Disruption of learning and long-term retention of prose passages in patients with focal epilepsy

Abstract

Recent investigations of accelerated long-term forgetting, a condition in which newly acquired memory is normal initially but decays rapidly over days or weeks, indicate that multiple factors might influence whether this phenomenon is seen in patients with epilepsy. Test-based differences such as learning condition or type of memory measure (e.g., recall vs recognition) as well as epilepsy variables (e.g., side, site, or frequency of epileptiform activity) may be important. The present study sought to characterize factors affecting learning and memory for prose passages in patients with focal epilepsy. We enrolled 21 patients with temporal lobe epilepsy, with and without hippocampal lesions, 11 patients with extratemporal epilepsy (ETE), and 29 healthy controls. Two matched passages were used to compare effects of initial learning condition (one exposure versus learning-to-criterion) on subsequent patterns of retention. Recall and recognition were tested at different delays (i.e., immediately, 30min, 24h, and 4days). Regression analyses and one-way ANOVAs indicated that having a left-hemisphere epileptic focus had a negative impact on learning, whilst presence of a hippocampal lesion (irrespective of side) was associated with deterioration in recall for intervals up to 24h postencoding. Learning condition affected patterns of memory decay in that the ETE group showed significant decline in recall between 24h and 4days only when stories were learned to criterion. In contrast with recall, no changes over time were evident in recognition memory, as patients with hippocampal lesions were impaired from 30min onward. Epilepsy variables other than side and site of epilepsy/lesion did not influence performance. In conclusion, the left hemisphere is involved in learning of prose material, and the hippocampus is involved in the consolidation of this material mainly for the first 24h. After this, cortical regions outside the hippocampus become important for recall.

Keywords: Accelerated long-term forgetting; Epilepsy; Hippocampal lesion; Verbal memory.