Abstracts

Rationale: Epilepsy has long been known to impact both short- and long-term memory. In more recent years, a specific long-term memory deficit known as accelerated long-term forgetting (ALF) has been commonly described in temporal lobe epilepsy patients (TLE). ALF is characterized by what appears to be a normal initial retention of learned information, followed by an accelerated rate of memory decay (Kapur et al., Brain Cogn 1997), and impacts both recall and recognition of previously learned information (Narayanan et al., Epilepsy Behav 2012). However, the precise point at which ALF becomes pathologic is still unclear, as delay intervals and memory types vary across research designs in the literature. Additionally, the neural underpinnings of this memory loss has not yet been elucidated. Thus, we aimed to quantify and characterize long-term memory in epilepsy using a video task and relate these findings to individual subjects' interictal epileptiform activity (IEA) rate.

Methods: Subjects undergoing intracranial monitoring watched a documentary then were tested on their retrieval of the film’s episodic, visual, and verbal content immediately after viewing and at delay points of 24, 48, 72, and 96 hours. In order to test both recall and recognition memory, question style was randomized between free recall and forced recognition. IEA during encoding was captured using a StimTracker and the Natus EEG monitoring system, then individual spikes were detected using our automated detector and processing pipeline (Quon et al., Clinical Neurophysiology 2022).

Results: Our preliminary results show a significant effect of retrieval type on memory. Epilepsy subjects had significantly worse free recall memory overall compared to forced recognition. Additionally, there was a significant effect of delay for both retrieval types. Recall memory showed significant decay at the 48-hour mark, while recognition had a later decline at 96 hours. Finally, individual spike rates had a significant impact on recall but not recognition memory. Specifically, higher rates of IEA were associated with reduced likelihood of correct recall at the 96-hour delay point.

Conclusions: Overall, subjects with epilepsy displayed ALF for both recall and recognition memory. Free recall was more sensitive to ALF, as significant forgetting began at the 48-hour mark while recognition memory did not show impairments until 96 hours after viewing. Additionally, IEA during encoding was significantly associated with increased forgetting in recall memory. These results further support the need for more effective therapies aimed at reducing IEA burden and improving memory in epilepsy and more sensitive neuropsychological testing to capture ALF.

Funding: This work was supported by the National Science Foundation (Award Number 1632738) and by the Louis and Ruth Frank Professorship of Neurosciences award.