Context-dependent interpretation of words: evidence for interactive neural processes

The meaning of a word usually depends on the context in which it occurs. This study investigated the neural mechanisms involved in computing word meanings that change as a function of syntactic context. Current semantic processing theories suggest that word meanings are retrieved from diverse cortical regions storing sensory-motor and other types of semantic information and are further integrated with context in left inferior frontal gyrus (LIFG). Our fMRI data indicate that brain activity in an area sensitive to motion and action semantics--the posterior middle temporal gyrus (PMTG)--is modulated by a word's syntactic context. Ambiguous words such as bowl were presented in minimal disambiguating contexts indicating object (the bowl) or action (to bowl) meanings and were compared to low-ambiguity controls. Ambiguous words elicited more activity than low-ambiguity controls in LIFG and various meaning-related areas such as PMTG. Critically, ambiguous words also elicited more activity in to--contexts than the--contexts in PMTG and LIFG, suggesting that contextual integration strengthened the action meaning in both areas. The pattern of results suggests that the activation of lexical information in PMTG was sensitive to contextual disambiguating information and that processing context-dependent meanings may involve interactions between frontal and posterior areas.     

Linking semantic priming effect in functional MRI and event-related potentials

The aim of this study is to examine the neural substrates involved in semantic priming using a combined event-related functional magnetic resonance imaging (fMRI) and event-related potentials (ERP) study. Twelve subjects were instructed to judge whether the presented target word was a real word or a nonword. Under the related condition, target words were preceded by a semantically related prime word. On the other hand, under the unrelated condition, prime words did not have semantic relatedness with the target word. The reaction time for reaching a judgment was longer under the unrelated condition than under the related condition, indicating that the recognition of target words is promoted by semantic priming under the related condition. In the fMRI results, we found reduced activity in the dorsal and ventral left inferior frontal gyrus, the anterior cingulate, and left superior temporal cortex for related versus unrelated conditions (i.e., the repetition suppression effect). ERP analysis revealed that the amplitude of the N400 component was reduced under the related condition compared with the unrelated condition (i.e., the N400 priming effect). Correlation analysis between the BOLD repetition suppression effect and the N400 priming effect decomposed by independent component analysis (ICA) across subjects showed significant correlation in the left superior temporal gyrus. This finding is consistent with the recent MEG data suggesting that the source of N400 is judged to be the bilateral superior temporal lobe. We discussed this finding herein in relation to the modulation of access to the phonological representation caused by semantic priming.     

From N400 to N300: variations in the timing of semantic processing with repetition

The present study aimed to explore the variations of semantic processing according to the number of target words (i.e., 4, 12 and 24) and according to the number of repetitions (i.e, 1 to 15). The number of targets had no impact on the N400 brain potential, the index of semantic processing, nor on the late positive component (LPC), an index of episodic encoding and retrieval. Analyses of the effects of the number of repetitions showed that the duration of semantic processes--assessed by measuring N400 latency--was linearly shortened along repetitions while their extent--as indexed by N400 amplitude--remained constant after the second presentation. In contrast, the extent of episodic processes--as indexed by LPC amplitude--was found to increase linearly with repetition. By showing that N400 latency may be much less stable than previously thought, these results bring new constraints on the functional correlates of this key stage in the processing of semantic information. They also suggest that semantic processes can be studied at high repetition rates whatever the number of target stimuli. Finally, our findings show that each episode of prior presentation has an impact on the late processing of a stimulus despite the absence of an explicit memory task.     

Multiple brain signatures of integration in the comprehension of degraded speech

When listening to speech under adverse conditions, expectancies resulting from semantic context can have a strong impact on comprehension. Here we ask how minimal variations in semantic context (cloze probability) affect the unfolding comprehension of acoustically degraded speech. Three main results are observed in the brain electric response. First, auditory evoked responses to a degraded sentence's onset (N100) correlate with participants' comprehension scores, but are generally more vigorous for more degraded sentences. Second, a pronounced N400 in response to low-cloze sentence-final words, reflecting the integration effort of words into context, increases linearly with improving speech intelligibility. Conversely, transient enhancement in Gamma band power (γ, ~40-70 Hz) during high-cloze sentence-final words (~600 ms) reflects top-down-facilitated integration. This γ-band effect also varies parametrically with signal quality. Third, a negative correlation of N100 amplitude at sentence onset and the later γ-band response is found in moderately degraded speech. This reflects two partly distinct neural strategies when dealing with moderately degraded speech; a more "bottom-up," resource-allocating, and effortful versus a more "top-down," associative and facilitatory strategy. Results also emphasize the non-redundant contributions of phase-locked (evoked) and non-phase-locked (induced) oscillatory brain dynamics in auditory EEG.     

