The enigma of long-term forgetting

The evolution of accelerated long-term forgetting: Evidence from the TIME study

OBJECTIVE

Accelerated long-term forgetting (ALF) occurs when newly learned information decays faster than normal over extended delays. It has been recognised most frequently in temporal lobe epilepsy, including Transient Epileptic Amnesia (TEA), but can also be drug-induced. Little is known about the evolution of ALF over time and its impacts upon other memory functions, such as autobiographical memory (ABM). Here we investigate the long-term outcome of ALF and ABM in a group of patients with TEA and a single case of baclofen-induced ALF.

METHODS

Study 1 involved a longitudinal follow-up of 14 patients with TEA over a 10-year period. Patients repeated a neuropsychological battery, three ALF measures (with free recall probed at 30-min and 1-week), and a modified Autobiographical Memory Interview (MAMI). Performance was compared with a group of healthy age-matched controls. In Study 2, patient CS, who previously experienced baclofen-induced ALF, was followed over 4 years, and re-tested now, 18 months after ceasing baclofen. CS repeated a neuropsychological battery, three ALF experimental tasks (each probed after 30 min and 1 week), and a modified autobiographical interview (AI). Her performance was compared with healthy age-matched controls.

RESULTS

On ALF measures, the TEA group performed significantly below controls, but when analysed individually, 4 of the 7 patients who originally showed ALF no longer did so. In two, this was accompanied by improvements in ABM for recent but not remote memory. Patient CS no longer demonstrated ALF on standard lab-based tests and now appeared to retain new episodic autobiographical events with a similar degree of episodic richness as controls.

CONCLUSION

Long-term follow up suggests that ALF can resolve, with improvements translating to recent ABM in some cases.

Novel forms of forgetting in temporal lobe epilepsy

Transient Epileptic Amnesia (TEA) is a recently defined subtype of temporal lobe epilepsy, principally affecting people in middle age with a male predominance. Its key manifestation is the occurrence of recurring episodes of transient amnesia, usually lasting less than an hour and often occurring on waking. One-third of patients have exclusively amnestic attacks, while in two-thirds, at least some attacks are accompanied by other manifestations of epilepsy, especially olfactory hallucinations. Several lines of evidence point to a seizure focus in the medial temporal lobes. Transient Epileptic Amnesia is accompanied by a striking loss of autobiographical memories in two-thirds of sufferers, accelerated loss of memories which had been acquired successfully in around one half, and topographical amnesia in around one-third. This paper reviews the findings of the TIME project (The Impairment of Memory in Epilepsy - http://sites.pcmd.ac.uk/time/tea.php) in relation to TEA, accelerated long-term forgetting, and remote memory impairment.

Accelerated long-term forgetting in temporal lobe epilepsy: verbal, nonverbal and autobiographical memory

Patients with temporal lobe epilepsy (TLE) often present with memory complaints despite performing within normal limits on standard memory tests. One possible explanation for this phenomenon is accelerated long-term forgetting (ALF). The present study investigated material-specific ALF in patients with unilateral TLE and also examined whether ALF could be demonstrated on a novel, standardized anterograde autobiographical memory (ABM) task. Fourteen patients with TLE and 17 controls were administered verbal, nonverbal and ABM event memory tasks. The participants were tested for immediate recall, recall and recognition at 30-minute delay, and recall and recognition after four weeks. The extent of ALF was calculated based on the percentage decay of memory from the 30-minute delay trial to the four-week delay trial. Patients with left TLE showed significantly greater ALF for verbal material and a trend towards greater forgetting of ABM. Patients with right TLE showed a non-significant trend towards greater ALF for nonverbal material. Patients with unilateral hippocampal abnormalities showed greater ALF compared to patients without hippocampal abnormalities. Patients with seizures that generalize had more global memory deficits and greater ALF. We conclude that patients with unilateral TLE show material-specific ALF, which appears to be more pronounced with an abnormal hippocampus or seizures that secondarily generalize.