Medial temporal lobe amnesia impairs performance on a free association task

A growing body of evidence suggests that the hippocampus contributes to performance (or is implicated) in non-memory domains from perception to problem solving. In a previous study we found that hippocampal contribution to exemplar generation in a fluency task was determined jointly by the open-endedness of the task and its ability to elicit episodic memories (Sheldon and Moscovitch (2012) Hippocampus 22:1451-1466). In the current study, we extend these observations by exploring the role of the hippocampus in generative, goal-directed open-ended thought in patients with medial temporal lobe (MTL) amnesia on a free association task (think of words as they come to mind). Patients and control participants were asked to associate freely for one minute to cue words that varied in the open-endedness of the responses they elicited (greater for low- than high-frequency words), and in the ease with which episodic memories were evoked (greater for high imageable than low imageable words). As predicted, MTL amnesia patients generated fewer words than control participants when cues were highly imageable and low in frequency, but performed equivalently to them in the other conditions. These results support our prediction that the hippocampus contributes to free association, and possibly more generally to other generative tasks that are open-ended, creative, or that elicit the use of contextual and likely episodic memories in order to derive relevant information.