Relational verbal memory processing in patients with temporal lobe epilepsy

Patients with left temporal lobe epilepsy (LTLE) have been shown to demonstrate impaired categorization in semantic memory tasks leaving open the question whether this is due to problems with active categorization or to problems with consolidation/retrieval of relations. 35 patients with left or right temporal lobe epilepsy (LTLE, RTLE) and 20 healthy controls performed three verbal learning and memory tests differing in the relational distance between the items to be learned (unrelated, low associative, high associative). In order to separate the memory component in relational memory processing, we avoided problems in active categorization and perception of semantic relations by presenting category members already clustered in the order of category membership, and by checking whether the patients had actually noticed the categories. In controls and in patients with RTLE, both semantic conditions significantly improved the test performance. LTLE-patients showed the worst test performance independent of the test condition. Their performance improved under the high associative condition, but they did not at all profit from the low associative condition. Although the semantic tasks had no demand on active clustering and although all LTLE-patients noticed the relational properties very well, they could not make use of the offered clusters. They demonstrated significantly more "item specific" than "relational" encoding than the other groups. This was pronounced in the low associative condition but became evident also in the high associative condition. In contrast to the other groups, the order in recall (clustering) deteriorated with passing time in LTLE-patients. In conclusion the data provide evidence of impaired semantic relational processing in LTLE. The impairment depends on the relational properties of the material to be learned, in that it increases with the relational distance between the memory contents. Because the tasks did not demand active clustering, we can attribute this impairment to deficits in relational encoding/retrieval which would be consistent with current opinions of the role of temporolimbic structures in relational memory processing.