Accelerated forgetting in patients with epilepsy: evidence for an impairment in memory consolidation

Patients with epilepsy frequently complain of memory difficulties yet perform normally on standard neuropsychological tests of memory. It has been suggested that this may be due to an impairment of very long-term memory consolidation processes, beyond those normally assessed in the neuropsychological clinic. We carried out a prospective study of verbal memory over a long-term retention interval of 8 weeks in patients with epilepsy and in controls. Results were compared with performance on conventional tests of memory. Despite normal learning and retention over 30 min, patients with epileptic foci in the left temporal lobe performed disproportionately poorly on the long-term test compared with both patients with epileptic foci in the right temporal lobe and controls. Our findings provide evidence for an extended period of memory consolidation and point to the critical region for this process, at least for verbal material, in the left temporal lobe. The implications of our findings for clinical assessment and therapeutic management of patients with epilepsy are discussed.