Current Concepts of Memory Disorder in Epilepsy: Edging Towards a Network Account

The network is more important than the node: stereo-EEG evidence of neurocognitive networks in epilepsy

Introduction

Neuropsychological assessment forms an integral part of the presurgical evaluation for patients with medically refractory focal epilepsy. Our understanding of cognitive impairment in epilepsy is based on seminal lesional studies that have demonstrated important structure-function relationships within the brain. However, a growing body of literature demonstrating heterogeneity in the cognitive profiles of patients with focal epilepsy (e.g., temporal lobe epilepsy; TLE) has led researchers to speculate that cognition may be impacted by regions outside the seizure onset zone, such as those involved in the interictal or "irritative" network.

Methods

Neuropsychological data from 48 patients who underwent stereoelectroencephalography (SEEG) monitoring between 2012 and 2023 were reviewed. Patients were categorized based on the site of seizure onset, as well as their irritative network, to determine the impact of wider network activity on cognition. Neuropsychological data were compared with normative standards (i.e., z = 0), and between groups.

Results

There were very few distinguishing cognitive features between patients when categorized based purely on the seizure onset zone (i.e., frontal lobe vs. temporal lobe epilepsy). In contrast, patients with localized irritative networks (i.e., frontal or temporal interictal epileptiform discharges [IEDs]) demonstrated more circumscribed profiles of impairment compared with those demonstrating wider irritative networks (i.e., frontotemporal IEDs). Furthermore, the directionality of propagation within the irritative network was found to influence the manifestations of cognitive impairment.

Discussion

The findings suggest that neuropsychological assessment is sensitive to network activity beyond the site of seizure onset. As such, an overly focal interpretation may not accurately reflect the distribution of the underlying pathology. This has important implications for presurgical work-up in epilepsy, as well as subsequent surgical outcomes.

Cognitive phenotypes in patients with drug-resistant temporal lobe epilepsy: Relationships with cortisol and affectivity

OBJECTIVE

Drug-resistant temporal lobe epilepsy (TLE) is a neurological disorder characterized by cognitive deficits. This study examined whether patients with TLE and different cognitive phenotypes differ in cortisol levels and affectivity while controlling for demographic and clinical variables. Methods: In this cross-sectional study, 79 adults with TLE underwent neuropsychological evaluation in which memory, language, attention/processing speed, executive function, and affectivity were assessed. Six saliva samples were collected in the afternoon to examine the ability of the hypothalamic-pituitary-adrenal (HPA) axis to descend according to the circadian rhythm (C1 to C6). The cortisol area under the curve concerning ground (AUCg) was computed to examine global cortisol secretion.

RESULTS

Three cognitive phenotypes were identified: memory impairment, generalized impairment, and no impairment. The memory-impairment phenotype showed higher cortisol levels at C4, C5, and C6 than the other groups (p = 0.03, η2 = 0.06), higher cortisol AUCg than the generalized-impairment phenotype (p = 0.004, η2 = 0.14), and a significant reduction in positive affectivity after the evaluation (p = 0.026, η2 = 0.11). Higher cortisol AUCg and reductions in positive affectivity were significant predictors of the memory-impairment phenotype (p < 0.001; Cox and Snell R2 = 0.47).

CONCLUSIONS

Patients with memory impairment had a slower decline in cortisol levels in the afternoon, which could be interpreted as an inability of the HPA axis to inhibit itself. Thus, chronic stress may influence hippocampus-dependent cognitive function more than other cognitive functions in patients with TLE.

The effects of slow wave sleep characteristics on semantic, episodic, and procedural memory in people with epilepsy

