In four different tasks (memory for names, faces, three object locations, and six object locations), we found concordance between the performance of patients with medial temporal lobe damage and the effect of distraction on controls. The patients were intact on tasks in which distraction disrupted control performance, and the patients were impaired on tasks in which distraction minimally affected control performance. These findings suggest that an active maintenance process (working memory) contributed substantially to control performance when patients performed well and less so or not at all when patients were impaired. These results suggest that the active maintenance process is intact after medial temporal lobe damage. It is true that patients with medial temporal lobe damage can be impaired at remembering some kinds of stimuli after quite brief delays, even when no stimuli intervene between study and test (Hannula et al., 2006
; Nichols et al., 2006
; Olson et al., 2006a
; Hartley et al., 2007
; Ezzyat and Olson, 2008
). However, we suggest that these findings reflect an early dependence on long-term memory, not an impairment in working memory. Working memory is limited by its low capacity and by the ease with which information can be actively maintained through rehearsal. The length of the study–test interval is not the important factor.
Our findings also address the suggestion that relational memory is critically dependent on the medial temporal lobe, regardless of whether performance depends on long-term memory or working memory. The strong version of this view holds that patients with medial temporal lobe damage should be impaired in maintaining relational information even over short delays (Olson et al., 2006a
). Contrary to this idea, we found that patients with hippocampal damage, and even the two patients with extensive medial temporal lobe damage, successfully maintained up to six object–location associations (relational information) for 1 s and up to four object–location associations for 8 s. We suggest that relational information can be maintained as long as the material is amenable to rehearsal and does not exceed the capacity of working memory.
Despite similarity in the tasks, the performance of our patients differed in some respects from the performance of patients in previous studies (Nichols et al., 2006
; Olson et al., 2006a
). In the faces test, our patients performed as well as controls at the 2 s and 7 s delays and were impaired only at the 14 s delay. In a previous study (Nichols et al., 2006
), patients were impaired at the 7 s delay and performed numerically worse than our patients at that delay. In another study of memory for faces, patients were impaired when trying to remember a single face for 4 s (Olson et al., 2006b
). In the object location test, our patients performed well at remembering up to six object locations for 1 s and up to four object locations for 8 s. In a previous study (Olson et al., 2006a
), patients were impaired when remembering three object locations for 1 s (in one of two experiments) and for 8 s (in two of two experiments).
Differences between patient groups might account for these differences in severity of impairment. The damage in our patients was measured using quantitative volumetric analysis of MR images (Bayley et al., 2005b
; Gold and Squire, 2005
). The damage in the previous studies resulted from a variety of etiologies and reportedly included diencephalic (Nichols et al., 2006
) and medial temporal lobe structures (Nichols et al., 2006
; Olson et al., 2006a
). Descriptions of the damage were based on visual inspection of MR images, or on etiology in the absence of MRI evidence. In the absence of quantitative measurements, the possibility remains that the patients had additional damage.
It is also possible that differences in testing procedure or in the construction of test stimuli could account for the modest differences between our study and the previous ones. First, the faces in Olson et al. (2006b)
were presented without hair, which can make face recognition rather difficult. Our faces were presented with hair (as in Nichols et al., 2006
). In addition, in Olson et al. (2006a)
, there was not a subject-paced pause between trials (instead, there was a 0.5 s intertrial interval). In our experience, amnesic patients can become confused about what they are supposed to do or whether they are in the study phase or the test phase. Therefore, we included a pause between trials, so that the patients would not be disadvantaged in their knowledge about the task compared with controls. This difference in procedure might explain why our patients performed a little better than the patients in Olson et al. (2006a)
Using a measure of working memory that is unrelated to the effects of medial temporal lobe damage, we have resolved a circularity inherent in the working memory construct. The findings support a brain-based distinction between working memory and long-term memory, as well as the idea that working memory is independent of medial temporal lobe structures. We suggest that working memory depends on persistent activity in distributed regions of neocortex, including frontal, lateral temporal, and parietal cortical areas that are known to be important in the perception and initial processing of new information (Fuster, 2003
; Postle, 2006