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

Abstract

Summary: Purpose: Temporal lobe epilepsy patients are well known to present deficits on explicit verbal memory procedures (e.g., recall, recognition). The integrity of implicit memory procedures in these patients is not established. Previous studies in this area used implicit memory measures contaminated by the effects of explicit memory.Methods: We examined the integrity of verbal implicit and explicit memory in left temporal lobe epilepsy (LTLE) patients and hypothesized that a clear dissociation in performance would be found with a relative preservation of implicit memory. TLE patients (n = 15) and age-and education-matched healthy normal patients (n = 15) were shown a 40-word study list, followed by a test phase requiring completion of word stems based on the study words or new/unseen words. Experimental conditions involved instructions to provide either the old (study) words or novel/nonlist words when completing the stem. Measures of automaticity and recollection provided uncontaminated indices of implicit and explicit memory, respectively.Results: The data showed a significant difference (p < 0.001) between the patients (Recollection, 0.12; SD, 0.18) and controls (0.50, SD, 0.15) on the measure of explicit memory. In contrast, the patients (Automaticity, 0.51; SD, 0.11) and controls (0.45, SD, 0.18) performed similarly on the implicit memory measure, with patient scores clearly at normative levels based on other Process Dissociation Procedure data.Conclusions: The data demonstrate the integrity of implicit memory in LTLE patients. Finding a dissociation between the two forms of verbal memory in LTLE patients provides evidence that they rely on different neuroanatomic systems.

Dissociated process for implicit and explicit recognition of novel events

“…Our findings confirm previous studies showing that implicit and explicit recognition processes are underpinned by different information processing systems [2]. It has been proposed that the hippocampus is important to explicit memory mechanism, which requires intentional and conscious recollection [8]. They also reported that the medial temporal lobe is involved in implicit memory measures contaminated by the effect of explicit memory.…”

Lexical-semantic processing in the semantic priming paradigm in aphasic patients

“…Priming effects concern implicit, unconscious memory abilities of past experiences, which have an indirect influence on future behavior 16 . The assessment of such effects is done by means of indirect tests, that is, those which involve motor or cognitive activities related to a learning event, but in which the instructions do not directly refer to that.…”

Impaired perceptual networks in temporal lobe epilepsy revealed by resting fMRI

“…In addition to the well-addressed impairment of cognitive functions such as memory [15,47] and language [1], functional alteration of perceptions in mTLE are often reported. A series of behavioral and electrophysiological studies by Grant et al and other authors revealed that somatosensory [4,18,22,25] and auditory [16,24] perceptions are impaired.…”

“…With better temporal and spatial resolutions, BOLD-fMRI has been widely used for investigation of epilepsy [21]. However, the current fMRI studies of mTLE have been limited to the evaluation of memory and language deficits [1,15,47] or detection of abnormal epileptic activations in the brain [2,33,34]; changes in the perceptual systems have seldom or not yet specifically been addressed in patients with mTLE.…”