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.     

N400-like magnetoencephalography responses modulated by semantic context,word frequency,and lexical class in sentences

Words have been found to elicit a negative potential at the scalp peaking at approximately 400 ms that is strongly modulated by semantic context. The current study used whole-head magnetoencephalography (MEG) as male subjects read sentences ending with semantically congruous or incongruous words. Compared with congruous words, sentence-terminal incongruous words consistently evoked a large magnetic field over the left hemisphere, peaking at approximately 450 ms. Source modeling at this latency with conventional equivalent current dipoles (ECDs) placed the N400 m generator in or near the left superior temporal sulcus. A distributed solution constrained to the cortical surface suggested a sequence of differential activation, beginning in Wernicke's area at approximately 250 ms, spreading to anterior temporal sites at approximately 270 ms, to Broca's area by approximately 300 ms, to dorsolateral prefrontal cortices by approximately 320 ms, and to anterior orbital and frontopolar cortices by approximately 370 ms. Differential activity was exclusively left-sided until >370 ms, and then involved right anterior temporal and orbital cortices. At the peak of the N400 m, activation in the left hemisphere was estimated to be widespread in the anterior temporal, perisylvian, orbital, frontopolar, and dorsolateral prefrontal cortices. In the right hemisphere, the orbital, as well as, weakly, the right anterior temporal cortices were activated. Similar but weaker field patterns were evoked by intermediate words in the sentences, especially to low-frequency words occurring in early sentence positions where there is little preceding context. The locations of the N400 m sources identified with the distributed solution correspond well with those previously demonstrated with direct intracranial recordings, and suggested by functional magnetic resonance imaging (fMRI). These results help identify a distributed cortical network that supports online semantic processing.     

Syntax and the brain: disentangling grammar by selective anomalies

Many paradigms employed so far with functional imaging in language studies do not allow a clear differentiation of the semantic, morphological, and syntactic components, as traditionally defined within linguistic theory. In fact, many studies simply consider the brain's response to lists of unrelated words, rather than to syntactic structures, or do not neutralize the confounding effect of the semantic component. In the present PET experiment, we isolated the functional correlates of morphological and syntactic processing. The neutralization of the access to the lexical-semantic component was achieved by requiring the detection of anomalies in written sentences consisting of pseudowords. In both syntactic and morphosyntactic processing, the involvement of a selective deep component of Broca's area and of a right inferior frontal region was detected. In addition, within this system, the left caudate nucleus and insula were activated only during syntactic processing, indicating their role in syntactic computation. These findings provide original in vivo evidence that these brain structures, whose individual contribution has been highlighted by clinical studies, constitute a neural network selectively engaged in morphological and syntactic computation.     

Specialisation in Broca's region for semantic, phonological, and syntactic fluency?

The literature suggests that that semantic and phonological fluency tasks selectively activate left Brodmann's area (BA) 45 and 44, respectively, in Broca's speech region. We used functional MRI to test this hypothesis. Subjects performed a semantic and a phonological fluency task. In addition, a syntactic fluency task (e.g. "Generate nouns with masculine gender") was included. Resting blocks were included as a low-level control condition. The exact localisation of the effects was tested with cytoarchitectonic probability maps of BA 44 and BA 45. Participants generated fewer words in the syntactic than in both the semantic and the phonological condition, which did not differ from each other. Compared to rest, all language tasks activated the well-known language network in the left hemisphere including both left BA 44 and BA 45. In the direct contrasts between the different verbal fluency tasks, phonological fluency activated BA 44 more strongly than semantic or syntactic fluency. However, semantic fluency did not elicit higher activation than the phonological fluency tasks in any part of Broca's region. No differences were observed between syntactic and semantic fluency. Thus, the activation in BA 45 observed during verbal fluency tasks seems to be not restricted to semantic processing as suggested by the literature. In contrast, phonological verbal fluency additionally involved the left BA 44. In conclusion, different parts of Broca's region support task-specific and more general processes in verbal fluency.     

