orking memory and amnesia : The role of stimulus novelty athan

Despite the traditional view that damage to the hippocampus and/or surrounding areas of the medial temporal lobe (MTL) does not impair short-term or working memory (WM), recent research has shown MTL amnesics to be impaired on WM tasks that require maintaining a small amount of information over brief retention intervals (e.g., maintenance of a single face for one second). However, the types of tasks that have demonstrated WM impairments in amnesia tend to have involved novel stimuli. We hypothesized that WM may be impaired in amnesia for tasks that require maintaining novel information, but may be preserved for more familiar material, particularly if the material can be easily rehearsed. To test this hypothesis, patient HC, a 22-year-old developmental amnesic with relatively preserved semantic memory and 20 age and education matched controls performed a delayed match-to-sample task that required maintaining a single famous or non-famous face for 1–8 s, digit span and reading span tasks, and a modified Brown–Peterson task that required maintaining a single highor low-frequency word or a non-word for 4–8 s. HC’s performance was impaired for non-famous faces but preserved for famous faces, impaired for the reading span task but preserved for digit span, and it was impaired for non-words and unfamiliar low-frequency words but preserved for familiar words. These results support the hypothesis that an intact hippocampus is necessary for maintaining a single novel stimulus in WM. However, stimulus familiarity and rehearsal support WM via cortical regions independent of the MTL. © 2011 Elsevier Ltd. All rights reserved. Patients with amnesia as a result of damage to their hipocampus and/or surrounding structures of the medial temporal obe (MTL) have impaired long-term memory (LTM), but are traitionally thought to have preserved short-term memory (STM; .g., Baddeley & Warrington, 1970), or what many would now all working memory (WM). This dissociation has provided one f the strongest pieces of support for the notion that there are istinct brain systems devoted to memory over the short-term nd long-term. Despite the long-held belief that WM is preserved n amnesia, but LTM is impaired, a surprising number of studes have shown that, under certain circumstances, people with mnesia are impaired at maintaining small amounts of information ver short delays (for reviews, see Jonides et al., 2008; Ranganath Blumenfeld, 2005). Findings of WM task impairments in MTL mnesics have strong implications for how researchers should onceptualize the distinction between WM and LTM. If amnesics annot reliably maintain a single item for just four seconds, then ∗ Corresponding author at: Rotman Research Institute, Baycrest Centre for Geritric Care, 3560 Bathurst Street, Toronto, Ontario, Canada M6A 2E1. el. : +1 416 785 2500x3596. E-mail address: nrose@rotman-baycrest.on. ca (N.S. Rose). 028-3932/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. oi:10.1016/j.neuropsychologia. 2011.10. 016 this suggests that the MTL is necessary to perform both WM and LTM tasks, which casts doubt upon the independence between WM and LTM “systems. ” Several studies have shown that amnesics are impaired on various WM tasks that involve maintaining material such as a single face, abstract shapes/patterns, or a scene over delays of just a few seconds (Buffalo, Reber, & Squire, 1998; Ezzyat & Olson, 2008; Hannula, Tranel, & Cohen, 2006; Holdstock, Shaw, & Aggleton, 1995; Olson, Moore, Stark, & Chatterjee, 2006; Owen, Sahakian, Semple, Polkey, & Robbins, 1995; Warrington & Taylor, 1973). For example, Olson et al. (2006) conducted an experiment in which three amnesics with bilateral MTL damage and matched controls performed a task that required maintaining the image of a single face for four seconds before being presented a recognition probe on each trial. The amnesics demonstrated a striking deficit: corrected recognition was 86% for the controls and 31% for the amnesics. These studies suggest a processing-based approach, whereby MTL involvement in a task (regardless of whether it is categorized by researchers as an STM, WM, or LTM task) likely depends on various factors. One common aspect among many of the tasks that have demonstrated STM or WM impairments in amnesia is that they required short-term retention of novel types of visuospatial information. In contrast, the classic studies that