Syntactic and semantic processing in the healthy and aphasic human brain

The role and use of event-related potentials in aphasia: A scoping review

Event-related potentials (ERPs) can provide important insights into underlying language processes in both unimpaired and neurologically impaired populations and may be particularly useful in aphasia. This scoping review was conducted to provide a comprehensive summary of how ERPs have been used with people with aphasia (PWA), with the goal of exploring the potential clinical application of ERPs in aphasia assessment and treatment. We identified 117 studies that met inclusionary criteria, reflecting six thematic domains of inquiry that relate to understanding both unimpaired and aphasic language processing and the use of ERPs with PWA. In these studies, a wide variety of ERP components were reported. Inconsistencies in reporting of participant characteristics and study protocols limit our ability to generalize beyond the individual studies and understand implications for clinical applicability. We discuss the potential roles of ERPs in aphasia management and make recommendations for further developing ERPs for clinical utility in PWA.

Auditory word recognition of verbs: Effects of verb argument structure on referent identification

Word recognition includes the activation of a range of syntactic and semantic knowledge that is relevant to language interpretation and reference. Here we explored whether or not the number of arguments a verb takes impinges negatively on verb processing time. In this study, three experiments compared the dynamics of spoken word recognition for verbs with different preferred argument structure. Listeners' eye movements were recorded as they searched an array of pictures in response to hearing a verb. Results were similar in all the experiments. The time to identify the referent increased as a function of the number of arguments, above and beyond any effects of label appropriateness (and other controlled variables, such as letter, phoneme and syllable length, phonological neighborhood, oral and written lexical frequencies, imageability and rated age of acquisition). The findings indicate that the number of arguments a verb takes, influences referent identification during spoken word recognition. Representational complexity and amount of information generated by the lexical item that aids target identification are discussed as possible sources of this finding.

Therapy-Induced Neuroplasticity of Language in Chronic Post Stroke Aphasia: A Mismatch Negativity Study of (A)Grammatical and Meaningful/less Mini-Constructions

Clinical language performance and neurophysiological correlates of language processing were measured before and after intensive language therapy in patients with chronic (time post stroke >1 year) post stroke aphasia (PSA). As event-related potential (ERP) measure, the mismatch negativity (MMN) was recorded in a distracted oddball paradigm to short spoken sentences. Critical 'deviant' sentence stimuli where either well-formed and meaningful, or syntactically, or lexico-semantically incorrect. After 4 weeks of speech-language therapy (SLT) delivered with high intensity (10.5 h per week), clinical language assessment with the Aachen Aphasia Test battery demonstrated significant linguistic improvements, which were accompanied by enhanced MMN responses. More specifically, MMN amplitudes to grammatically correct and meaningful mini-constructions and to 'jabberwocky' sentences containing a pseudoword significantly increased after therapy. However, no therapy-related changes in MMN responses to syntactically incorrect strings including agreement violations were observed. While MMN increases to well-formed meaningful strings can be explained both at the word and construction levels, the neuroplastic change seen for 'jabberwocky' sentences suggests an explanation in terms of constructions. The results confirm previous reports that intensive SLT leads to improvements of linguistic skills in chronic aphasia patients and now demonstrate that this clinical improvement is associated with enhanced automatic brain indexes of construction processing, although no comparable change is present for ungrammatical strings. Furthermore, the data confirm that the language-induced MMN is a useful tool to map functional language recovery in PSA.

Sentence Processing in Traumatic Brain Injury: Evidence From the P600

PURPOSE

Sentence processing can be affected following a traumatic brain injury (TBI) due to linguistic or cognitive deficits. Language-related event-related potentials (ERPs), particularly the P600, have not been described in individuals with TBI history.

METHOD

Four young adults with a history of closed head injury participated. Two had severe injuries, and 2 had mild-moderate injuries more than 24 months prior to testing. ERPs were recorded while participants read sentences designed to be grammatically correct or incorrect. Participants also completed cognitive and sentence comprehension measures.

RESULTS

One participant with TBI was significantly different than the control group on several behavioral sentence measures and 1 cognitive measure. However, none of the participants with TBI had a reliable P600 effect. Nonparametric bootstrapping indicated that the ERP was reliable in 10 control participants but no participants with TBI history.

