Neuropsychology of accelerated long-term forgetting in temporal lobe epilepsy
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Patients with temporal lobe epilepsy (TLE) often complain of a fading of new memories over days to weeks. This is particularly the case for patients with transient epileptic amnesia (TEA), a subtype of TLE. Objective memory testing sometimes corroborates this complaint, demonstrating normal or near-normal recall after standard delays (10-30 minutes), followed by a rapid decline in recall over longer delays (i.e. 1 week). This ‘nonstandard’ form of memory impairment has been termed accelerated long-term forgetting (ALF). It may reflect impairment of memory encoding, consolidation or retrieval. The aim of this thesis was to characterise the cognitive basis of ALF in TEA/TLE. The objectives were to: (a) determine the time scale of ALF of words (Chapter 3), (b) establish whether ALF affects picture recognition (Chapter 4), (c) establish whether ALF is affected by repeated retrieval (Chapter 2), number of learning trials (Chapter 5) and post-learning sensory stimulation (interference) (Chapter 5), (d) investigate ALF under incidental encoding conditions (Chapter 6), and (e) examine ALF associated with baclofen, a GABAB – receptor agonist (Chapter 7). A range of experimental paradigms and materials were applied to test memory function in several samples of TEA/TLE patients complaining of ALF and in healthy controls. The experiments revealed the following: ALF for word lists became apparent after 3–8 hours of daytime wakefulness, suggesting that disturbance of sleep related consolidation processes is not necessary for ALF to emerge in TEA. ALF for verbal information occurred both under incidental and intentional encoding conditions, and this rapid forgetting was not prevented by cued or recognition tests or by the matching of encoding conditions for patients and controls. This suggests that ALF is not associated primarily with an encoding or retrieval deficit. Although multiple learning trials and reduced sensory stimulation after learning reduced early forgetting (over 15-30 minutes) in TEA/TLE, neither factor reduced long-term forgetting. Moreover, in contrast to verbal recall, picture recognition was impoverished after minutes, but declined normally thereafter, demonstrating a subtle ‘early’ memory deficit in TEA, which might or might not be related to ALF. Overall, the present research suggests that ALF reflects a consolidation deficit, which results in accelerating forgetting the first few hours to days after memory acquisition, without a requirement for intervening sleep.