Disturbance of semantic processing in temporal lobe epilepsy demonstrated with scalp ERPs

Event-related potentials reveal visual episodic memory deficits in patients with temporal lobe epilepsy

OBJECTIVE

Temporal lobe epilepsy (TLE) patients usually suffer from impaired episodic memory (EM), but its underlying electrophysiologic mechanism and impacted cognitive performance are unclear. We aim to investigate the association between episodic memory reserve and physiological measures of memory workload in TLE patients using Event-related potentials (ERP).

METHODS

A change detection task with image stimuli assesses visual episodic memory. During the memory encoding and decoding phases, the ERP signals were analyzed from twenty-nine TLE patients (twelve with left TLE patients, seventeen with TLE), and thirty healthy controls. Given that EM is a complex process involving many fundamental cognitive processes, the amplitudes and latencies of EM-related ERP (FN400, late positive potential (LPC), and late posterior negativity (LPN)), and the ERP reflecting the fundamental processes (P100, N100, P200, and P300) were calculated. Then we used a three-by-two factorial design on the ERP metrics for interaction and main effects. The correlation analysis among Wechsler Memory Scales-Chinese Revision (WMS-RC) results, behavioral data, and the ERPs was carried out.

RESULTS

The TLE patients performed worse in WMS-RC and the memory task. The increased P200 and decreased P300 amplitudes were observed in the TLE patients, and LPN was abnormal in only LTLE patients. For EM-related components, differences were observed in both the LTLE and RTLE patients: the lack of the FN400 effect, the lack of the reversed LPC effect, and the reduced FN400. No significant inter-group difference was detected for the latencies of all the ERPs. Additionally, there were significant correlations among WMS-RC scores, behaviors, and some ERP amplitudes.

CONCLUSIONS

The impaired EM is linked to the increased P200 and decreased P300 amplitudes. LPN seems to be sensitive to left temporal lobe dysfunction. More importantly, the abnormal old or new effects of the FN400 and LPC, and the reduced FN400 amplitude might be associated with the visual EM deficit in the TLE patients. These findings may assist in the deep understanding of the EM disorder and the evaluation of the side effects of antiepileptic drugs.

Characteristics of N400 component elicited in patients who have migraine with aura

Background

This study aimed to examine the N400 effect and event-related potentials (ERPs) elicited from congruent and incongruent stimuli in patients who have migraines with aura (MwA).

Methods

A total of 33 MwA patients and 20 healthy controls (HCs) were studied. They were balanced in age (35.12 ± 8.94 vs 34.70 ± 9.59 years, p = 0.872) and sex (69.7 vs 75.0% females, p = 0.761). ERPs were measured in response to both stimuli, where pictures were preceded with an object name that either matched or mismatched with the object. Averaged amplitudes, peaks, peak latencies, difference waves and topography were compared between MwA and HCs.

Results

MwA patients had significantly lower averaged amplitudes at the Fz and F4 sites during incongruent stimuli, as well as reduced peaks at the C3 and Pz sites. Topography showed a more widespread N400 effect over scalp relative to HCs. The difference ERP waveforms did not differ in the N400 effect between groups, but the P600 effect was significantly stronger in the HCs group relative to the MwA group at the Pz (6.52 ± 2.57 vs. 3.50 ± 3.15, p = 0.001) and P4 (5.86 ± 2.79 vs. 3.95 ± 3.64, p = 0.040) sites.

Conclusions

Picture-word matching tasks could serve as a potential new method for the investigation of semantic processing in MwA patients.

Cognitive-Linguistic Functions in Adults With Epilepsy: Preliminary Electrophysiological and Behavioral Findings

Purpose Cognition and language difficulties are frequently reported in both children and adults with epilepsy. The majority of the existing research has focused on pediatric epilepsy, documenting impairments in learning, academics, and social-emotional functioning. In comparison, language deficits in younger and older adults with epilepsy have received less empirical attention. Given recently identified limitations in the extant literature regarding assessing epilepsy-related language problems in adults (Dutta et al., 2018), the current exploratory study described in this research note investigated the cognitive-linguistic abilities of adults with focal or generalized types of epilepsy. Method Twelve participants with epilepsy and 11 age- and education-matched healthy controls completed a cognitive-linguistic test battery. Event-related potential (ERP) procedures were also employed to assess the integrity of neural activity supporting psycholinguistic processing in both groups using a lexical decision task. Results No significant performance differences between epilepsy and healthy control groups were noted on basic language tasks; however, group differences were evident on the more complex language measures, including spoken discourse. Even though both groups performed the lexical decision task similarly in terms of accuracy, individuals with epilepsy demonstrated longer reaction times and some atypical ERP characteristics compared to controls. Conclusion The cognitive-linguistic assessment and ERP findings suggested that, compared to neurotypical adults, individuals with epilepsy demonstrate slower processing times and greater difficulty with high-level language and spoken discourse production, despite performing within typical limits on basic language tests. Preliminary results from this research are significant in providing new knowledge about language functioning in adults with epilepsy.

