Aneta Kielar

Oscillatory responses to semantic and syntactic violations

EEG studies employing time–frequency analysis have revealed changes in theta and alpha power in a variety of language and memory tasks. Semantic and syntactic violations embedded in sentences evoke well-known ERPs, but little is known about the oscillatory responses to these violations. We investigated oscillatory responses to both kinds of violations, while monolingual and bilingual participants performed an acceptability judgment task. Both violations elicited power decreases (event-related desynchronization, ERD) in the 8–30 Hz frequency range, but with different scalp topographies. In addition, semantic anomalies elicited power increases (event-related synchronization, ERS) in the 1–5 Hz frequency band. The 1–5 Hz ERS was strongly phase-locked to stimulus onset and highly correlated with time domain averages, whereas the 8–30 Hz ERD response varied independently of these. In addition, the results showed that language expertise modulated 8–30 Hz ERD for syntactic violations as a function of the executive demands of the task. When the executive function demands were increased using a grammaticality judgment task, bilinguals but not monolinguals demonstrated reduced 8–30 Hz ERD for syntactic violations. These findings suggest a putative role of the 8–30 Hz ERD response as a marker of linguistic processing that likely represents a separate neural process from those underlying ERPs.

Graded effects of regularity in language revealed by n400 indices of morphological priming

Differential electrophysiological effects for regular and irregular linguistic forms have been used to support the theory that grammatical rules are encoded using a dedicated cognitive mechanism. The alternative hypothesis is that language systematicities are encoded probabilistically in a way that does not categorically distinguish rule-like and irregular forms. In the present study, this matter was investigated more closely by focusing specifically on whether the regular–irregular distinction in English past tenses is categorical or graded. We compared the ERP priming effects of regulars (baked–bake), vowel-change irregulars (sang–sing), and “suffixed” irregulars that display a partial regularity (suffixed irregular verbs, e.g., slept–sleep), as well as forms that are related strictly along formal or semantic dimensions. Participants performed a visual lexical decision task with either visual (Experiment 1) or auditory prime (Experiment 2). Stronger N400 priming effects were observed for regular than vowel-change irregular verbs, whereas suffixed irregulars tended to group with regular verbs. Subsequent analyses decomposed early versus late-going N400 priming, and suggested that differences among forms can be attributed to the orthographic similarity of prime and target. Effects of morphological relatedness were observed in the later-going time period, however, we failed to observe true regular–irregular dissociations in either experiment. The results indicate that morphological effects emerge from the interaction of orthographic, phonological, and semantic overlap between words.

Neural correlates of covert and overt production of tense and agreement morphology: Evidence from fMRI.

Most neuroimaging studies examining verb morphology have focused on verb tense, with fewer examining agreement morphology, and no previous fMRI studies have investigated distinctions between past and present tense inflection. However, models of language representation and processing suggest differences in where these inflections are instantiated in the phrase structure as well as differences in the linguistic functions they serve, suggesting unique neural networks for these forms. In addition, results of available neuroimaging studies of grammatical morphology vary considerably due to methodological differences. Some studies have used overt production tasks, whereas others have used covert tasks. In the present study we examined brain activation associated with past tense and present tense/agreement morphology under overt and covert production conditions in 13 healthy adults using an event-related functional magnetic resonance imaging (fMRI) design. Production of verbs inflected for past tense (V + -ed) and present tense/agreement (V -s) was elicited using temporal adverbs (i.e. Yesterday, Nowadays). Results showed that in healthy adults inflecting both past tense and agreement morphology (compared to a verb stem production condition) recruited not only left inferior frontal structures, but also motor and premotor cortices, and posterior parietal regions. Activation also was observed in the basal ganglia, thalamus, and the cingulate gyrus. Past tense and present tense/agreement recruited partially overlapping tissue in these regions, with distinctions observed for the two forms in frontal and parietal brain areas. We also found that activation varied with task demands, with more extensive frontal activation noted in the overt compared to the covert verb inflection task. These results are consistent with the hypothesis that the neural signatures for verb inflection differ from that for verb stems alone and involve a distributed frontal and parietal network of brain regions. Further, the neural tissue recruited for instantiation of past tense versus present tense/agreement morphology is distinct, supporting linguistic theories that differentiate the two forms.

