Direct and indirect measures of verbal relational memory following anterior temporal lobectomy

Medial temporal lobe (MTL) structures are implicated in forming conjunctions between events in order to form enduring relational memories; these memories are not evident using direct measures with varieties of amnesic subjects. Extratemporal brain structures are thought to be responsible for preserved memories, which are sometimes detectable using indirect measures. The present study tests this theory of multiple memory systems by examining whether preserved learning can be demonstrated for relational material in MTL-disordered subjects using an indirect measure which minimises conscious mediation of performance. The subjects had undergone anterior temporal lobectomy for relief of temporal lobe epilepsy: left-sided (LATL) cases had a mild verbal amnesia and right-sided (RATL) cases had better verbal memory, forming a comparison group. A direct measure of verbal relational memory was provided by successive trials of cued recall in a specially-constructed paired associate learning task with arbitrarily paired words; pairs consisted of either concrete or abstract words. LATL subjects performed worse than RATL subjects, and particularly so with abstract words. Following direct testing, memory for the pairings was measured indirectly using a masked recognition priming technique. RATL subjects showed savings in RT, demonstrating that masked priming can reveal evidence of the formation of conjunctions. Critically, LATL subjects showed no evidence of preserved learning with priming. Thus when MTL structures are damaged, relational memory appears to be affected without exception, consistent with the tenets of multiple memory systems theory.