Deep Brain Stimulation of Anterior Nuclei of the Thalamus and Hippocampal Seizure Rate Modulate Verbal Memory Performance

Verbal memory impairment is a common symptom of temporal lobe epilepsy (TLE). The ability to encode and recall verbal memories can be probed with the classic free recall task. We hypothesized that increased seizure frequency will lead to decreased verbal memory. We investigated this hypothesis in patients receiving chronic stimulation to bilateral anterior nuclei of the thalamus (ANT) deep brain stimulation (DBS). Recordings were obtained from individuals with drug resistant mesial TLE implanted with an investigational Medtronic Summit RC+S sensing and stimulation device. As patient seizure diaries are notoriously inaccurate, the patient's intracranial electroencephalography (iEEG) was scored for seizures using a validated seizure classifier with trained epileptologist review, resulting in a reliable seizure diary. The subjects completed free recall memory tasks in their home environment with iEEG and behavioral data streamed to a handheld device and cloud repository. Using multivariate analysis of variance, we were able to determine that ANT stimulation modulates memory performance. Further, changes in the seizure rate were predictive of changes in memory performance as captured by a generalized linear model (GLM). Emerging devices for electrical stimulation of the brain enabling continuous streaming of the iEEG data are crucial for long-term studies of underlying brain dynamics during cognitive tasks. However, not all patients are candidates for devices with chronic sensing capabilities, and this study suggests that changes in verbal memory performance may provide a suitable metric of therapy effectiveness in lieu of a reliable, objective seizure diary.