Localization of electrophysiological responses to semantic and syntactic anomalies in language comprehension with MEG

Syntactically and semantically anomalous words encountered during sentence comprehension are known to elicit dissociable electrophysiological responses, which are thought to reflect distinct aspects of language processing. However, the sources of these responses have not been well characterized. We used beamforming analysis of magnetoencephalography (MEG) data to map generators of electrophysiological responses to linguistic anomalies. Anomalous words occurred in the context of a sentence acceptability judgement task conducted in both visual and auditory modalities. Time-frequency analysis revealed that both kinds of violations elicited event-related synchronization (ERS) in the delta-theta frequency range (1-5 Hz), and desynchronization (ERD) in the alpha-beta range (8-30 Hz). In addition, these responses were differentially modulated by violation type and presentation modality. 1-5 Hz responses were consistently localized within medial prefrontal cortex and did not vary significantly across violation types, but were stronger for visual presentation. In contrast, 8-30 Hz ERD occurred in different regions for different violation types. For semantic violations the distribution was predominantly in the bilateral occipital cortex and left temporal and inferior frontal regions, and these effects did not differ for visual and auditory presentation. In contrast, syntactic responses were strongly affected by presentation modality. Under visual presentation, syntactic violations elicited bilateral 8-30 Hz ERD extending into dorsal parietal and frontal regions, whereas effects were much weaker and mostly statistically insignificant in the auditory modality. These results suggest that delta-theta ERS reflects generalized increases in working memory demands related to linguistic anomaly detection, while alpha-beta ERD reflects specific activation of cortical regions involved in distinct aspects of linguistic processing, such as semantic vs. phonological short-term memory. Beamforming analysis of time-domain average signals (ERFs) revealed an N400m effect for semantic anomalies in both modalities, localized to left superior temporal and posterior frontal regions, and a later P600-like effect for syntactic anomalies in both modalities, widespread over bilateral frontal, posterior temporal, and parietal regions. These results indicate that time-domain averaged responses and induced oscillatory responses have distinct properties, including localization and modality dependence, and likely reflect dissociable and complementary aspects of neural activity related to language comprehension and additional task-related processes.

Electrophysiological responses to argument structure violations in healthy adults and individuals with agrammatic aphasia

Sentence comprehension requires processing of argument structure information associated with verbs, i.e. the number and type of arguments that they select. Many individuals with agrammatic aphasia show impaired production of verbs with greater argument structure density. The extent to which these participants also show argument structure deficits during comprehension, however, is unclear. Some studies find normal access to verb arguments, whereas others report impaired ability. The present study investigated verb argument structure processing in agrammatic aphasia by examining event-related potentials associated with argument structure violations in healthy young and older adults as well as aphasic individuals. A semantic violation condition was included to investigate possible differences in sensitivity to semantic and argument structure information during sentence processing. Results for the healthy control participants showed a negativity followed by a positive shift (N400-P600) in the argument structure violation condition, as found in previous ERP studies (Friederici & Frisch, 2000; Frisch, Hahne, & Friederici, 2004). In contrast, individuals with agrammatic aphasia showed a P600, but no N400, response to argument structure mismatches. Additionally, compared to the control groups, the agrammatic participants showed an attenuated, but relatively preserved, N400 response to semantic violations. These data show that agrammatic individuals do not demonstrate normal real-time sensitivity to verb argument structure requirements during sentence processing.

Functional reorganization of language networks for semantics and syntax in chronic stroke: Evidence from MEG.

Using magnetoencephalography, we investigated the potential of perilesional and contralesional activity to support language recovery in patients with poststroke aphasia. In healthy young controls, left‐lateralized ventral frontotemporal regions responded to semantic anomalies during sentence comprehension and bilateral dorsal frontoparietal regions responded to syntactic anomalies. Older adults showed more extensive bilateral responses to the syntactic anomalies and less lateralized responses to the semantic anomalies, with decreased activation in the left occipital and parietal regions for both semantic and syntactic anomalies. In aphasic participants, we observed compensatory recruitment in the right hemisphere (RH), which varied depending on the type of linguistic information that was processed. For semantic anomalies, aphasic patients activated some preserved left hemisphere regions adjacent to the lesion, as well as homologous parietal and temporal RH areas. Patients also recruited right inferior and dorsolateral frontal cortex that was not activated in the healthy participants. Responses for syntactic anomalies did not reach significance in patients. Correlation analyses indicated that recruitment of homologous temporoparietal RH areas is associated with better semantic performance, whereas higher accuracy on the syntactic task was related to bilateral superior temporoparietal and right frontal activity. The results suggest that better recovery of semantic processing is associated with a shift to ventral brain regions in the RH. In contrast, preservation of syntactic processing is mediated by dorsal areas, bilaterally, although recovery of syntactic processing tends to be poorer than semantic. Hum Brain Mapp 37:2869–2893, 2016.