Impaired semantic processing during sentence reading in children with dyslexia: combined fMRI and ERP evidence

Developmental dyslexia is a specific disorder of reading acquisition characterized by a phonological core deficit. Sentence reading is also impaired in dyslexic readers, but whether semantic processing deficits contribute is unclear. Combining spatially and temporally sensitive neuroimaging techniques to focus on semantic processing can provide a more comprehensive characterization of sentence reading in dyslexia. We recorded brain activity from 52 children (16 with dyslexia, 31 controls) with functional magnetic resonance imaging (fMRI) and event-related potentials (ERP) in two separate counterbalanced sessions. The children silently read and occasionally judged simple sentences with semantically congruous or incongruous endings. fMRI and ERP activation during sentence reading and semantic processing was analyzed across all children and also by comparing children with dyslexia to controls. For sentence reading, we analyzed the response to all words in a sentence; for semantic processing, we contrasted responses to incongruous and congruous endings. Sentence reading was characterized by activation in a left-lateralized language network. Semantic processing was characterized by activation in left-hemispheric regions of the inferior frontal and superior temporal cortex and by an electrophysiological N400 effect after 240 ms with consistent left anterior source localization. Children with dyslexia showed decreased activation for sentence reading in inferior parietal and frontal regions, and for semantic processing in inferior parietal regions, and during the N400 effect. Together, this suggests that semantic impairment during sentence reading reduces dyslexic children's response in left anterior brain regions underlying the more phasic N400 effect and subsequently modulates the more sustained BOLD response in left inferior parietal regions.     

听觉呈现条件下汉语双字词语义和语音启动的事件相关电位研究

背景:在关于语义启动的事件相关电位研究中,N400(在刺激呈现后400ms左右出现的负波)成分占据重要的地位,被普遍认为反映了语义加工过程。不少研究证实,语义启动会导致N400波幅降低。而语音启动的研究较少,尤其是通过听觉呈现刺激的方式进行的的事件相关电位研究。目的:观察听觉呈现条件下汉语双字词的语音启动和语义启动事件相关电位特征,验证关于听觉词汇识别的相关理论。设计:重复测量实验。单位:天津师范大学心理与行为研究中心。对象:实验于2003-08/10在天津师范大学进行。选择17名天津师范大学在读大学生为被试,男生8名,女生9名,年龄19～23岁,身体健康,听力正常,母语为汉语,普通话标准。除1名男生外,其余均为右利手。所有受试大学生均对检测项目知情同意。方法:①本实验采用词汇决定任务,判断继时听觉呈现的词对中,后一个词为真词还是假词。要求被试进行按键反应。刺激材料为640对双字词组成的词对(包括词义相关、首音相同、尾音相同、音义无关及真假词对5类)。实验中5类词对以随机顺序呈现。②使用Stim呈现实验材料,该软件同时记录行为数据,对被试在4种实验条件下(词对间语义相关、首音相同、尾音相同、音义无关)的反应的错误率和平均反应时进行计算。对真假词对不进行分析(作为填充)。被试头戴美国Neuroscan公司Quick-Cap32导脑电帽(国际10-20系统),参考电极为双侧乳突连线,经Synamps放大器放大15000倍,采样频率500Hz,然后送入电脑由Scan4.3软件记录。对数据的离线分析包括利用SCAN软件中的OcularArtifactReduction功能去除连续文件中的眼电干扰、分段(分析时程从触发点前200ms至触发点之后1500ms)、基线矫正(-100～0ms为标准)、去伪迹(超过±100μV的数据作为伪迹排除)、叠加。主要观察指标:不同实验条件下目标词进行反应时的错误率、反应时间及引起的N400波形的变化。结果:17名被试的数据均被纳入结果分析。①语义相关条件下被试错误率低于音义无关(P<0.05),反应时短于音义无关(P<0.01);首音相同及尾音相同条件下被试错误率高于音义无关(P<0.01)。将错误的反应剔除,语义相关、首音相同、尾音相同和音义无关4种条件下的平均反应时分别为830,891,864和903ms。语义相关与音义无关相比差异显著(P<0.01);尾音相同与音义无关及首音相同差异明显(P<0.01)。②因为FCZ引发的N400最为显著,所以对4种实验条件下FCZ点引发的N400的波幅和潜伏期进行统计分析。LSD多重比较发现,音义无关波幅显著高于语义相关(P<0.01),音义无关峰潜时最长,其次分别为语义相关、尾音相同,时间最短的是首音相同。LSD多重比较显示,首音相同具有峰潜时缩短的趋势,与音义无关差异显著(P<0.05)。③考察不同实验条件下N400头皮分布的差异,除FCZ点之外,又选取了FZ、CZ、PZ、T7、T8分别作为前区、顶区、后区、左区和右区的代表点,对波幅进行4(实验条件)×2(电极位置)两因素重复测量方差分析。不同电极位置的主效应非常显著(P=0.000)。波幅最高的是FCZ(-7.946μV),其次分别为CZ(-7.285μV)、FZ(-6.849μV)、PZ(-4.324μV)、T8(-3.501μV)、T7(-3.484μV)。LSD多重比较表明,除CZ外,FCZ波幅高于其他代表点。结论:①在词汇决定实验中,通过听觉通道呈现汉语双字词,启动词与目标词之间存在语义联系可导致目标词的加工得到改善,表现为错误率降低、反应时缩短,以及N400波幅减小。②启动词与目标词之间的语音一致对目标词的加工既有干扰作用,又有促进作用。表现为错误率提高,但反应时趋向缩短,N400波幅亦呈下降趋势。③启动词与目标词首音相同比尾音相同有更大的干扰作用。这为交股理论所认为的在听觉词汇识别过程中首音具有特殊意义的看法提供了支持,不过该理论应用于汉语双字词识别需要进行一定的修正。④本实验没有发现语义启动语音启动具有不同脑内源的证据。     