Slow wave sleep (SWS) is highly relevant for verbal and non-verbal/spatial memory in healthy individuals, but also in people with epilepsy. However, contradictory findings exist regarding the effect of seizures on overnight memory retention, particularly relating to procedural and non-verbal memory, and thorough examination of episodic memory retention with ecologically valid tests is missing. This research explores the interaction of SWS duration with epilepsy-relevant factors, as well as the relation of spectral characteristics of SWS on overnight retention of procedural, verbal, and episodic memory. In an epilepsy monitoring unit, epilepsy patients (N = 40) underwent learning, immediate and 12 h delayed testing of memory retention for a fingertapping task (procedural memory), a word-pair task (verbal memory), and an innovative virtual reality task (episodic memory). We used multiple linear regression to examine the impact of SWS duration, spectral characteristics of SWS, seizure occurrence, medication, depression, seizure type, gender, and epilepsy duration on overnight memory retention. Results indicated that none of the candidate variables significantly predicted overnight changes for procedural memory performance. For verbal memory, the occurrence of tonic-clonic seizures negatively impacted memory retention and higher psychoactive medication load showed a tendency for lower verbal memory retention. Episodic memory was significantly impacted by epilepsy duration, displaying a potential nonlinear impact with a longer duration than 10 years negatively affecting memory performance. Higher drug load of anti-seizure medication was by tendency related to better overnight retention of episodic memory. Contrary to expectations longer SWS duration showed a trend towards decreased episodic memory performance. Analyses on associations between memory types and EEG band power during SWS revealed lower alpha-band power in the frontal right region as significant predictor for better episodic memory retention. In conclusion, this research reveals that memory modalities are not equally affected by important epilepsy factors such as duration of epilepsy and medication, as well as SWS spectral characteristics.

A Genetic Model of Epilepsy with a Partial Alzheimer's Disease-Like Phenotype and Central Insulin Resistance

Studies have suggested an important connection between epilepsy and Alzheimer's disease (AD), mostly due to the high number of patients diagnosed with AD who develop epileptic seizures later on. However, this link is not well understood. Previous studies from our group have identified memory impairment and metabolic abnormalities in the Wistar audiogenic rat (WAR) strain, a genetic model of epilepsy. Our goal was to investigate AD behavioral and molecular alterations, including brain insulin resistance, in naïve (seizure-free) animals of the WAR strain. We used the Morris water maze (MWM) test to evaluate spatial learning and memory performance and hippocampal tissue to verify possible molecular and immunohistochemical alterations. WARs presented worse performance in the MWM test (p < 0.0001), higher levels of hyperphosphorylated tau (S396) (p < 0.0001) and phosphorylated glycogen synthase kinase 3 (S21/9) (p < 0.05), and lower insulin receptor levels (p < 0.05). Conversely, WARs and Wistar controls present progressive increase in amyloid fibrils (p < 0.0001) and low levels of soluble amyloid-β. Interestingly, the detected alterations were age-dependent, reaching larger differences in aged than in young adult animals. In summary, the present study provides evidence of a partial AD-like phenotype, including altered regulation of insulin signaling, in a genetic model of epilepsy. Together, these data contribute to the understanding of the connection between epilepsy and AD as comorbidities. Moreover, since both tau hyperphosphorylation and altered insulin signaling have already been reported in epilepsy and AD, these two events should be considered as important components in the interconnection between epilepsy and AD pathogenesis and, therefore, potential therapeutic targets in this field.

Compromised future thinking: another cognitive cost of temporal lobe epilepsy

The ability to mentally travel forward through time allows humans to envisage a diverse array of possible events taking place in the future, helping us to choose which pathway to take in life. In epilepsy, we assume that patients use this cognitive ability when deciding between various treatment options, but this assumption has not been robustly tested. The temporal lobes are key contributors to this 'future thinking' and its building blocks include cognitive functions commonly impaired in temporal lobe epilepsy such as memory and language, giving rise to a hypothesis that 'future thinking' is impaired in this patient cohort. Participants were 68 adults: 37 with neurosurgically-naïve, unilateral temporal lobe epilepsy (51% right lateralized) and 31 healthy controls of similar age, sex and intellectual ability to the participants with epilepsy. Future thinking was measured using an imagined experiences task validated in other neurological populations. Tools well-established in temporal lobe epilepsy were used to measure potential cognitive correlates of future thinking. Analysis of variance revealed significantly impoverished future thinking in both left and right temporal lobe epilepsy relative to controls (P = 0.001, η p 2=0.206), with no difference between temporal lobe epilepsy groups (P > 0.05). Future thinking deficits in left temporal lobe epilepsy were paralleled by deficits in scene construction, whereas impoverished future thinking in right temporal lobe epilepsy occurred in the setting of intact scene construction. Deficits in future thinking were associated with reductions in lexical access and episodic autobiographic memory in both epilepsy groups. In sum, future thinking is compromised in both left and right temporal lobe epilepsy. The deficit in left temporal lobe epilepsy is largely explainable by dysfunction in verbal cognitive processes including scene construction. While the basis of the deficits observed with right temporal foci shares features with that of left temporal lobe epilepsy, their intact scene construction raises questions about the role of the left and right temporal lobes in future thinking and scene construction and the relationship between these two constructs, including whether right temporal lobe might play a specific role in future thinking in terms of creative processing. Clinicians should take impaired future thinking into account when counselling temporal lobe epilepsy patients about various treatment options, as they may struggle to vividly imagine what different outcomes might mean for their future selves.