Neuropsychological differentiation of late onset schizophrenia and frontotemporal dementia

Frontaltemporal dementia (FTD) and schizophrenia are characterised by disturbances in cognition, personality, behaviour, and social functioning often leading to a decline in activities in daily living. Deterioration of comportment and disturbances in attention are typical in both disorders and manifest behaviourally in terms of withdrawal, isolation, lack of volition, emotional unresponsiveness, and poverty of speech. Accordingly, the considerable overlap in behavioural expression between patients with schizophrenia and FTD makes a differential diagnosis difficult. This different diagnosis is especially difficult when the age of onset of schizophrenia is late (i.e., 45 +). The purpose of this study was to identify which neuropsychological tests best differentiate patients with late onset schizophrenia from patients with FTD. Hence, neurocognitive test results from a total of 12 patients with FTD and 32 patients with schizophrenia were analysed using test sensitivity statistics (i.e., Cohens U2% overlap). The results support a test battery composed of the WAIS-R Vocabulary, Information, Digit Span, and Comprehension subtests, and the Hooper Visual Organisation test as being the most sensitive measures to diagnostic differentiation between patients with FTD and those with late onset schizophrenia.

Identifying Dysfunctional Cortex: Dissociable Effects of Stroke and Aging on Resting State Dynamics in MEG and fMRI

Spontaneous signals in neuroimaging data may provide information on cortical health in disease and aging, but the relative sensitivity of different approaches is unknown. In the present study, we compared different but complementary indicators of neural dynamics in resting-state MEG and BOLD fMRI, and their relationship with blood flow. Participants included patients with post-stroke aphasia, age-matched controls, and young adults. The complexity of brain activity at rest was quantified in MEG using spectral analysis and multiscale entropy (MSE) measures, whereas BOLD variability was quantified as the standard deviation (SDBOLD), mean squared successive difference (MSSD), and sample entropy of the BOLD time series. We sought to assess the utility of signal variability and complexity measures as markers of age-related changes in healthy adults and perilesional dysfunction in chronic stroke. The results indicate that reduced BOLD variability is a robust finding in aging, whereas MEG measures are more sensitive to the cortical abnormalities associated with stroke. Furthermore, reduced complexity of MEG signals in perilesional tissue were correlated with hypoperfusion as assessed with arterial spin labeling (ASL), while no such relationship was apparent with BOLD variability. These findings suggest that MEG signal complexity offers a sensitive index of neural dysfunction in perilesional tissue in chronic stroke, and that these effects are clearly distinguishable from those associated with healthy aging.

Behavioural and neuroimaging changes after naming therapy for semantic variant primary progressive aphasia

The objectives of this study were to examine the effects of a successful naming intervention on naming performance and brain activity in individuals with the semantic variant of primary progressive aphasia (svPPA). Four participants with svPPA were scanned while performing phonologically- and semantically-based tasks before and after an intense, 20-h naming therapy that followed the principles of errorless learning whereby errors were eliminated from the learning process. Five healthy control participants were scanned at the outset of the study and did not receive treatment. The results showed that in svPPA participants, successful re-learning of forgotten vocabulary was accompanied by activation of a larger network in bilateral brain regions and that the level of activation in the left anterior lobe may be inversely correlated with severity of semantic impairment. Our findings have implications for treatment in svPPA patients and suggest that semantic cues can improve naming, in spite of significant semantic impairment. The results indicate that intensive language therapy can lead to behavioural gains and neuroplastic changes even in individuals with more advanced anterior temporal lobe atrophy.

Morphological aspects of agrammatic aphasia

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