Intact Working Memory for Relational Information after Medial Temporal Lobe Damage

Working memory has traditionally been viewed as independent of the hippocampus and related medial temporal lobe structures. Yet memory-impaired patients with medial temporal lobe damage are sometimes impaired at remembering relational information (e.g., an object and its location) across delays as short as a few seconds. This observation has raised the possibility that medial temporal lobe structures are sometimes critical for maintaining relational information, regardless of whether the task depends on working or long-term memory. An alternative possibility is that these structures are critical for maintaining relational information only when the task exceeds working memory capacity and depends instead on long-term memory. To test these ideas, we drew on a method used previously in a classic study of digit span in patient HM that distinguished immediate memory from long-term memory. In two experiments, we assessed the ability of four patients with medial temporal lobe lesions to maintain varying numbers of object-location associations across a 1 s retention interval. In both experiments, the patients exhibited a similar pattern of performance. They performed similarly to controls when only a small number of object-location associations needed to be maintained, and they exhibited an abrupt discontinuity in performance with larger set sizes. This pattern of results supports the idea that maintenance of relational information in working memory is intact after damage to the hippocampus and related medial temporal lobe structures and that damage to these structures impairs performance only when the task depends on long-term memory.