Semantic Processing Impairment in Patients with Temporal Lobe Epilepsy

The impairment in episodic memory system is the best-known cognitive deficit in patients with temporal lobe epilepsy (TLE). Recent studies have shown evidence of semantic disorders, but they have been less studied than episodic memory. The semantic dysfunction in TLE has various cognitive manifestations, such as the presence of language disorders characterized by defects in naming, verbal fluency, or remote semantic information retrieval, which affects the ability of patients to interact with their surroundings. This paper is a review of recent research about the consequences of TLE on semantic processing, considering neuropsychological, electrophysiological, and neuroimaging findings, as well as the functional role of the hippocampus in semantic processing. The evidence from these studies shows disturbance of semantic memory in patients with TLE and supports the theory of declarative memory of the hippocampus. Functional neuroimaging studies show an inefficient compensatory functional reorganization of semantic networks and electrophysiological studies show a lack of N400 effect that could indicate that the deficit in semantic processing in patients with TLE could be due to a failure in the mechanisms of automatic access to lexicon.

Temporal lobe epilepsy surgery modulates the activity of auditory pathway

The purpose of this paper is to evaluate the effects of the anterior temporal lobectomy on the functional state of the auditory pathway in a group of drug-resistant epileptic patients, linking the electrophysiological results to the resection magnitude. Twenty-seven patients with temporal lobe epilepsy and a matched control group were studied. Auditory brainstem and middle latency responses (ABR and MLR respectively) were carried out before and after 6, 12 and 24 months surgical treatment. The volume and longitude of temporo-mesial resected structures were estimated on magnetic resonance images taken 6 months after surgery. Before the intervention the patients showed a significant delay of latency in waves III, V, Pa and Nb, with an increase in duration of I-V interval in comparison with healthy subjects (Mann-Whitney U-test, p<0.05). After resection, additional significant differences in waves I and Na latency were observed. Na and Pa waveforms showed a tendency to increase in amplitude, which became statistically significant 12 months after surgery for right hemisphere lobectomized patients in the midline electrode, and in Pa waveform for all patients in the temporal electrodes ipsilateral to resection (Wilcoxon test, p<0.05). In general, latency variations of MLR correlated with resection longitude, while changes in amplitude correlated with the volume of the resection in the middle temporal pole and amygdala (Pearson' correlation test, p<0.05). As a result, we assume that anterior temporal lobectomy provokes functional modifications into the auditory pathway, probably related to an indirect modulation of its activity by the temporo-mesial removed structures.

Event-related potentials in clinical research: guidelines for eliciting, recording, and quantifying mismatch negativity, P300, and N400

This paper describes recommended methods for the use of event-related brain potentials (ERPs) in clinical research and reviews applications to a variety of psychiatric and neurological disorders. Techniques are presented for eliciting, recording, and quantifying three major cognitive components with confirmed clinical utility: mismatch negativity (MMN), P300, and N400. Also highlighted are applications of each of the components as methods of investigating central nervous system pathology. The guidelines are intended to assist investigators who use ERPs in clinical research, in an effort to provide clear and concise recommendations and thereby to standardize methodology and facilitate comparability of data across laboratories.

Sex differences in two event-related potentials components related to semantic priming

Although sex differences in language processing are well documented in behavioral studies, only a few electrophysiological studies have explored this topic. We analyzed sex differences in two language-related components of event-related potentials (ERPs): the N400 and the Late Positive Complex (LPC). Ten men and 10 women, matched by age and handedness, participated in the study. Two semantic priming paradigms were presented: word pairs (60 congruent and 60 incongruent) and sentences (50 with congruent and 50 with incongruent ending words). In addition, the effect of context was investigated by a comparison between ERP effects obtained in single word priming and sentential priming. The N400 effect was earlier and larger in women, and the LPC effect was larger in men. Furthermore, the LPC effect in men, but not in women, was much larger with sentence priming than with word priming, suggesting that the LPC effect may be more dependent on context in men than in women. The opposite sex difference on the two ERP components indicates different underlying mechanisms. While the LPC effect is thought to be generated by purely explicit mechanisms, such as postlexical integrative processes, the N400 effect may be also governed by prelexical implicit and explicit mechanisms. Our results were consistent with the notion of more automated processing of language in women than in men.

