The Oscillatory Basis of Working Memory Function and Dysfunction in Epilepsy

Does epilepsy differentially affect different types of memory?

Despite the recognition that epilepsy can substantially disrupt memory, there are few published accounts of whether and how this disruption varies across different types of memory and/or different types of epilepsy. This review explores four main questions: (1) Are working, episodic and semantic memory differentially affected by epilepsy? (2) Do various types of epilepsy, and their treatment, have different, specifiable effects on memory? (3) Are the usual forms of neuropsychological assessments of memory - many or most designed for other conditions - appropriate for patients with epilepsy? (4) How can research on epilepsy contribute to our understanding of the neuroscience of memory? We conclude that widespread and multifactorial problems are seen in working memory in all patient groups, while patients with temporal lobe epilepsy seem particularly prone to episodic memory deficit, and those with frontal lobe epilepsy to executive function deficits that may in turn impair semantic control. Currently, it is difficult to make individual patient predictions about likely memory deficits based on seizure aetiology and type, but it is possible to guide and tailor neuropsychological assessments in an individualised way. We make recommendations for future directions in validating and optimising neuropsychological assessments, and consider how to approach effective shared decision making about the pros and cons of seizure treatment strategies, especially at crucial educational stages such as adolescence.

Scalp Electroencephalogram-Derived Involvement Indexes during a Working Memory Task Performed by Patients with Epilepsy

Electroencephalography (EEG) wearable devices are particularly suitable for monitoring a subject's engagement while performing daily cognitive tasks. EEG information provided by wearable devices varies with the location of the electrodes, the suitable location of which can be obtained using standard multi-channel EEG recorders. Cognitive engagement can be assessed during working memory (WM) tasks, testing the mental ability to process information over a short period of time. WM could be impaired in patients with epilepsy. This study aims to evaluate the cognitive engagement of nine patients with epilepsy, coming from a public dataset by Boran et al., during a verbal WM task and to identify the most suitable location of the electrodes for this purpose. Cognitive engagement was evaluated by computing 37 engagement indexes based on the ratio of two or more EEG rhythms assessed by their spectral power. Results show that involvement index trends follow changes in cognitive engagement elicited by the WM task, and, overall, most changes appear most pronounced in the frontal regions, as observed in healthy subjects. Therefore, involvement indexes can reflect cognitive status changes, and frontal regions seem to be the ones to focus on when designing a wearable mental involvement monitoring EEG system, both in physiological and epileptic conditions.

Complex executive functions assessed by the trail making test (TMT) part B improve more than those assessed by the TMT part A or digit span backward task during vagus nerve stimulation in patients with drug-resistant epilepsy

Introduction

There is a paucity of clinical studies examining the long-term effects of vagus nerve stimulation (VNS) on cognition, although a recent study of patients with drug-resistant epilepsy (DRE) treated with VNS therapy demonstrated significant improvement in executive functions as measured by the EpiTrack composite score. The present study aimed to investigate performance variability in three cognitive tests assessing executive functions and working memory in a cohort of DRE patients receiving VNS therapy during a follow-up duration of up to 5 years.

Methods

The study included 46 DRE patients who were assessed with the Trail Making Test (TMT) (Parts A and B) and Digit Span Backward (DB) task prior to VNS implantation, 6 months and 12 months after implantation, and yearly thereafter as a part of the clinical VNS protocol. A linear mixed-effects (LME) model was used to analyze changes in test z scores over time, accounting for variations in follow-up duration when predicting changes over 5 years. Additionally, we conducted descriptive analyses to illustrate individual changes.

Results

On average, TMT-A z scores improved by 0.024 units (95% confidence interval (CI): 0.006 to 0.042, p = 0.009), TMT-B z scores by 0.034 units (95% CI: 0.012 to 0.057, p = 0.003), and DB z scores by 0.019 units per month (95% CI: 0.011 to 0.028, p < 0.001). Patients with psychiatric comorbidities achieved the greatest improvements in TMT-B and DB z scores among all groups (0.0058 units/month, p = 0.036 and 0.028 units/month, p = 0.003, respectively). TMT-A z scores improved the most in patients taking 1-2 ASMs as well as in patients with psychiatric comorbidities (0.042 units/month, p = 0.002 and p = 0.003, respectively).

