Intact implicit verbal relational memory in medial temporal lobe amnesia

Abstract

To elucidate the role of the hippocampus in unaware relational memory, the present study examined the performance of amnesic patients with medial temporal lobe (MTL) lesions on a cued category-exemplar generation task. In contrast to a prior study in which amnesic patients showed impaired performance (Verfaellie et al., Cognitive, Affective, and Behavioral Neuroscience, 2006, 6, 91-101), the current study employed a task that required active processing of the context word at test. In this version of the task, amnesic patients, like control participants, showed enhanced category exemplar priming when the context word associated with the target at study was reinstated at test. The finding of intact implicit memory for novel associations following hippocampal lesions in a task that requires flexible use of retrieval cues is inconsistent with a relational memory view that suggests that the hippocampus is critical for all forms of relational memory, regardless of awareness. Instead, it suggests that unaware memory for within-domain associations does not require MTL mediation.

Fig. 1

Reconstruction of lesions based on CT and MRI scans.

Fig. 2

Experimental Design. (A) Participant read word pair; sentence was then read aloud and the participant rated its plausibility. (B) The participant saw on the screen, “When I read [context word backwards] the first [category name] that come to mind are …”. The second half of the sentence was presented after the participant correctly identified the context word. The participant was then given 30 s to come up with four examples from the given category.

Fig. 3

Associative priming in individual amnesic patients and in controls. Associative priming reflects the difference in the proportion of target words generated in the old compared to the recombined condition.