CONCLUSIONS

There were few behavioral differences between individuals with TBI history and the control group, though all reported subjective difficulty with reading. The P600 was absent in the TBI group in this study. Given the heterogeneity of individuals with TBI and the difficulty in assessing subtle language impairments, exploring the P600 further may provide useful insight into language processing difficulties.

Differentiating hemispheric contributions to syntax and semantics in patients with left-hemisphere lesions

Understanding the relationship between brain and cognition critically depends on data from brain-damaged patients since these provide major constraints on identifying the essential components of brain-behavior systems. Here we relate structural and functional fMRI data with behavioral data in 21 human patients with chronic left hemisphere (LH) lesions and a range of language impairments to investigate the controversial issue of the role of the hemispheres in different language functions. We address this issue within a dual neurocognitive model of spoken language comprehension in which core linguistic functions, e.g., syntax, depend critically upon an intact left frontotemporal system, whereas more general communicative abilities, e.g., semantics, are supported by a bilateral frontotemporal system and may recover from LH damage through normal or enhanced activity in the intact right hemisphere. The fMRI study used a word-monitoring task that differentiated syntactic and semantic aspects of sentence comprehension. We distinguished overlapping interactions between structure, neural activity, and performance using joint independent components analysis, identifying two structural-functional networks, each with a distinct relationship with performance. Syntactic performance correlated with tissue integrity and activity in a left frontotemporal network. Semantic performance correlated with activity in right superior/middle temporal gyri regardless of tissue integrity. Right temporal activity did not differ between patients and controls, suggesting that the semantic network is degenerately organized, with regions in both hemispheres able to perform similar computations. Our findings support the dual neurocognitive model of spoken language comprehension and emphasize the importance of linguistic specificity in investigations of language recovery in patients.

Constrained versus unconstrained intensive language therapy in two individuals with chronic, moderate-to-severe aphasia and apraxia of speech: behavioral and fMRI outcomes

PURPOSE

This Phase I study investigated behavioral and functional MRI (fMRI) outcomes of 2 intensive treatment programs to improve naming in 2 participants with chronic moderate-to-severe aphasia with comorbid apraxia of speech (AOS). Constraint-induced aphasia therapy (CIAT; Pulvermüller et al., 2001) has demonstrated positive outcomes in some individuals with chronic aphasia. Whether constraint to the speech modality or treatment intensity is responsible for such gains is still under investigation. Moreover, it remains to be seen whether CIAT is effective in individuals with persistent severe nonfluent speech and/or AOS.

METHOD

A single-subject multiple-baseline approach was used. Both participants were treated simultaneously, first with Promoting Aphasics' Communicative Effectiveness (PACE; Davis & Wilcox, 1985) and then with CIAT. Pre-/posttreatment testing included an overt naming fMRI protocol. Treatment effect sizes were calculated for changes in probe accuracy from baseline to posttreatment phases and maintenance where available.

RESULTS

Both participants made more and faster gains in naming following CIAT. Treatment-induced changes in BOLD activation suggested that better naming was correlated with the recruitment of perilesional tissue.

CONCLUSION

Participants produced more target words accurately following CIAT than following PACE. Behavioral and fMRI results support the notion that the intense and repetitive nature of obligatory speech production in CIAT has a positive effect on word retrieval, even in participants with chronic moderate-to-severe aphasia with comorbid AOS.

ERP Correlates of Word Production before and after Stroke in an Aphasic Patient

Changes in brain activity characterizing impaired speech production after brain damage have usually been investigated by comparing aphasic speakers with healthy subjects because prestroke data are normally not available. However, when interpreting the results of studies of stroke patients versus healthy controls, there is an inherent difficulty in disentangling the contribution of neuropathology from other sources of between-subject variability. In the present work, we had an unusual opportunity to study an aphasic patient with severe anomia who had incidentally performed a picture naming task in an ERP study as a control subject one year before suffering a left hemisphere stroke. The fortuitous recording of this patient's brain activity before his stroke allows direct comparison of his pre- and poststroke brain activity in the same language production task. The subject did not differ from other healthy subjects before his stroke, but presented major electrophysiological differences after stroke, both in comparison to himself before stroke and to the control group. ERP changes consistently appeared after stroke in a specific time window starting about 250 msec after picture onset, characterized by a single divergent but stable topographic configuration of the scalp electric field associated with a cortical generator abnormally limited to left temporal posterior perilesional areas. The patient's pattern of anomia revealed a severe lexical–phonological impairment and his ERP responses diverged from those of healthy controls in the time window that has previously been associated with lexical–phonological processes during picture naming. Given that his prestroke ERPs were indistinguishable from those of healthy controls, it seems highly likely that the change in his poststroke ERPs is due to changes in language production processes as a consequence of stroke. The patient's neurolinguistic deficits, combined with the ERPs results, provide unique evidence for the role of left temporal cortex in lexical–phonological processing from about 250 to 450 msec during word production.