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.     

Event-related brain potentials during the monitoring of speech errors

When we perceive speech, our goal is to extract the meaning of the verbal message which includes semantic processing. However, how deeply do we process speech in different situations? In two experiments, native Dutch participants heard spoken sentences describing simultaneously presented pictures. Sentences either correctly described the pictures or contained an anomalous final word (i.e. a semantically or phonologically incongruent word). In the first experiment, spoken sentences were task-irrelevant and both anomalous conditions elicited similar centro-parietal N400s that were larger in amplitude than the N400 for the correct condition. In the second experiment, we ensured that participants processed the same stimuli semantically. In an early time window, we found similar phonological mismatch negativities for both anomalous conditions compared to the correct condition. These negativities were followed by an N400 that was larger for semantic than phonological errors. Together, these data suggest that we process speech semantically, even if the speech is task-irrelevant. Once listeners allocate more cognitive resources to the processing of speech, we suggest that they make predictions for upcoming words, presumably by means of the production system and an internal monitoring loop, to facilitate lexical processing of the perceived speech.     

Reading anomalous sentences: an event-related fMRI study of semantic processing

We report a random-effects analysis of an event-related fMRI study (n = 28) of cerebral activity during the reading of sentences that ended with a word that was either congruent or incongruent with the previous sentence context. Event-related potential studies have shown that this task elicits a late negativity peaking around 400 ms poststimulus (N400) that is larger for incongruent than for congruent sentence endings. A direct comparison of the activation for incongruent words versus that for congruent words revealed significantly greater activation for incongruent words than congruent words in bilateral inferior frontal and inferio-medial temporal cortex, left lateral frontal cortex, left posterior fusiform gyrus, bilateral motor cortex, and supplementary motor area. These results are consistent with data from intracranial electrical recording studies of the N400 electrical potential. The results are discussed as they relate to the localization of the cerebral sites underlying semantic processing in general and the localization of the scalp recorded N400 event-related potential in particular.     