Conclusion

Performance in all three tests improved at the group level during the follow-up period, with the most robust improvement observed in TMT-B, which requires inhibition control and set-switching in addition to the visuoperceptual processing speed that is crucial in TMT-A and working-memory performance that is essential in DB. Moreover, the improvement in TMT-B was further enhanced if the patient had psychiatric comorbidities.

Long-term effect associated with seizures and dynamic effect associated with treatment on cognitive dysfunction of adult patients with focal epilepsy as evaluated by the Trail Making Test

OBJECTIVE

This study was performed to clarify the utility of the Trail Making Test (TMT) in evaluating the effects of the course of epilepsy on cognitive function by evaluating the course of epileptic seizures and the results of the TMT over time.

METHODS

We performed the TMT twice at a 1-year interval for each patient with focal epilepsy. We performed multiple regression analyses with the first TMT scores as dependent variables and clinical features as independent variables. Next, we performed a multivariate analysis of covariance (MANCOVA) to evaluate the difference between the first and second TMT scores for patients in each seizure prognosis group.

RESULTS

We enrolled 132 adult patients in this study. Multiple regression analyses showed that longer active seizure periods were associated with worse first TMT-B performance (β = .318, p < .001) and B-A (β = .377, p < .001) and that the number of antiseizure medicines was associated with worse first TMT-A performance (β = .186, p = .025). In addition, topiramate and zonisamide adversely affected TMT performance. MANCOVA showed an interaction between the prognosis of TMT-B performance and the seizure prognosis [F(2, 120) = 3.68, p = .028]. Subeffect tests revealed that the second TMT-B performance improved only in the seizure improvement group [F(1, 10) = 10.07, p = .01].

SIGNIFICANCE

Epileptic seizures were shown to be associated with both long-term and dynamic adverse effects on cognitive function evaluated with the TMT in adult patients with focal epilepsy. Seizure control is important for improving the cognitive function of patients with epilepsy; however, the potential adverse effects of polypharmacy and some antiseizure medicines such as zonisamide and topiramate on cognitive function should be considered.

Epilepsy disrupts hippocampal phase precision and impairs working memory

OBJECTIVE

Working memory deficits are prevalent in childhood epilepsy. Working memory processing is thought to be supported by the phase of hippocampal neural oscillations. Disruptions in working memory have previously been linked to the occurrence of transient epileptic activity. This study aimed to resolve the associations between oscillatory neural activity, transient epileptiform events, and working memory in children with epilepsy.

METHODS

Intracranial recordings were acquired from stereotactically-implanted electrodes in the hippocampi, epileptogenic zones, and working memory-related networks of children with drug-resistant epilepsy during a 1-back working memory task. Interictal epileptic activity was captured using automated detectors. Hippocampal phase and interregional connectivity within working memory networks were indexed by Rayleigh Z and the phase difference derivative respectively. Trials with and without transient epileptiform events were compared.

RESULTS

Twelve children (mean age of 14.3 ± 2.8 years) with drug-resistant epilepsy were included in the study. In the absence of transient epileptic activity, significant delta and theta hippocampal phase resetting occurred in response to working memory stimulus presentation (Rz = 9, Rz = 8). Retrieval trials that were in-phase with the preferred phase angle were associated with faster reaction times (p = 0.01, p = 0.03). Concurrently, delta and theta coordinated interactions between the hippocampi and working memory-related networks were enhanced (PDD z-scores = 6-11). During retrieval trials with pre-encoding or pre-retrieval transient epileptic activity, phase resetting was attenuated (Rz = 5, Rz = 1), interregional connectivity was altered (PDD z-scores = 1-3), and reaction times were prolonged (p = 0.01, p = 0.03).

SIGNIFICANCE

This work highlights the role of hippocampal phase in working memory. We observe post-stimulus hippocampal phase resetting coincident with enhanced interregional connectivity. The precision of hippocampal phase predicts optimal working memory processing, and transient epileptic activity prolongs working memory processing. These findings can help guide future treatments aimed at restoring memory function in this patient population.

Connectomic Profiles and Cognitive Trajectories After Epilepsy Surgery in Children

OBJECTIVE

Neurocognitive outcomes following surgery for temporal lobe epilepsy in childhood are variable. Postoperative changes are not directly predicted by seizure-freedom and associations between epilepsy, neuropsychological function, and developing neural networks are poorly understood. Here, we leveraged whole-brain connectomic profiling in magnetoencephalography (MEG) to retrospectively study associations between brain connectivity and neuropsychological function in children with temporal lobe epilepsy undergoing resective surgery.

