Sentence Context Prevails Over Word Association in Aphasia Patients with Spared Comprehension: Evidence from N400 Event-Related Potential

Electrophysiological correlates of basic semantic composition in people with aphasia

The neuroanatomical correlates of basic semantic composition have been investigated in previous neuroimaging and lesion studies, but research on the electrophysiology of the involved processes is scarce. A large literature on sentence-level event-related potentials (ERPs) during semantic processing has identified at least two relevant components - the N400 and the P600. Other studies demonstrated that these components are reduced and/or delayed in people with aphasia (PWA). However, it remains to be shown if these findings generalize beyond the sentence level. Specifically, it is an open question if an alteration in ERP responses in PWA can also be observed during basic semantic composition, providing a potential future diagnostic tool. The present study aimed to elucidate the electrophysiological dynamics of basic semantic composition in a group of post-stroke PWA. We included 20 PWA and 20 age-matched controls (mean age 58 years) and measured ERP responses while they performed a plausibility judgment task on two-word phrases that were either meaningful ("anxious horse"), anomalous ("anxious wood") or had the noun replaced by a pseudoword ("anxious gufel"). The N400 effect for anomalous versus meaningful phrases was similar in both groups. In contrast, unlike the control group, PWA did not show an N400 effect between pseudoword and meaningful phrases. Moreover, both groups exhibited a parietal P600 effect towards pseudoword phrases, while PWA showed an additional P600 over frontal electrodes. Finally, PWA showed an inverse correlation between the magnitude of the N400 and P600 effects: PWA exhibiting no or even reversed N400 effects towards anomalous and pseudoword phrases showed a stronger P600 effect. These results may reflect a compensatory mechanism which allows PWA to arrive at the correct interpretation of the phrase. When compositional processing capacities are impaired in the early N400 time-window, PWA may make use of a more elaborate re-analysis process reflected in the P600.

Context matters: How do task demands modulate the recruitment of sensorimotor information during language processing?

Many theories of semantic representation propose that simulations of sensorimotor experience contribute to language processing. This can be seen in the body-object interaction effect (BOI; how easily the human body can interact with a word's referent). Words with high BOI ratings (e.g., ball) are processed more quickly than words with low BOI ratings (e.g., cloud) in various language tasks. This effect can be modulated by task demands. Previous research established that when asked to decide if a word is an object (entity condition), a BOI effect is observed, but when asked to decide if a word is an action (action condition), there is no BOI effect. It is unclear whether the null behavioral effect in the action condition reflects top-down modulation of task-relevant sensorimotor information or the absence of bottom-up activation of sensorimotor simulations. We investigated this question using EEG. In Experiment 1 we replicated the previous behavioral findings. In Experiment 2, 50 participants were assigned to either the entity or action conditions and responded to the same word stimuli. In both conditions we observed differences in ERP components related to the BOI effect. In the entity condition the P2 mean amplitude was significantly more positive for high compared to low BOI words. In the action condition the N400 peak latency was significantly later for high compared to low BOI words. Our findings suggest that BOI information is generated bottom-up regardless of task demands and modulated by top-down processes that recruit sensorimotor information relevant to the task decision.

What is Functional Communication? A Theoretical Framework for Real-World Communication Applied to Aphasia Rehabilitation

Aphasia is an impairment of language caused by acquired brain damage such as stroke or traumatic brain injury, that affects a person’s ability to communicate effectively. The aim of rehabilitation in aphasia is to improve everyday communication, improving an individual’s ability to function in their day-to-day life. For that reason, a thorough understanding of naturalistic communication and its underlying mechanisms is imperative. The field of aphasiology currently lacks an agreed, comprehensive, theoretically founded definition of communication. Instead, multiple disparate interpretations of functional communication are used. We argue that this makes it nearly impossible to validly and reliably assess a person’s communicative performance, to target this behaviour through therapy, and to measure improvements post-therapy. In this article we propose a structured, theoretical approach to defining the concept of functional communication. We argue for a view of communication as “situated language use”, borrowed from empirical psycholinguistic studies with non-brain damaged adults. This framework defines language use as: (1) interactive, (2) multimodal, and (3) contextual. Existing research on each component of the framework from non-brain damaged adults and people with aphasia is reviewed. The consequences of adopting this approach to assessment and therapy for aphasia rehabilitation are discussed. The aim of this article is to encourage a more systematic, comprehensive approach to the study and treatment of situated language use in aphasia.

The electrophysiology of aphasia: A scoping review

OBJECTIVE

To systematically assess the body of literature using N400 and P600 as they relate to people with aphasia. The primary aim was to reveal patterns in the literature which could be used to direct future research in the development of clinically relevant Event-Related Potentials (ERPs) for language assessment, while also identifying gaps in existing knowledge and highlight areas of further inquiry.

METHODS

A literature search was performed on studies published before May 2021. Relevant studies on aphasia and the two ERPs of interest were assessed for quality, and the relationship between aphasia and these ERPs was explored.