Comparing electrophysiological correlates of word production in immediate and delayed naming through the analysis of word age of acquisition effects

Most EEG studies analysing speech production with event related brain potential (ERP) have adopted silent metalinguistic tasks or delayed or tacit picture naming in order to avoid possible artefacts during motor preparation. A central issue in the interpretation of these results is whether the processes involved in those tasks are comparable to those involved in overt speech production. In the present study we addressed a methodological issue about the integration of stimulus-aligned and response-aligned ERPs in immediate overt picture naming in comparison to delayed production, coupled with a theoretical point on the effect of word Age of Acquisition (AoA). High density EEG recordings were used and waveform analyses and spatio-temporal segmentation were combined on stimulus-aligned and response-aligned ERPs. The same sequence and duration of topographic maps appeared in the immediate and delayed production until around 350 ms after picture onset, revealing similar encoding processes until the beginning of phonological encoding, but modulations linked to word AoA were only observed in the immediate production. Considering stimulus-aligned and response-aligned ERPs together allowed to identify that a stable topography starting around 350 ms lasts 30 ms longer for late-acquired than for early-acquired words. This difference falls within the time-window of phonological encoding and its modulation can be linked to the longer production latencies for late-acquired words.

Time course of evoked-potential changes in different forms of anomia in aphasia

Impaired word production after brain damage can be due to impairment at lexical-semantic or at lexical-phonological levels of word encoding. These processes are thought to involve different brain regions and to have different time courses. The present study investigated the time course of electrophysiological correlates of anomia in 16 aphasic speakers, divided in two subgroups according to their anomic pattern (8 with lexical-semantic impairment and 8 with lexical-phonological impairment), in comparison to 16 healthy control subjects performing the same picture naming task. Differences in amplitudes and in topographic maps between groups were differently distributed when the whole heterogeneous group of aphasic patients was compared to the control group and when the two more homogeneous subgroups of anomic patients were analyzed. The entire aphasic group expressed different waveforms and topographic patterns than the control group starting about 100 msec after picture presentation. When two subgroups of aphasic patients are considered according to the underlying cognitive impairment, early event-related potential (ERP) abnormalities (100-250 msec) appeared only in the lexical-semantic subgroup, whereas later ERP abnormalities (300-450 msec) occurred only in the lexical-phonological subgroup. These results indicate that the time windows of ERP abnormalities vary depending on the underlying anomic impairment. Moreover, the findings give support to current hypotheses on the time course of processes involved in word production during picture naming.

Syntactic gender processing in the human brain: a review and a model

Despite the increasing number of neuroimaging studies of syntactic gender processing no model is currently available that includes data from visual and auditory language comprehension and language production. This paper provides a systematic review of the neural correlates of syntactic gender processing. Based on anatomical information from cytoarchitectonic probability maps it is argued that the left BA 44 plays a central role for the active use of gender information, e.g., for explicit decisions as well as for subsequent morphological encoding. The left BA 45 is involved in the strategic generation of morphological cues that facilitate gender processing. Model implications for aphasic patients with lesions including or excluding parts of Broca's speech region are discussed.

