Kaia L. Vilberg

We investigated whether time courses of fMRI BOLD activity in recollection-sensitive brain regions varied according to the time over which recollected information was maintained. Human subjects studied word-picture pairs and were subsequently tested with studied and unstudied pictures during a scanned test phase. The test requirement was to judge whether each picture was old or new and, if old, to retrieve its study associate and hold it in mind until a response cue appeared. The interval between the test item and cue varied between two and eight seconds. Separate responses were required when items were deemed new or the associate was not retrieved. Whereas recollection-related activity in the posterior cingulate, medial temporal, and medial prefrontal cortices was transient and unrelated to the maintenance interval, activity in the left anterior angular gyrus (aLAG) tracked the interval. Thus, as in a prior study, recollection-sensitive regions could be temporally dissociated.

Prior research has identified several regions where neural activity is enhanced when recollection of episodic information is successful. Here, we investigated whether these regions dissociate according to whether recollection-related activity is transient or sustained across the time that recollected information must be maintained before a behavioral judgment. Human subjects studied a series of word-picture pairs under the requirement to judge which of the denoted objects was smaller. Following each of 4 study sessions, a scanned test phase occurred in which a series of studied and unstudied words was presented. The requirement at test was to judge whether each word was old or new and, if judged old, to retrieve the associated study picture and hold it in mind until a cue appeared. The delay interval varied between two and eight seconds. The cue instructed subjects which of three different judgments should be applied to the retrieved picture. Separate responses were required when words were either deemed new or the associated image was not retrieved. Relative to studied words for which the associated picture could not be retrieved, words giving rise to successful recollection elicited transient responses in the hippocampus/parahippocampal cortex and retrosplenial cortex, and to sustained activity in prefrontal cortex, the intraparietal sulcus, the left angular gyrus and the inferior temporal gyrus. The finding that recollection-related activity in the angular gyrus tracked the period over which recollected information was maintained is consistent with the proposal that this region contributes to the online representation of recollected information.