Neural correlates of verbal semantic memory in patients with temporal lobe epilepsy

Functional imaging data suggest that the core network engaged in verbal semantic memory (SM) processing encompasses frontal and temporal lobe structures, with a strong left lateralization in normal right handers. The impact of long term temporal lobe epilepsy (TLE) on this network has only partly been elucidated. We studied verbal SM in 50 patients with chronic, intractable TLE (left TLE=26, right TLE=24) and 35 right handed normal controls using a verbal fMRI semantic decision paradigm. All patients had language lateralized to the left hemisphere, as verified by the intracarotid amobarbital procedure. Within and between group analyses showed remarkable, group-specific activation profiles. The control group activated frontal and temporal areas bilaterally, with a strong left predominance. Left TLE patients showed a shift of activations of left frontal and medial temporal areas to homologous regions in the right hemisphere. Furthermore, left TLE subjects utilized subcortical structures such as the thalamus and putamen to accomplish the verbal SM task. Contrastively, the activation pattern of right TLE patients resembled that of normal controls, but exhibited "hypofrontality" with a shift from frontal to posterior regions in the temporal, parietal and occipital lobe. Our results show that chronic epileptic activity originating from temporal seizure foci is associated with an alteration of neural circuits which support semantic language processing and that side of seizure focus has a specific impact on the resulting activation network. These findings presumably result from morphological changes and from functional reorganization which are both inherent to chronic TLE.

Neural localization of semantic context effects in electromagnetic and hemodynamic studies

Measures of electrical brain activity (event-related potentials, ERPs) have been useful in understanding language processing for several decades. Extant data suggest that the amplitude of the N400 component of the ERP is a general index of the ease or difficulty of retrieving stored conceptual knowledge associated with a word, which is dependent on both the stored representation itself, and the retrieval cues provided by the preceding context. Recordings from patients with brain damage, intracranial recordings, and magnetoencephalographic data implicate a (probably large portion of) the left temporal lobe as the largest source of the N400 semantic context effect, with a substantial but lesser contribution from the right temporal lobe. Event-related functional magnetic resonance (fMRI) studies using semantic context manipulations are dominated by observations of greater hemodynamic activity for incongruent sentence completions or semantically unrelated words than congruent or related words, consistent with the direction of the ERP effect. The locations of the hemodynamic effects show some variability across studies, but one commonly identified region is the left superior temporal gyrus, which is compatible with the electrophysiological results. A second commonly identified region in the fMRI studies is the left inferior frontal gyrus, which does not appear to make a substantial contribution to the N400 effect.

Frontal and posterior sources of event-related potentials in semantic comprehension

An important question in brain and language research is how activity in multiple brain networks is coordinated over time during semantic comprehension. To address this question, we applied spatiotemporal source analysis to event-related potentials (ERPs) recorded as subjects read words that were meaningful or incongruous in the context of a sentence (N400 paradigm). The incongruous word was placed either early in the sentence or at the end. Source analysis showed activity in language areas of the left hemisphere, right temporal cortex and medial limbic cortex. The initial detection of semantic incongruity (approximately 250 ms) engaged the left prefrontal cortex and left anterior cingulate. In the critical (300-500 ms) interval, regional sources in left and right lateral prefrontal cortex, right temporal cortex, and both anterior and posterior cingulate were responsive to the semantic manipulation. Left hemisphere activity preceded right hemisphere activity, and semantic effects in frontal regions began earlier and were more sustained than the transient effects within posterior cortical regions. Findings are discussed with respect to recent theories of corticothalamic and corticolimbic networks in attention and semantic processing.

Levels of processing with free and cued recall and unilateral temporal lobe epilepsy

This study investigates the role of the temporal lobes in levels-of-processing tasks (phonetic and semantic encoding) according to the nature of recall tasks (free and cued recall). These tasks were administered to 48 patients with unilateral temporal epilepsy (right "RTLE"=24; left "LTLE"=24) and a normal group (n=24). The results indicated that LTLE patients were impaired for semantic processing (free and cued recall) and for phonetic processing (free and cued recall), while for RTLE patients deficits appeared in free recall with semantic processing. It is suggested that the left temporal lobe is involved in all aspects of verbal memory, and that the right temporal lobe is specialized in semantic processing. Moreover, our data seem to indicate that RTLE patients present a retrieval processing impairment (semantic condition), whereas the LTLE group is characterized by encoding difficulties in the phonetic and semantic condition.