Very long-term amnesia in association with temporal lobe epilepsy: evidence for multiple-stage consolidation processes.

The temporal fractionation of long-term retention remains a relatively uncharted area in human memory research, and in particular there is little in the way of neuropsychological data that address this issue. We describe a patient with temporal lobe epilepsy who complained of amnesia for important events that had occurred in the previous 3-24 months, but who reported that her short-term and medium-term memory were normal. She displayed normal performance on traditional tests of short-term and long-term retention, performing at a very similar level to that of age- and sex-matched healthy control subjects on immediate and half-hour delayed recall measures. Forty days later, however, she showed a dense amnesia for recall of such information, whereas control subjects could readily recall much of the original stimuli. She also showed evidence of memory loss for news events that had occurred over the previous few years. MRI scanning and EEG brain mapping indicated left temporal lobe pathology, with a possible epileptogenic focus in the left anterior hippocampus. These data provide empirical evidence for the existence of a distinct very long-term consolidation process in human episodic memory and point to its neural correlates in the temporal lobe. Transfer of information into a permanent long-term memory store may entail multiple-stage consolidation processes rather than a single-stage, unitary consolidation process.