Aphasia therapy on a neuroscience basis

BACKGROUND: Brain research has documented that the cortical mechanisms for language and action are tightly interwoven and, concurrently, new approaches to language therapy in neurological patients are being developed that implement language training in the context of relevant linguistic and non-linguistic actions, therefore taking advantage of the mutual connections of language and action systems in the brain. A further well-known neuroscience principle is that learning at the neuronal level is driven by correlation; consequently, new approaches to language therapy emphasise massed practice in a short time, thus maximising therapy quantity and frequency and, therefore, correlation at the behavioural and neuronal levels. Learned non-use of unsuccessful actions plays a major role in the chronification of neurological deficits, and behavioural approaches to therapy have therefore employed shaping and other learning techniques to counteract such non-use. AIMS: Advances in theoretical and experimental neuroscience have important implications for clinical practice. We exemplify this in the domain of aphasia rehabilitation. MAIN CONTRIBUTION: Whereas classical wisdom had been that aphasia cannot be significantly improved at a chronic stage, we here review evidence that one type of intensive language-action therapy (ILAT)-constraint-induced aphasia therapy-led to significant improvement of language performance in patients with chronic aphasia. We discuss perspectives for further improving speech-language therapy, including drug treatment that may be particularly fruitful when applied in conjunction with behavioural treatment. In a final section we highlight intensive and rapid therapy studies in chronic aphasia as a unique tool for exploring the cortical reorganisation of language. CONCLUSIONS: We conclude that intensive language action therapy is an efficient tool for improving language functions even at chronic stages of aphasia. Therapy studies using this technique can open new perspectives for research into the plasticity of human language circuits.

ERP correlates of noun and verb processing in preschool-age children

Indices of discrimination between words of different syntactic classes were examined in a group of 22 preschool-age children utilizing event-related potential (ERP) procedures. Video taped scenes depicted an actor performing actions with a set of toys. ERPs were recorded to spoken nouns or verbs that either matched or failed to match the action name or object name in the video scene. ERPs were analyzed using peak amplitude-ANOVA procedures and principal components analysis-ANOVA. Results indicated good convergence across analysis approaches. More specifically, P100 and N220 varied bilaterally over frontal electrode sites and discriminated between different syntactic classes for both match and mismatch situations. Based on comparisons with adult data from previous work [Molfese, D.L., Burger-Judisch, L., Gill, L.A., Golinkoff, R.M., Hirsh-Pasek, K., 1996. Electrophysiological correlates of noun-verb processing in adults. Brain and Language 54 388-413], it is suggested that the pattern of brain involvement underlying syntactic class discriminations undergo developmental changes between the preschool years and adulthood.

In your right mind: right hemisphere contributions to language processing and production

The verbal/nonverbal account of left and right hemisphere functionality is the prevailing dichotomy describing the cerebral lateralization of function. Yet the fact that the left hemisphere is the superior language processor does not necessarily imply that the right hemisphere is completely lacking linguistic ability. This paper reviews the growing body of research demonstrating that, far from being nonverbal, the right hemisphere has significant language processing strength. From prosodic and paralinguistic aspects of speech production, reception, and interpretation, to prelexical, lexical and postlexical components of visual word recognition; strong involvement of the right hemisphere is implicated. The evidence reviewed challenges the notion that language is solely a function of the "verbal" left hemisphere, indicating that the right cerebral hemisphere makes significant and meaningful contributions to normal language processing as well.

Language lateralization in phonological, semantic and orthographic tasks: A slow evoked potential study

Most of literature on language has shown how different word-classes activate distinct neural networks within linguistic cortical areas. The present investigation aimed to demonstrate that, by means of slow evoked potentials and using the same set of words in different tasks, it is possible to activate cortical networks that are spatially and temporally distinguished. Twenty healthy subjects had to evaluate, in a word pair matching session, whether two words rhymed (phonological task), were semantically related (semantic task) or were written in the same letter case (orthographic task). Slow wave amplitude was computed in three relevant time windows: the last 0.5 s of first word presentation (W1), the initial contingent negative variation (iCNV) and the terminal CNV (tCNV). During W1 and iCNV intervals, both the orthographic and the phonological tasks were left lateralized. Furthermore, the phonological task was more lateralized than the orthographic because of a greater inhibition of the right hemisphere, whereas the orthographic task was characterized by a greater bilateral posterior activation. During the tCNV, only the phonological task remained left lateralized while orthographic and semantic were bilaterally distributed. Although the use of the same set of words tends to activate widely overlapped networks, in the present research task manipulation was effective in demonstrating task dependent differences in brain lateralization. Thus, the present paradigm and the adopted tasks are especially suited for studying deficit and recovery of language in patients affected by linguistic disorders such as developmental dyslexia and aphasia.

