Episodic memory in left temporal lobe epilepsy: a functional MRI study

Abstract

Left medial temporal lobe epilepsy (MTLE) is associated with verbal memory impairment usually related to hippocampal damage. We used functional MRI (fMRI) to investigate the patterns of functional activity in healthy volunteers and MTLE patients engaged in verbal episodic memory tasks to look for evidence of a reallocation of verbal memory in epileptic patients. fMRI data were collected from seven MTLE patients with left-sided hippocampal sclerosis and 10 healthy right-handed control subjects on a 3T scanner. Subjects were instructed to learn a list of 17 words (encoding) and then to recall them (retrieval) on successive trials. Healthy volunteers and patients both exhibited bilateral activation (right higher than left) of the parahippocampal gyrus during the retrieval. This effect was more marked in the control subjects. In contrast to the control subjects, patients exhibited consistent and extensive left prefrontal activations in all the memory tasks. These findings show that verbal memory tasks did not involve the same functional patterns in patients and healthy volunteers. This may be interpreted as a dysfunctional response due to the epilepsy and left hippocampal sclerosis, and could reflect the early onset and progressive course of the disease.

The use of neuroimaging to study behavior in patients with epilepsy

“…Similar results were obtained using event-related fMRI in left TLE (76). In another study, extensive activations were observed in left prefrontal regions during encoding and retrieval of verbal information in patients with left TLE, but not controls (77). Still other studies demonstrate a widespread decrease in functional activity in frontal and temporal regions during recall in TLE relative to controls (78,79).…”

fMRI language dominance and FDG-PET hypometabolism

“…As the MFG is implicated in working memory, this finding may be a nonspecific effect of increased working memory demands as a result of generally compromised brain function, reflect compensation for greater demands on routine language function (in the way tasks perceived to be harder increase verbal working memory demands), or reflect compensation by working memory systems for disturbed hippocampal declarative memory function. [33][34][35] The interplay between the seizure focus and widespread regional networks is complex and pervasive. There are functional consequences of focal pathology that may have widespread effects on brain function.…”

A Dissociation between Implicit and Explicit Verbal Memory in Left Temporal Lobe Epilepsy

“…Patients with left temporal lobe epilepsy (TLE) show evidence of mesial temporal lobe hippocampal atrophy, as measured through magnetic resonance imaging (MRI) volumetry (16)(17)(18)(19)(20). Their deficits in explicit memory are well documented (21)(22)(23)(24)(25).…”