Cognitive deficits in temporal lobe epilepsy. An investigation of memory functioning.

Abstract

An experiment is described in which memory functioning is investigated in two comparable groups of epileptics. This study was undertaken with the aim of discriminating between the performance of temporal lobe and centrencephalic epileptics on the Wechsler Memory Scale (WMS) and of contributing some information concerning cerebral functioning and organization. The short term memory deficit which is thought to be associated with temporal lobe epilepsy is studied in relation to a secondary distraction task. Experimental groups are comprised of 30 subjects with chronic unilateral temporal lobe epilepsy and 30 chronic centrencephalic epileptics. A further group of 23 normal subjects is used to control for the effects of the distraction task on the memory test. The results indicate that a general memory impairment can be related to the temporal lobes. However, more detailed analysis of the data reveals that the only specific area of memory functioning in which the temporal lobe epileptics are markedly inferior to the centrencephalic epileptics is immediate recall of narrative texts. That is, the temporal lobe epileptics have particular difficulty in retaining meaningful verbal material even in the immediate present. Other subtests of the WMS do not discriminate effectively between the two epileptic groups and do not contribute significant information about the nature of the memory impairment. The performance of both epileptic groups on the WMS is adversely affected by a divided attention situation, but the distraction task does not significantly differentiate between them. An attempt is made to relate these findings to theories of cognitive functioning within the brain and to other empirical findings. A tentative hypothesis, based on an assumption of subictal discharge is proposed to explain the disturbance of memory functioning in temporal lobe epileptics. It is contended that a frequent, minimal electrical discharge disturbs the course of memory functioning in the stages of registration and consolidation by sensitizing the temporal lobe cortex (particularly the hippocampal zone) to the effects of external distraction and by directly disrupting the mechanisms involved in the consolidation of new information.