Functional activation studies of word processing in the recovery from aphasia

Some reviews on theories of recovery in aphasia put an emphasis on neural network models based on empirical data from evoked-potentials in aphasia as an approach to mapping recovery of cognitive function to neural structure. We will focus here on what we call an "anatomical" approach to look at recovery in aphasia. "Anatomical" theories of recovery stated by classical aphasiologists have contributed to the understanding of language representations in the human brain. But many aspects of these theories can only be investigated by using modern techniques of lesion analysis, psychometric assessment and functional imaging. Whereas structure-function relations have been primarily established by looking for the association of deficit symptoms with certain lesions, functional activation methods offer a means to study more directly the functional anatomy of recovered or retained functions in neuropsychological patients. To falsify or build up anatomical theories of recovery we will propose a stepwise approach of inference. The methodological pitfalls of this approach will be discussed by focussing on anatomical hypotheses of semantic word comprehension and its impairment and recovery in aphasia.

Therapy-related reorganization of language in both hemispheres of patients with chronic aphasia

The brain processes of language recovery after stroke are poorly understood, partly because past research did not allow to differentiate the effects of spontaneous restitution processes from those of learning-related cortical reorganization. Here, we use a new approach offered by recently developed intense neuropsychological therapy methods, which allow for improving language functions within a short time period. Stroke patients with chronic aphasia received intense language therapy for 2 weeks and, over this period, improved their language performance as assessed using clinical tests. Neurophysiological activity elicited by words and pseudowords was measured before and after treatment. Over the therapy interval, early word evoked potentials (latency 250-300 ms) became significantly stronger whereas pseudoword responses did not change. Word-specific changes were documented by analyses of ERP amplitudes and root mean square values, which revealed interactions of the factors Assessment time (before vs. after therapy) and Wordness (word vs. pseudoword). Source localization using Minimum Norm Current Estimates showed that bilateral cortical sources activated by word stimuli contributed to the change, suggesting that neuronal networks distributed over both hemispheres are the substrate of cortical reorganization of language processing in intense aphasia therapy. Word-evoked differences in source strengths were significantly correlated with performance on a clinical language test, demonstrating a link between behavioral and neurophysiological changes. We suggest that the early word-evoked negativity might represent an index of reorganization of language after stroke and thus an aphasia recovery potential.

Stimulus-dominance effects and lateral asymmetries for language in normal subjects and in patients with a single functional hemisphere

The assessment of language laterality by the dichotic fused-words test may be impaired by interference effects revealed by the dominant report of one member of the stimuli-pair. Stimulus-dominance and ear asymmetry were evaluated in normal population (48 subjects of both sex and handedness) and in 2 patients with a single functional hemisphere. Results show that in normals the number of stimulus-dominated responses is five times higher than in patients, and is negatively correlated to the index of laterality. It is suggested that dichotic stimuli may interfere one with another during the subcortical acoustic processing and at cortical level, when competing for verbal output. Subcortical interference accounts for stimulus-dominance in the single-hemisphere patients. In normal subjects, the dichotic discrimination is disturbed mainly during the hemispheric cross-talk needed for the oral-verbal processing of dichotic inputs. The frequency of 'interhemispheric' interference, as well as the extents of ear asymmetry, may both depend on differences in the processing stage of the competing inputs, and then they may be influenced by differential verbal skills of LH and RH. Very unequal levels of verbal analysis of the dichotic stimuli may hinder reciprocal interference during the hemispheric cross-talk thus yielding large ear asymmetries associated to small dominance effects.

Neurophysiological correlates of word and pseudo-word processing in well-recovered aphasics and patients with right-hemispheric stroke

Neurophysiological correlates of language recovery after stroke were investigated. Neurological patients with single focal lesions in their left or right hemisphere and healthy control subjects made lexical decisions on written words and pseudo-words while EEG responses were recorded. At the time of testing, patients did not show clinically apparent language dysfunction, although those with left-hemisphere lesions had suffered from aphasia in the first months after their stroke. A P3-like positive deflection of the event-related potential (ERP) was reduced in the patients relative to healthy controls, this reduction being most pronounced over the hemisphere affected by stroke. Consistent with earlier research, healthy control subjects showed more positive ERPs to words than to pseudo-words. This pattern was reversed in both patient groups, where words elicited more negative-going ERPs than pseudo-words already 160-320 ms after stimulus onset. Because ERPs showed between-group differences only for words, these word-specific neurophysiological signatures altered in stroke patients with well-recovered language functions may be a correlate of cortical lesions or an index of reorganization of language after stroke.