Neurobehavioural comorbidities of epilepsy: towards a network-based precision taxonomy

Cognitive and behavioural comorbidities are prevalent in childhood and adult epilepsies and impose a substantial human and economic burden. Over the past century, the classic approach to understanding the aetiology and course of these comorbidities has been through the prism of the medical taxonomy of epilepsy, including its causes, course, characteristics and syndromes. Although this 'lesion model' has long served as the organizing paradigm for the field, substantial challenges to this model have accumulated from diverse sources, including neuroimaging, neuropathology, neuropsychology and network science. Advances in patient stratification and phenotyping point towards a new taxonomy for the cognitive and behavioural comorbidities of epilepsy, which reflects the heterogeneity of their clinical presentation and raises the possibility of a precision medicine approach. As we discuss in this Review, these advances are informing the development of a revised aetiological paradigm that incorporates sophisticated neurobiological measures, genomics, comorbid disease, diversity and adversity, and resilience factors. We describe modifiable risk factors that could guide early identification, treatment and, ultimately, prevention of cognitive and broader neurobehavioural comorbidities in epilepsy and propose a road map to guide future research.

Memory complaints, clinical aspects, and self-esteem in adult people with epilepsy

Memory complaints are common in adult people with epilepsy (PWEs). However, the associated clinical aspects are not yet fully understood.

Objective

This study aims to relate the occurrence of memory complaints in PWEs with clinical aspects and self-esteem.

Methods

To relate the data obtained from the Memory Complaint Questionnaire (MAC-Q) with clinical aspects, 71 PWEs were assessed using the Rosenberg Self-Esteem Scale (SES), the Mini Mental State Examination (MMSE), and the Brief Cognitive Battery-Edu. These data were compared with 55 individuals in a control group (CG).

Results

Memory complaints (MAC-Q≥25) were significantly higher in PWEs, when compared with individuals in the CG [35 (49.3%) vs. 15 (27.2%); Student's t-test; p=0.012]. Objective cognitive performance was lower in PWEs. Memory complaints were associated with a lower educational level, the presence of depression, SES, MMSE, incidental memory, and the clock-drawing test scores in PWEs.

Conclusions

Memory complaints were more frequent in PWEs than in individuals in the CG, and there was a relationship with cognitive deficit, educational level, depression, and low self-esteem.

Cannabidiol (CBD) and cognition in epilepsy

Anecdotal reports of the benefits of cannabis and its components in the treatment of epilepsy have been reported for millennia. However, only recently randomized controlled trial data in support of cannabidiol (CBD) became available resulting in its FDA approval for the treatment of seizures and epilepsy. One of the most common and debilitating comorbidities of epilepsy is cognitive impairment. This impairment has a multifactorial etiology including network dysfunction due to seizures, negative cognitive side effects from anti-seizure medications (ASMs), and mood disturbances. Knowing the effects of a particular ASM (either positive or negative) is vital for providers to counsel patients on expected side effects, and may result in choosing a particular regimen over the other if the patient already suffers from significant cognitive deficits. Unlike most other ASMs and other well-studied cannabinoids such as Δ9-tetrahydrocannabinol, CBD has been shown to have additional mechanisms of action (MOA) that result in neuroprotective, anti-inflammatory, anti-oxidant, and neurogenesis effects. These additional MOAs suggest that the use of CBD could lead to other actions including positive effects on cognition that may be independent of seizure control. This targeted review discusses the currently available data on CBD's effects on cognition in epilepsy. First, we review the proposed mechanisms by which CBD could exert effects on cognition. Then, we present the pre-clinical/animal data investigating cognitive effects of CBD in seizure/epilepsy models. Finally, we discuss the available human data, including the studies in people with epilepsy that included cognitive evaluations pre- and on-CBD, and studies investigating if CBD has any effects on brain structure or function in areas pertinent to memory and cognitive functions.