METHODS

Clinical and MEG data were retrospectively analyzed for children who underwent temporal lobe epilepsy surgery at the Hospital for Sick Children from 2000 to 2021. Resting-state connectomes were constructed from neuromagnetic oscillations via the weighted phase lag index. Using a partial least-squares (PLS) approach, multidimensional associations between patient connectomes, neuropsychological scores, and clinical covariates were assessed. Bootstrap resampling statistics were performed to assess statistical significance.

RESULTS

A total of 133 medical records were reviewed, and 5 PLS analyses were performed. Each PLS analysis probed a particular neuropsychological domain and the associations between its baseline and post-operative scores and the connectomic data. In each PLS analysis, a significant latent variable was identified, representing a specific percentage of the variance in the data, and relating neural networks to clinical covariates, which included changes in rote verbal memory (N=41, p = 0.01, σ² = 0.38), narrative/verbal memory (N=57, p = 0.00, σ² = 0.52), visual memory (N=51, p = 0.00, σ² = 0.43), working memory (N=44, p = 0.00, σ² = 0.52), and overall intellectual function (N=59, p = 0.00, σ² = 0.55). Children with more diffuse, bilateral intrinsic connectivity across several frequency bands showed lower scores on all neuropsychological assessments but demonstrated a greater propensity for gains following resective surgery.

CONCLUSION

Here, we report that connectomes characterized by diffuse connectivity, reminiscent of developmentally immature networks, are associated with lower pre-operative cognition and post-operative cognitive improvement. These findings provide a potential means to understand neurocognitive function in children with temporal lobe epilepsy and expected changes post-operatively.

Visuomotor errors drive step length and step time adaptation during 'virtual' split-belt walking: the effects of reinforcement feedback

Precise foot placement is dependent on changes in spatial and temporal coordination between two legs in response to a perturbation during walking. Here, we used a 'virtual' split-belt adaptation task to examine the effects of reinforcement (reward and punishment) feedback about foot placement on the changes in error, step length and step time asymmetry. Twenty-seven healthy adults (20 ± 2.5 years) walked on a treadmill with continuous feedback of the foot position and stepping targets projected on a screen, defined by a visuomotor gain for each leg. The paradigm consisted of a baseline period (same gain on both legs), visuomotor adaptation period (split: one high = 'fast', one low = 'slow' gain) and post-adaptation period (same gain). Participants were divided into 3 groups: control group received no score, reward group received increasing score for each target hit, and punishment group received decreasing score for each target missed. Re-adaptation was assessed 24 ± 2 h later. During early adaptation, the slow foot undershot and fast foot overshot the stepping target. Foot placement errors were gradually reduced by late adaptation, accompanied by increasing step length asymmetry (fast < slow step length) and step time asymmetry (fast > slow step time). Only the punishment group showed greater error reduction and step length re-adaptation on the next day. The results show that (1) explicit feedback of foot placement alone drives adaptation of both step length and step time asymmetry during virtual split-belt walking, and (2) specifically, step length re-adaptation driven by visuomotor errors may be enhanced by punishment feedback.

Relationship Between Epilepsy and Dreaming: Current Knowledge, Hypotheses, and Perspectives

The interactions between epilepsy and sleep are numerous and the impact of epilepsy on cognition is well documented. Epilepsy is therefore likely to influence dreaming as one sleep-related cognitive activity. The frequency of dream recall is indeed decreased in patients with epilepsy, especially in those with primary generalized seizures. The content of dreams is also disturbed in epilepsy patients, being more negative and with more familiar settings. While several confounding factors (anti-seizure medications, depression and anxiety disorders, cognitive impairment) may partly account for these changes, some observations suggest an effect of seizures themselves on dreams. Indeed, the incorporation of seizure symptoms in dream content has been described, concomitant or not with a focal epileptic discharge during sleep, suggesting that epilepsy might directly or indirectly interfere with dreaming. These observations, together with current knowledge on dream neurophysiology and the links between epilepsy and sleep, suggest that epilepsy may impact not only wake- but also sleep-related cognition.