Semantic combinatorial processing of non-anomalous expressions

In this study, semantic meaning composition is investigated focusing on the ERP correlates of reading of minimal Spanish noun-adjective pairs. Comprehending these constructs requires combining the concepts expressed by nouns with the - not contextually expected - semantic feature expressed by the adjectives. Previous studies have mainly focused on the processing of either semantic anomalies or contextually expected target words; we focus on the comprehension of more natural expressions, manipulating the typicality of the noun-adjective relation. In two different ERP experiments, compared to neutral pairs (monstruo solitario, lonely monster), only anomalous adjectives (monstruo geográfico, geographic monster) elicited increased N400, the effect classically associated with lexical/semantic processing. Low-typicality adjectives that apparently contrasted with the inherent prototypical features associated with the noun (monstruo hermoso, lovely monster) elicited a long-lasting frontal late positive effect (LPC, 550-750 ms) in Experiment 1; high-typicality adjectives that expressed redundant information (monstruo horrible, horrible monster) elicited a similar, but shorter, positivity (550-650 ms) in Experiment 2. These findings suggest that the combination of non-anomalous noun-adjective pairs involves additional neurocognitive resources to the ones represented by the N400. Correlation analyses indicate that the LPC is associated with a meta-linguistic analysis of the expressions: a significant inverse correlation was found between the stimuli's naturality and the amplitude of the LPC.     

Retrieval of visual, auditory, and abstract semantics

Conceptual knowledge is thought to be represented in a large distributed network, indexing a range of different semantic features (e.g., visual, auditory, functional). We investigated the anatomical organization of these features, using PET, by contrasting brain activity elicited by heard words with (i) visual (e.g., blue), (ii) auditory (e.g., noise), or (iii) abstract (e.g., truth) meaning. The activation task was either repetition or semantic decision (e.g., does the meaning of the word relate to religion?). In the baseline conditions, the sound track of the words was reversed and subjects had to say "OK" (control for repetition) or make an acoustic decision (control for semantic decision). Irrespective of task, words relative to their corresponding controls activated the left posterior inferior temporal and inferior frontal cortices. In addition, semantic decisions on words with sensory (visual and auditory) meanings enhanced activation in a ventral region of the left anterior temporal pole. These results are consistent with neuropsychological studies showing that anterior temporal lobe damage can cause deficits for items that are mainly defined by their sensory features (i.e., concrete, particularly living items). Since modality-specific activation was observed only during the semantic decision task, we discuss whether it reflects retrieval of sensory semantics per se or the degree to which semantic associations are triggered during effortful retrieval.     

Time course of semantic processes during sentence comprehension: an fMRI study

The ability to create new meanings from combinations of words is one important function of the language system. We investigated the neural correlates of combinatorial semantic processing using fMRI. During scanning, participants performed a rating task on auditory word or pseudoword strings that differed in the presence of combinatorial and word-level semantic information. Stimuli included normal sentences comprised of thematically related words that could be readily combined to produce a more complex meaning, semantically incongruent sentences in which content words were randomly replaced with other content words, pseudoword sentences, and versions of these three sentence types in which syntactic structure was removed by randomly re-ordering the words. Several regions showed greater BOLD signal for stimuli with words than for those with pseudowords, including the left angular gyrus, left superior temporal sulcus, and left inferior frontal gyrus, suggesting that these areas are involved in semantic access at the single word level. In the angular and inferior frontal gyri these differences emerged early in the course of the hemodynamic response. An effect of combinatorial semantic structure was observed in the left angular gyrus and left lateral temporal lobe, which showed greater activation for normal compared to semantically incongruent sentences. These effects appeared later in the time course of the hemodynamic response, beginning after the entire stimulus had been presented. The data indicate a complex spatiotemporal pattern of activity associated with computation of word and sentence-level semantic information, and suggest a particular role for the left angular gyrus in processing overall sentence meaning.     

Native and non-native reading of sentences: an fMRI experiment

The processing of syntactic and semantic information in written sentences by native (L1) and non-native (L2) speakers was investigated in an fMRI experiment. This was done by means of a violation paradigm, in which participants read sentences containing either a syntactic, a semantic, or no violation. The results of this study were compared to those of a previous fMRI study, in which auditory sentence processing in L1 and L2 was investigated. The results indicate greater activation for L2 speakers as compared to L1 speakers when reading sentences in several language- and motor-related brain regions. The processing of syntactically incorrect sentences elicited no reliably greater activation in language areas in L2 speakers. In L1 speakers, on the other hand, syntactic processing, as compared to semantic processing, was associated with increased activation in left mid to posterior superior temporal gyrus. In response to the processing of semantically incorrect sentences, both L2 and L1 speakers demonstrated increased involvement of left inferior frontal gyrus. The results of this study were compared to a previously conducted fMRI study, which made use of identical sentence stimuli in the auditory modality. Results from the two studies are in general agreement with one another, although some differences in the response of brain areas very proximal to primary perceptual processing areas (i.e. primary auditory and visual cortex) were observed in conjunction with presentation in the different modalities. The combined results provide evidence that L1 and L2 speakers rely on the same cortical network to process language, although with a higher level of activation in some regions for L2 processing.     

