Encoding-related brain activity in patients with Transient Epileptic Amnesia-associated Accelerated Long-term Forgetting

Atherton, K. 1 , Nobre, A. C. . 1 , Filippini, N. . 1 , Zeman, A. . 2 & Butler, C. 1

1 University of Oxford
2 University of Exeter

Patients with transient epileptic amnesia (TEA, a sub-type of medial temporal lobe epilepsy) often complain of accelerated long-term forgetting (ALF), despite performing within the normal range on standard neuropsychological tests and having clinically normal structural brain imaging. These patients typically exhibit normal learning and initial retention, but forget rapidly over subsequent days and weeks. ALF is presumed to reflect a deficit in hippocampus-dependent memory consolidation. However, it remains possible that these patients suffer from a subtle encoding abnormality. In this fMRI study, we presented multiple photographs to a group of patients with TEA, who complained of ALF, and a group of controls. The patients actually demonstrated abnormally poor performance on a recognition test (outside the scanner) within 45 minutes of encoding. They produced more false alarms than the controls on both this test and another test four days later. In the left hippocampus, the patients showed a greater activity difference between subsequently remembered and forgotten items than that seen in the controls. Subsequently forgotten items were associated with less activity in this region in the patients than the controls. These results demonstrate that there are brain activity abnormalities at the stage of encoding in patients with ALF.