NeuropsychologyLong-term memory deficits in temporal lobe epilepsy
Introduction
Memory complaints and deficits are commonly seen in patients with epilepsy, especially temporal lobe epilepsy (TLE), where memory-related brain structures are directly involved by seizures. Numerous studies and reviews have delineated the clinical neuropsychology of TLE, particularly with respect to lateralization of seizures within the classic visual/verbal memory framework and in comparisons of pre- and post-surgery memory performance, in attempts to reliably predict postsurgical outcomes [1]. The material-specific model of memory relies on the idea that, in right-handers, the left temporal lobe sustains verbal memories, while the right temporal lobe sustains non-verbal memories (for a recent review, see Willment and Golby [2]). However, over the years, neuropsychological and neuroimaging studies have progressively challenged this model. For instance, non-verbal deficits have been less consistently associated with right TLE than verbal memory has been with left TLE, and aberrant lateralization of activation patterns have been demonstrated during material-specific memory tasks in functional neuroimaging studies.
In recent years, additional issues challenging the traditional neuropsychological approach to TLE have emerged. First, the severity of memory complaints has not been consistently captured by standardized neuropsychological assessment, with many TLE patients performing at average levels or above [3]. Standardized memory tests typically assess the ability to retain new information over relatively short (from 20 min to 1 h) delays [4], whereas several lines of evidence suggest that TLE may also interfere with long-term consolidation, with successfully memorized information after short delays progressively fading over periods of days or weeks. Second, paralleling the progress in our understanding of declarative memory organization, the possibility that TLE memory deficits might be analyzed beyond the scope of the verbal/visual dichotomy has been raised [5], leading to the development of complementary assessment paradigms (for reviews, see Bell and Giovagnoli [6] and Butler and Zeman [7]). Finally, characterization of memory impairment in seizure-free patients has stimulated the development of a new area of research, focusing on the relationships between interictal activities and functional deficits.
Thus, the present report reviews all the available evidence on long-term memory deficits in TLE and discusses some of their putative pathophysiological mechanisms, along with potential strategies aimed at optimalizing memory functioning.
Section snippets
Remote memory in TLE
Remote memory deficits can have a considerable impact on psychological well-being and may represent one of the main complaints of TLE sufferers [8]. However, while the ability to acquire new information (anterograde memory) has been investigated in detail in TLE, relatively few studies have focused on remote memory, which is multifaceted, comprising memories encoded in the relatively distant past [7], [9], with episodic and semantic components. Episodic memory is typically autobiographical,
Accelerated forgetting
In “typical” cases of amnesia following brain insults, anterograde memory impairment is usually demonstrated within seconds or minutes of information exposure [39], [40]. Over the past two decades, however, a different pattern of amnesia has been identified in TLE patients, who demonstrate normal learning and retention of information over relatively brief delays, but an accelerated rate of forgetting over days or weeks. This slowly developing form of amnesia, only apparent over extended periods
A role of interictal spikes in memory disturbances?
Interictal spikes (ISs) are defined as high-amplitude (> 50 μV) fast EEG transients usually followed by a slow wave lasting several hundreds of ms. In intracellular recordings, this pattern is associated with an intracellular discharge, a paroxysmal depolarizing shift, characterized by a rapid sequence of fast action potentials superimposed on a slow depolarizing potential. ISs are followed by a refractory period lasting a few seconds and generally attributed to a post-spike inhibitory phenomenon
Impact of AEDs on memory functioning in TLE
Following the onset of treatment, a subjective improvement of memory is sometimes reported [8], [21], [22], but remains inconsistent. With regard to objective measures, few studies have provided direct comparisons of pre- and post-treatment memory performance in TLE. In a study of three TEA patients, a persistent decline in remote autobiographical memory was reported despite full resolution of amnestic seizures [87]. In a subset of our present patients for whom longitudinal data were available,
Impact of a non-pharmacological intervention on memory functioning in TLE
Several non-pharmacological protocols to alleviate the memory disturbances associated with epilepsy have been evaluated. Some studies focusing on psychosocial failures [88], [89], [90] have suggested that generic psychological support or psychotherapy may help to overcome psychosocial handicaps and seizure-related distress [91], [92]. Cognitive rehabilitation targeting specific cognitive impairments has been tried in addition to clinical care. However, the lack of homogeneity in methods of
Conclusion
The present report has reviewed the large body of evidence that TLE produces memory impairment with distinctive characteristics and features, with a particular impact on remote personal memory and very long-term storage of information. This form of memory disorder appears to be closely associated with the anatomical focus of epilepsy, especially in cases originating from MTL structures. Although the pathophysiological mechanisms have yet to be established, the role of interictal activities in
Disclosure of interest
The authors declare that they have no competing interest.
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