RESULTS

A total of 721 articles were identified, with 30 meeting inclusion criteria. Although there is significant variation in the literature, this scoping review revealed people with aphasia show reduced amplitude, delayed latency and different distribution compared to controls, and that ERPs are modulated by severity of aphasia.

CONCLUSIONS

To develop a relevant clinical tool for the management of aphasia, future research must strive to improve consistency within ERP methodology, with a greater number of diverse aphasia subtypes included in research.

SIGNIFICANCE

This scoping review reveals N400 and P600 represent promising potential biomarkers for the diagnosis and ongoing management of aphasia.

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.

Semantic Processing in Healthy Aging and Alzheimer's Disease: A Systematic Review of the N400 Differences

Semantic deficits are common in individuals with Alzheimer’s disease (AD). These deficits notably impact the ability to understand words. In healthy aging, semantic knowledge increases but semantic processing (i.e., the ability to use this knowledge) may be impaired. This systematic review aimed to investigate semantic processing in healthy aging and AD through behavioral responses and the N400 brain event-related potential. The results of the quantitative and qualitative analyses suggested an overall decrease in accuracy and increase in response times in healthy elderly as compared to young adults, as well as in individuals with AD as compared to age-matched controls. The influence of semantic association, as measured by N400 effect amplitudes, appears smaller in healthy aging and even more so in AD patients. Thus, semantic processing differences may occur in both healthy and pathological aging. The establishment of norms of healthy aging for these outcomes that vary between normal and pathological aging could eventually help early detection of AD.

Language-Related Brain Potentials in Patients With Disorders of Consciousness: A Follow-up Study to Detect “Covert” Language Disorders

Background. Language disorders may occur in patients with disorders of consciousness (DoCs), and they could interfere with the behavioral assessment of consciousness and responsiveness. Objective. In this study, we retrospectively explored whether ERP N400 was eventually associated with the presence of aphasia diagnosed in those patients who had evolved into Exit-Minimally Conscious State (E-MCS) at the clinical follow-up. Methods. In this retrospective cohort study, the ERPs elicited by an auditory sentences task were retrospectively examined in a sample of 15 DoC patients diagnosed according to the Coma Recovery Scale–Revised (CRS-R). All these 15 DoC patients underwent a (at least) 1-year clinical follow-up, which included a neuropsychological evaluation to assess language function among other cognitive functions. Ten healthy individuals also underwent the same paradigm to investigate the variability of ERPs characteristics. Results. The N400 ERP component with centroparietal topography was found in 9 of 10 healthy controls in response to the ill-formed sentences. Analysis of patients’ data revealed that (1) a significant N400 component could be detected in 64% (9 of 14 patients) of the DoC patients regardless of the type of DoC; (2) no significant N400 ERP component was retrospectively detected in those E-MCS patients who showed aphasia at the follow-up; and (3) the presence/absence of the N400-ERP component was consistent with the brain lesion side and significantly predict the recovery. Conclusion. These preliminary findings indicate that the absence of N400 was significantly associated with the presence of aphasia diagnosed at the clinical follow-up in E-MCS patients.

Event Related Potential Study of Language Interaction in Bilingual Aphasia Patients

Half of the global population can be considered bilingual. Nevertheless when faced with patients with aphasia, clinicians and therapists usually ignore the patient's second language (L2) albeit its interference in first language (L1) processing has been shown. The excellent temporal resolution by which each individual linguistic component can be gaged during word-processing, promoted the event-related potential (ERP) technique for studying language processing in healthy bilinguals and monolingual aphasia patients. However, this technique has not yet been applied in the context of bilingual aphasia. In the current study, we report on L2 interference in L1 processing using the ERP technique in bilingual aphasia. We tested four bilingual- and one trilingual patients with aphasia, as well as several young and older (age-matched with patients) healthy subjects as controls. We recorded ERPs when subjects were engaged in a semantic association judgment task on 122 related and 122 unrelated Dutch word-pairs (prime and target words). In 61 related and 61 unrelated word-pairs, an inter-lingual homograph was used as prime. In these word-pairs, when the target was unrelated to the prime in Dutch (L1), it was associated to the English (L2) meaning of the homograph. Results showed a significant effect of homograph use as a prime on early and/or late ERPs in response to word-pairs related in Dutch or English. Each patient presented a unique pattern of L2 interference in L1 processing as reflected by his/her ERP image. These interferences depended on the patient's pre- and post-morbid L2 proficiency. When the proficiency was high, the L2 interference in L1 processing was higher. Furthermore, the mechanism of interference in patients that were pre-morbidly highly proficient in L2 additionally depended on the frequency of pre-morbid L2 exposure. In summary, we showed that the mechanism behind L2 interference in L1 processing in bilingual patients with aphasia depends on a complex interaction between pre- and post-morbid L2 proficiency, pre- and post-morbid L2 exposure, impairment and the presented stimulus (inter-lingual homographs). Our ERP study complements the usually adopted behavioral approach by providing new insights into language interactions on the level of individual linguistic components in bilingual patients with aphasia.