Recovery of semantic word processing in global aphasia: a functional MRI study

One important issue concerning the recovery of higher cognitive functions-such as word comprehension in aphasia-is to what extent impairments can be compensated for by intact parts of the network of areas normally involved in a closely related function ("redundancy recovery"). In a previous functional MRI investigation, we were able to show that left hemispheric redundancy recovery within a distributed system of related lexical-semantic functions was the most probable basis of recovery of comprehension from transcortical sensory aphasia. The question remained, however, whether redundancy recovery may play a more general role in the recovery of comprehension after large left hemispheric lesions and severe aphasia. We had the possibility, using the same fMRI paradigm, to study seven cases with left middle cerebral artery (MCA) infarction and partial recovery of comprehension > or =6 months after presentation with global aphasia on acute assessment. Lateralization of activation did not differ significantly between patients and controls. The most consistent regions of activation included the left extrasylvian posterior temporal and the right posterior parietal cortex. Recovery of language comprehension was associated predominantly with activations in regions, which were also activated in several normal subjects. We suggest that a redundancy recovery mechanism within multiple representations of closely related functions was more important than take-over of function by previously unrelated areas (vicariation) as the basis of recovery of word comprehension in our patients in spite of extensive left hemispheric damage. We conclude that redundancy within the lexical-semantic system seems to make an important contribution to recovery of comprehension even in severe aphasia.

Intensivierung, Fokussierung und Verhaltensrelevanz

Zusammenfassung: Gehirnwissenschaftliche Erkenntnisse der vergangenen Jahre haben neue neuropsychologische Therapieformen hervorgebracht, deren allgemeine Prinzipien sich mit den Termini Intensivierung, Verhaltensrelevanz und Fokussierung umreißen lassen (genannt Constraint-Induced Aphasia Therapy - CIA Therapie). Intensivierung der Therapie bedeutet, dass die Intervention mit besonders hoher Frequenz durchgeführt wird (engl.: massed practice). Die im therapeutischen Setting implementierten Interaktionstypen gleichen denjenigen im Alltag (behavioral relevance). “Fokussierung” heißt, dass der Patient durch Hilfsmittel und Regeln der therapeutischen Interaktion zu Handlungsweisen gebracht wird, die er aufgrund operanter Konditionierungen in Folge der Gehirnschädigung sonst vermeiden würde (constraints to avoid learned nonuse). In einer kontrollierten Effektivitätsstudie konnten wir beweisen, dass CIA Therapie auch bei chronischer Aphasie zu einer signifikanten Verbesserung der sprachlichen Leistungen führen kann. Wir gehen hier auf die historischen Wurzeln ein und diskutieren Fragen für zukünftige Forschungen.

A brain perspective on language mechanisms: from discrete neuronal ensembles to serial order

Language is constituted by discrete building blocks, sounds and words, which can be concatenated according to serial order principles. The neurobiological organization of these building blocks, in particular words, has been illuminated by recent metabolic and neurophysiological imaging studies. When humans process words of different kinds, various sets of cortical areas have been found to become active differentially. The old concept of two language centers processing all words alike must therefore be replaced by a model according to which words are organized as discrete distributed neuron ensembles that differ in their cortical topographies. The meaning of a word, more precisely, aspects of its reference, may be crucial for determining which set of cortical areas becomes involved in its processing. Whereas the serial order of sounds constituting a word may be established by serially aligned sets of neurons called synfire chains, different mechanisms are necessary for establishing word order in sentences. The serial order of words may be organized by higher-order neuronal sets, called sequence detectors here, which are being activated by sequential excitation of neuronal sets representing words. Sets of sequence detectors are proposed to process aspects of the syntactic information contained in a sentence. Other syntactic rules can be related to general features of the dynamics of cortical activation and deactivation. These postulates about the brain mechanisms of language, which are rooted in principles known from neuroanatomy and neurophysiology, may provide a framework for theory-driven neuroscientific research on language.