Network, clinical and sociodemographic features of cognitive phenotypes in temporal lobe epilepsy

This study explored the taxonomy of cognitive impairment within temporal lobe epilepsy and characterized the sociodemographic, clinical and neurobiological correlates of identified cognitive phenotypes. 111 temporal lobe epilepsy patients and 83 controls (mean ages 33 and 39, 57% and 61% female, respectively) from the Epilepsy Connectome Project underwent neuropsychological assessment, clinical interview, and high resolution 3T structural and resting-state functional MRI. A comprehensive neuropsychological test battery was reduced to core cognitive domains (language, memory, executive, visuospatial, motor speed) which were then subjected to cluster analysis. The resulting cognitive subgroups were compared in regard to sociodemographic and clinical epilepsy characteristics as well as variations in brain structure and functional connectivity. Three cognitive subgroups were identified (intact, language/memory/executive function impairment, generalized impairment) which differed significantly, in a systematic fashion, across multiple features. The generalized impairment group was characterized by an earlier age at medication initiation (P < 0.05), fewer patient (P < 0.001) and parental years of education (P < 0.05), greater racial diversity (P < 0.05), and greater number of lifetime generalized seizures (P < 0.001). The three groups also differed in an orderly manner across total intracranial (P < 0.001) and bilateral cerebellar cortex volumes (P < 0.01), and rate of bilateral hippocampal atrophy (P < 0.014), but minimally in regional measures of cortical volume or thickness. In contrast, large-scale patterns of cortical-subcortical covariance networks revealed significant differences across groups in global and local measures of community structure and distribution of hubs. Resting-state fMRI revealed stepwise anomalies as a function of cluster membership, with the most abnormal patterns of connectivity evident in the generalized impairment group and no significant differences from controls in the cognitively intact group. Overall, the distinct underlying cognitive phenotypes of temporal lobe epilepsy harbor systematic relationships with clinical, sociodemographic and neuroimaging correlates. Cognitive phenotype variations in patient and familial education and ethnicity, with linked variations in total intracranial volume, raise the question of an early and persisting socioeconomic-status related neurodevelopmental impact, with additional contributions of clinical epilepsy factors (e.g., lifetime generalized seizures). The neuroimaging features of cognitive phenotype membership are most notable for disrupted large scale cortical-subcortical networks and patterns of functional connectivity with bilateral hippocampal and cerebellar atrophy. The cognitive taxonomy of temporal lobe epilepsy appears influenced by features that reflect the combined influence of socioeconomic, neurodevelopmental and neurobiological risk factors.

Contribution of autobiographic memory impairment to subjective memory complaints in focal epilepsy

"My memory is terrible!" is a common refrain among people with epilepsy, but such complaints are not reliably linked to poor performances on standard tests of memory. Negative affect like depression and anxiety are the most robust predictor of these complaints; however, neither do they entirely account for the phenomenon. The contribution of autobiographic memory impairment to subjective memory complaints in focal epilepsy has not been well-explored despite autobiographic memory impairments being common in patients with epilepsy, and the face validity of relating day-to-day memory failings to such a personally relevant form of memory. The current study sought to clarify whether autobiographic memory dysfunction contributes to subjective complaints in epilepsy, above and beyond negative affect, objective memory impairment, and epileptological factors in a large sample of patients with drug-resistant focal epilepsy relative to healthy controls (N = 135). Patients were stratified into groups with mesial temporal (MT; n = 40) versus nonmesial temporal (NMT; n = 46) foci. Compared to controls (n = 46), both patient groups reported more bitter subjective memory complaints (p < 0.001, large effect size), demonstrated poorer episodic (p = 0.001, large effect size) and semantic autobiographical recall (p = 0.004, medium effect size), and had higher levels of depressive symptomatology (p = 0.011, medium effect size), and trait neuroticism (p = 0.015, medium effect size). Contrary to expectations, multiple regression analyses revealed that autobiographic memory function was not an independent predictor of subjective memory complaints in either group with epilepsy. In people with epilepsy with MT foci, objective verbal memory dysfunction, neuroticism, and female gender predicted memory complaints (R² = 0.70, p = 0.015), whereas only neuroticism predicted memory complaints in people with epilepsy with NMT foci (R² = 0.21, p = 0.001). Although patients' poor recall of their autobiographical memories did not contribute to their concerns about their day-to-day memory function, the findings indicate that the location of the epileptogenic focus can provide clues as to the underlying contributors to subjective memory complaints in focal epilepsy. Important clinical implications to stem from these findings include the need for clinicians to adopt a patient-tailored, multifactorial lens when managing memory complaints in people with epilepsy, taking into account both psychological and cognitive factors.