The brain basis of syntactic processes: functional imaging and lesion studies

Language comprehension can be subdivided into three processing steps: initial structure building, semantic integration, and late syntactic integration. The two syntactic processing phases are correlated with two distinct components in the event-related brain potential, namely an early left anterior negativity (ELAN) and a late centroparietal positivity (P600). Moreover, ERP findings from healthy adults suggest that early structure-building processes as reflected by the ELAN are independent of semantic processes. fMRI results have revealed that semantic and syntactic processes are supported by separable temporofrontal networks, with the syntactic processes involving the left superior temporal gyrus (STG), the left frontal operculum, and the basal ganglia (BG) in particular. MEG data from healthy adults have indicated that the left anterior temporal region and the left inferior frontal region subserve the early structure building processes. ERP data from patients with lesions in the left anterior temporal region and from patients with lesions in the left inferior frontal gyrus support this view, as these patients do not demonstrate an ELAN, although they do demonstrate a P600. Further results from patients with BG dysfunction suggest that parts of this subcortical structure are involved in late syntactic integrational processes. The data from the different experiments lead to the notion of separable brain systems responsible for early and late syntactic processes, with the former being subserved by the inferior frontal gyrus and the anterior STG and the latter being supported by the BG and more posterior portions of the STG.     

Semantic ambiguity processing in sentence context: Evidence from event-related fMRI

Lexical semantic ambiguity is the phenomenon when a word has multiple meanings (e.g. 'bank'). The aim of this event-related functional MRI study was to identify those brain areas, which are involved in contextually driven ambiguity resolution. Ambiguous words were selected which have a most frequent, dominant, and less frequent, subordinate meaning. These words were presented in two types of sentences: (1) a sentence congruent with the dominant interpretation and (2) a sentence congruent with the subordinate interpretation. Sentences without ambiguous words served as a control condition. The ambiguous words always occurred early in the sentences and were biased towards one particular meaning by the final word(s) of the sentence; the event at the end of the sentences was modeled. The results indicate that a bilaterally distributed network supports semantic ambiguity comprehension: left (BA 45/44) and right (BA 47) inferior frontal gyri and left (BA 20/37) and right inferior/middle temporal gyri (BA 20). The pattern of activation is most consistent with a scenario in which initially a frequency-based probabilistic choice is made between the alternative meanings, and the meaning is updated when this interpretation does not fit into the final disambiguating context. The neural pattern is consistent with the results of other neuroimaging experiments which manipulated various aspects of integrative and context processing task demands. The presence of a bilateral network is also in line with the lesion and divided visual field literature, but contrary to earlier claims, the two hemispheres appear to play similar roles during semantic ambiguity resolution.     

Neuroimaging reveals automatic speech coding during perception of written word meaning

The extent to which visual word perception engages speech codes (i.e., phonological recoding) remains a crucial question in understanding mechanisms of reading. In this study, we used functional magnetic resonance imaging techniques combined with behavioral response measures to examine neural responses to focused versus incidental phonological and semantic processing of written words. Three groups of subjects made simple button-pressing responses in either phonologically (rhyming-judgment) or semantically (category-judgment) focused tasks or both tasks with identical sets of visual stimuli. In the phonological tasks, subjects were given both words and pseudowords separated in different scan runs. The baseline task required feature search of scrambled letter strings created from the stimuli for the experimental conditions. The results showed that cortical regions associated with both semantic and phonological processes were strongly activated when the task required active processing of word meaning. However, when subjects were actively processing the speech sounds of the same set of written words, brain areas typically engaged in semantic processing became silent. In addition, subjects who performed both the rhyming and the semantic tasks showed diverse and significant bilateral activation in the prefrontal, temporal, and other brain regions. Taken together, the pattern of brain activity provides evidence of a neural basis supporting the theory that in normal word reading, phonological recoding is automatic and facilitates semantic processing of written words, while rapid comprehension of word meaning requires devoted attention. These results also raise questions about including multiple cognitive tasks in the same neuroimaging sessions.