Context in free recall: multi-voxel pattern analysis of fMRI

Several researchers (e.g., Howard Kahana, 2002) have proposed that recalling an event is bound up with recall of that event's surrounding context, and that retrieved context information can be used to cue memory for other items from that context. In this study, we sought evidence for this contextual reinstatement process using fMRI. Specifically, we wanted to know whether the task being performed when forming a memory would be recalled along with that memory, and how this would influence subsequent recalls. Subjects studied lists of 24 words, performing either a size, animacy or pleasantness judgment task on each word. After a series of arithmetic distractors, subjects were asked to recall out loud and in any order the words from the most recent list. Since subjects were being scanned during both study and recall phases, we trained a classifier on the study period to distinguish which of the three tasks were being performed. We then tested this classifier during recall to estimate the degree to which each task representation was active in the subject's mind, moment by moment (Polyn et al, 2005). To analyze the recall data, we labelled each recall with its judgment task from the study period. These were predicted better than chance by the classifier's estimates of task activity at recall. We broke the data down further, looking at the transitions from one recall to the next. We found that high classifier activity for one kind of task judgment indicated that the next recall would be another item from that task, and that the inter-response latency would be small. In other words, a highly active task representation would facilitate recalls of other items from the same task. These results support the contextual reinstatement theory, suggesting that reinstating the context surrounding an event improves recall of other items that were studied in that context