Dissociation of verbal and nonverbal memory: A meta-analysis

Abstract

RATIONALE: Dissociations of both verbal and nonverbal memory in epileptic patients after temporal lobectomy have remained controversial. While some studies have found that such dissociations exist, others have found otherwise. The purpose of this study is to identify the overall consistency of these research findings and to what extent these findings support the presence of memory dissociations. METHODS: A meta-analysis was performed by reviewing an initial set of 54 articles on memory in temporal lobe epilepsy. Articles were excluded it they were characterized by one of the following: (1) absence of verbal or nonverbal measure, or both, (2) did not report usable statistics for meta-analytic techniques, (3) no explicit mentioning of cerebral dominance, (4) non-complimentarity of verbal and nonverbal measures, and (5) did not employ Weschler Memory Scale (WMS) as a primary memory measure. Five studies that qualified for subsequent analyses had left cerebral dominance (mostly right-handers). Logical memory (LM) and Visual Reproduction (VR) subscales of WMS indicated verbal and nonverbal memory respectively. Differences were calculated on both LM and VR between left- (LTL) and right temporal lobectomized (RTL) patients, as well as between pre- and post-lobectomy. RESULTS: Differences between LTL and RTL were found in both preand post-lobectomy. RTL had better scores than LTL on both immediate and delayed recall of LM and VR, with the exception of immediate recall on VR. In terms of within-subject differences from pre- to post-lobectomy, for both immediate and delayed recall, LTL had decreased LM, with an increase in VR. In contrast, RTL had an increased immediate and delayed recall on LM, but only a decrease in delayed recall on VR post-lobectomy. RTL had an increase in immediate recall on VR, though the effect size was neglible. CONCLUSIONS: The present findings support the presence of a double dissociation, though this pattern was more apparent for delayed than immediate recall on LM and VR. The possible presence of verbal components in nonverbal measures of memory may have rendered less apparent findings for nonverbal memory in this study.