyrus, left ventromedial PFC, and left anterior inferior parietal cortex. Our results suggest these regions intensify guilt where harm to others may incur a greater social cost.

Interpretation-mediated changes in neural activity during language comprehension

Using functional magnetic resonance imaging (fMRI), we identified cortical regions mediating interpretive processes that take place during language comprehension. We manipulated participants' interpretation of texts by asking them to focus on action-, space-, or time-related features while listening to identical short stories. We identify several cortical regions where activity varied significantly in response to this attention manipulation, even though the content being processed was exactly the same. Activity in the posterior and anterior sections of the left inferior frontal gyrus (IFG), which are thought to have different sensitivities to high-level language processing, was modulated by the listeners' attentional focus, but in ways that were quite different. The posterior left IFG (Pars Opercularis) showed different activity levels for the three conditions. However, a population coding analysis demonstrated similar distributions of activity across conditions. This suggests that while the gain of the response in the Pars Opercularis was modulated, its core organization was relatively invariant across the experimental conditions. In the anterior left IFG (Pars Triangularis), the analysis of population codes revealed different activity patterns between conditions: there was little similarity between activity during time–attention and action- and space–attention, however there were similar activity patterns while attending to space and action information. In addition, both the left superior temporal gyrus and sulcus showed greater activity in the space and action attention conditions when contrasted with time attention. We discuss these findings in light of work on the role of left IFG in processing semantic information in language, and in light of theories suggesting that temporal information in language is processed in the brain using similar mechanisms as spatial information. Our findings suggest that a substantial source of variance in neural activity during language comprehension emerges from the internally-driven, information-seeking preferences of listeners rather than the syntactic or semantic properties of a text.

Co‐speech gestures influence neural activity in brain regions associated with processing semantic information

Everyday communication is accompanied by visual information from several sources, including co-speech gestures, which provide semantic information listeners use to help disambiguate the speaker's message. Using fMRI, we examined how gestures influence neural activity in brain regions associated with processing semantic information. The BOLD response was recorded while participants listened to stories under three audiovisual conditions and one auditory-only (speech alone) condition. In the first audiovisual condition, the storyteller produced gestures that naturally accompany speech. In the second, the storyteller made semantically unrelated hand movements. In the third, the storyteller kept her hands still. In addition to inferior parietal and posterior superior and middle temporal regions, bilateral posterior superior temporal sulcus and left anterior inferior frontal gyrus responded more strongly to speech when it was further accompanied by gesture, regardless of the semantic relation to speech. However, the right inferior frontal gyrus was sensitive to the semantic import of the hand movements, demonstrating more activity when hand movements were semantically unrelated to the accompanying speech. These findings show that perceiving hand movements during speech modulates the distributed pattern of neural activation involved in both biological motion perception and discourse comprehension, suggesting listeners attempt to find meaning, not only in the words speakers produce, but also in the hand movements that accompany speech.

Functional Neuroanatomy of Meaning Acquisition from Context

An important issue in language learning is how new words are integrated in the brain representations that sustain language processing. To identify the brain regions involved in meaning acquisition and word learning, we conducted a functional magnetic resonance imaging study. Young participants were required to deduce the meaning of a novel word presented within increasingly constrained sentence contexts that were read silently during the scanning session. Inconsistent contexts were also presented in which no meaning could be assigned to the novel word. Participants showed meaning acquisition in the consistent but not in the inconsistent condition. A distributed brain network was identified comprising the left anterior inferior frontal gyrus (BA 45), the middle temporal gyrus (BA 21), the parahippocampal gyrus, and several subcortical structures (the thalamus and the striatum). Drawing on previous neuroimaging evidence, we tentatively identify the roles of these brain areas in the retrieval, selection, and encoding of the meaning.

Neural correlates of semantic and morphological processing of Hebrew nouns and verbs

Neuropsychological evidence regarding grammatical category suggests that deficits affecting verbs tend to localize differently from those affecting nouns, but previous functional imaging studies on healthy subjects fail to show consistent results that correspond to the clinical dissociation. In the current imaging study, we addressed this issue by manipulating not only the grammatical category but also the processing mode, using auditory presentation of Hebrew words. Subjects were presented with verbs and nouns and were instructed to make either a semantic decision ("Does the word belong to a given semantic category?") or a morphological decision ("Is the word inflected in plural?"). The results showed different patterns of activation across distinct regions of interest. With respect to grammatical category effects, we found increased activation for verbs in the posterior portion of the left superior temporal sulcus, left dorsal premotor area, and posterior inferior frontal gyrus. In each of these regions, the effect was sensitive to task. None of the ROIs showed noun advantage. With respect to task effects, we found a semantic advantage in left anterior inferior frontal gyrus, as well as in left posterior middle temporal gyrus. The results suggest that cerebral verb-noun dissociation is a result of localized and subtle processes that take place in a set of left frontal and temporal regions, and that the cognitive and neural processes involved in analyzing grammatical category depend on the lexical characteristics of the stimuli, as well as on task requirements. The discrepancy between functional imaging and patient data is also discussed.