Pre- and postoperative verbal memory and executive functioning in frontal versus temporal lobe epilepsy

There is accumulating evidence for considerable overlap in preoperatively affected cognitive functions in patients with temporal lobe epilepsy (TLE) and frontal lobe epilepsy (FLE). The current study investigated whether it is possible to differentiate between patients with FLE and TLE prior to surgery, based on measures of verbal memory and executive functioning. Furthermore, the postoperative cognitive development was compared. Pre- and postoperative data from 109 patients with FLE and 194 patients with TLE were retrospectively analyzed. Preoperatively, there were no differences in verbal memory, and postoperatively, no distinctive cognitive change was found between patients with FLE and TLE. However, patients with FLE performed worse on a cognitive switching task. Notably, irrespective of localization, patients with a presumed epileptogenic area in the language-dominant hemisphere performed worse than patients with seizures that originated in the nonlanguage-dominant hemisphere on measures of verbal memory, both pre- and postoperatively. In sum, the results suggest that verbal memory scores may be less valuable for differentiation between TLE and FLE, while measures of executive functioning may help identify patients with FLE. Additionally, rather than the localization, epilepsy lateralization critically impacts the evaluation of verbal memory functioning in both TLE and FLE. The results are discussed in light of the current frameworks of functional disturbances in epileptic networks.

Neuroanatomical Considerations in Preoperative Functional Brain Mapping

Task-based functional magnetic resonance imaging (fMRI) for the presurgical assessment of eloquent cortex is increasingly relied upon by surgeons, neurologists, and radiologists. The utility of fMRI stems from the lack of correlation between topographic anatomy and functional anatomy. fMRI can noninvasively reveal the functional anatomy of a given individual thereby allowing the surgeon to choose the most appropriate surgical trajectory, attain the most complete resection, and offer the best chance of preserving function. This dissociation between function and topography is even more critical to understand when disease distorts normal anatomic relations and when chronic evolution of pathology leads to reorganization of cortical function as can be seen with seizures or slow growing tumors. fMRI can demonstrate the functional anatomy of language, motor, vision, and memory systems. Accurate interpretation not only requires knowledge of the expected patterns of activations in the regions of interest but also demands an understanding of the many adjacent "bystander" activations that represent participatory neural activity but not the eloquent region in question. In addition, fMRI interpretation requires an understanding of the limitations of this technique when expected activity is either missing or seemingly displaced in location.

Challenges in epilepsy-The perspective of Norwegian epilepsy patients

OBJECTIVES

For most people with epilepsy (PWE), problems that are not directly related to seizures may constitute major challenges in everyday life. The purpose of this study was to determine the extent of these challenges and any risk factors for their occurrence among PWE in Norway, based on the patients' own perspective.

MATERIALS AND METHODS

We used a web-based survey to ask PWE visiting the homepage of the Norwegian Epilepsy Association about different everyday challenges. A link to the survey was accessible via the members' homepage for a 4-month period during 2017.

RESULTS

One thousand one hundred eighty-two PWE responded to the questionnaire. Although more than 40% of the cohort reported that they had been seizure free for at least 1 year, the majority reported that tiredness (71%), memory problems (70%), concentration problems (68%), headache or vertigo (51%), and feeling depressed (59%) continued to represent challenges. In addition, fear of being alone, sexual problems or difficulties in social settings were reported by about one-third of the patients. Reporting having these challenges was significantly associated with female gender, polytherapy, experiencing seizures during the previous 12 months and feeling blue or depressed.

CONCLUSIONS

The results of this study, reflecting a self-selected Norwegian population, provide insights into the challenges not directly associated with seizures that impact on the quality of life of PWE. The impacts of such challenges may be underestimated as components of the entire burden of epilepsy.

Music training is neuroprotective for verbal cognition in focal epilepsy

Focal epilepsy is a unilateral brain network disorder, providing an ideal neuropathological model with which to study the effects of focal neural disruption on a range of cognitive processes. While language and memory functions have been extensively investigated in focal epilepsy, music cognition has received less attention, particularly in patients with music training or expertise. This represents a critical gap in the literature. A better understanding of the effects of epilepsy on music cognition may provide greater insight into the mechanisms behind disease- and training-related neuroplasticity, which may have implications for clinical practice. In this cross-sectional study, we comprehensively profiled music and non-music cognition in 107 participants; musicians with focal epilepsy (n = 35), non-musicians with focal epilepsy (n = 39), and healthy control musicians and non-musicians (n = 33). Parametric group comparisons revealed a specific impairment in verbal cognition in non-musicians with epilepsy but not musicians with epilepsy, compared to healthy musicians and non-musicians (P = 0.029). This suggests a possible neuroprotective effect of music training against the cognitive sequelae of focal epilepsy, and implicates potential training-related cognitive transfer that may be underpinned by enhancement of auditory processes primarily supported by temporo-frontal networks. Furthermore, our results showed that musicians with an earlier age of onset of music training performed better on a composite score of melodic learning and memory compared to non-musicians (P = 0.037), while late-onset musicians did not differ from non-musicians. For most composite scores of music cognition, although no significant group differences were observed, a similar trend was apparent. We discuss these key findings in the context of a proposed model of three interacting dimensions (disease status, music expertise, and cognitive domain), and their implications for clinical practice, music education, and music neuroscience research.

Looking beyond lesions for causes of neuropsychological impairment in epilepsy

OBJECTIVE

Patients with temporal lobe epilepsy (TLE) are similar in their epileptology regardless of whether they have a lesion evident on MRI; this study aims to prospectively clarify whether they are also similar in their neuropsychological profiles.

METHODS

Participants comprised 152 adults: 79 patients with TLE and 73 healthy controls. Patients and controls did not differ in age, sex, or education (p > 0.05). Sixty-two percent of patients had an MRI-resolvable lesion (39% with presumed hippocampal sclerosis [HS-TLE], 61% with a lesion other than HS [MRI-positive TLE]); the remaining 38% of patients were lesion-negative. Psychometric measures well established in epilepsy were used.

RESULTS

Relative to controls, all 3 patient subgroups showed significantly impaired autobiographical, verbal, and visual memory (p < 0.05-0.001) and significantly more depression and anxiety (p < 0.05-0.01). Yet, contrary to expectations, the 3 TLE subgroups did not differ in their severity of memory or mood impairment (p > 0.05). Lower Full-Scale IQ predicted memory impairments across all TLE subtypes, with early age at seizure onset a predictor unique to MRI-negative TLE.

CONCLUSIONS

MRI-negative TLE is associated with memory and mood dysfunction equivalent to that seen in patients with hippocampal sclerosis and other MRI-resolvable pathologies. As such, neuropsychological impairments in TLE are not contingent on a macroscopic lesion and might be an intrinsic property of the underlying network disease.

Cognitive impairment in epilepsy: the role of reduced network flexibility

Objective

The dominant model of cognitive impairment in focal epilepsy has emphasised structural bases for cognitive deficits. Current theories of cognition in the healthy brain emphasise the importance of the reweighting of brain network interactions in support of task performance. Here, we explore the hypothesis that cognitive deficits in epilepsy arise through abnormalities of dynamic functional network interactions.

Method

We studied 19 healthy controls and 37 temporal lobe epilepsy (TLE) patients, using a behavioural measure of verbal fluency (the Controlled Oral Word Association Test) and an fMRI verbal fluency paradigm (Orthographic Lexical Retrieval).

Results

Behaviourally, verbal fluency was significantly impaired in TLE. Psychophysiological interaction analyses of the fMRI data, which capture state-dependent changes in network connectivity, revealed reduced task-dependent modulations of connectivity from left superior medial frontal cortex to left middle frontal gyrus in TLE patients. Individual differences in verbal fluency among TLE cases was correlated with task-dependent changes in connectivity from left posterior cingulate to left superior medial frontal cortex, and from left superior medial frontal cortex to a range of right predominant brain areas.

Interpretation

These data reveal that the typical pattern of task-driven shifts in network connectivity is not observed in TLE. Our observations go beyond simple structure-function associations and suggest that failure of network flexibility can be an important contributor to cognitive impairment in epilepsy.