Kaia L. Vilberg

Brain networks underlying episodic memory retrieval.

The importance of the medial temporal lobe to episodic memory has been recognized for decades. Recent human fMRI findings have begun to delineate the functional roles of different MTL regions, most notably the hippocampus, for the retrieval of episodic memories. Importantly, these studies have also identified a network of cortical regions--each interconnected with the MTL--that are also consistently engaged during successful episodic retrieval. Along with the MTL these regions appear to constitute a content-independent network that acts in concert with cortical regions representing the contents of retrieval to support consciously accessible representations of prior experiences.

Dissociation of the neural correlates of recognition memory according to familiarity, recollection, and amount of recollected information

The present experiment used fMRI to investigate whether neural correlates of recognition memory behave in a manner consistent with the proposal that recognition decisions are based on a unidimensional memory strength variable. A modified Remember/Know recognition test was used in which participants could indicate two levels of recollection. Participants were required to indicate whether a test item was new, familiar (known), elicited recollection of general contextual details from the study episode (R1 response), or elicited a specific recollection of the item with which it was paired at study (R2 response). Little evidence could be found to support the view that Remember/Know/New judgments reflect variations along a single strength dimension. Instead, the findings replicated prior research in indicating that the neural correlates of recollection and familiarity can be doubly dissociated. Two recollection-sensitive regions - left lateral inferior parietal and left fusiform cortex - were found to be sensitive to amount of information recollected, as operationalized in the contrast between R2 and R1 responses. It is proposed that these regions may support the representation of recollected information.

The relationship between electrophysiological correlates of recollection and amount of information retrieved

The electrophysiological correlates of recollection were investigated with a modified Remember/Know task in which subjects signaled whether they fully or partially recollected visual object information in each study episode. A positive-going ERP deflection--the left parietal old/new effect--was sensitive to the amount of information recollected, demonstrating greater amplitude when elicited by test items associated with full relative to partial recollection. These findings support prior proposals that the left parietal ERP old/new effect is sensitive to the amount of information recollected from episodic memory. An early-onsetting (ca. 150 ms), left frontal old/new effect differentiated items accorded correct old versus correct new responses regardless of whether the items were endorsed as familiar or recollected. This finding extends the range of circumstances under which early, frontally distributed old/new effects occur, and adds weight to previous suggestions that these effects are a neural correlate of familiarity-driven recognition memory.

Functional Significance of Retrieval-Related Activity in Lateral Parietal Cortex: Evidence From fMRI and ERPs

The present study addressed the question whether neural activity in left lateral parietal cortex is modulated by amount of information recollected. In two experiments (one using fMRI and the other ERPs), subjects first studied pairs of pictures presented for either 1 or 6 s. They then performed a standard “Remember/Know” recognition memory test in which the old items comprised one of the pictures from each studied pair. In both experiments, a surprise posttest indicated that subjects recollected more details about the study presentation of the items presented for the longer duration. In the fMRI experiment, recollection‐ and familiarity‐based recognition elicited activity in distinct cortical networks. Additionally, recollection‐related activity in left inferior parietal cortex was of greater magnitude for test items presented for 6 s than for 1 s. In the ERP study the “left‐parietal old/new effect”—a putative correlate of successful recollection—was likewise modulated by amount of information retrieved. Together, these findings provide further support for dual‐process models of recognition memory and add weight to the proposal that retrieval‐related activity in left inferior parietal cortex reflects processes supporting the online representation of retrieved episodic information. Hum Brain Mapp 2009.

The neural correlates of recollection: transient versus sustained fMRI effects

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.

Left parietal cortex is modulated by amount of recollected verbal information.

In two earlier experiments, we reported that left parietal cortex activity covaried with the amount of pictorial information recollected. The present experiment addressed the question whether our earlier results would generalize to verbal materials. Participants studied a series of word pairs and were then tested on individual old and new words in a modified remember/know task. In this task, participants were required to indicate whether recollection was accompanied by retrieval of study pairmates or not. As before, we operationally defined ‘amount recollected’ as the contrast between these two types of remember response. We found that the same left parietal region previously identified as sensitive to amount of recollected pictorial information is also sensitive to amount of recollected verbal information.

Overlap between the neural correlates of cued recall and source memory: Evidence for a generic recollection network?

Recall of a studied item and retrieval of its encoding context (source memory) both depend on recollection of qualitative information about the study episode. This study investigated whether recall and source memory engage overlapping neural regions. Participants (n = 18) studied a series of words, which were presented either to the left or right of fixation. fMRI data were collected during a subsequent test phase in which three-letter word-stems were presented, two thirds of which could be completed by a study item. Instructions were to use each stem as a cue to recall a studied word and, when recall was successful, to indicate the words study location. When recall failed, the stem was to be completed with the first word to come to mind. Relative to stems for which recall failed, word-stems eliciting successful recall were associated with enhanced activity in a variety of cortical regions, including bilateral parietal, posterior midline, and parahippocampal cortex. Activity in these regions was enhanced when recall was accompanied by successful rather than unsuccessful source retrieval. It is proposed that the regions form part of a “recollection network” in which activity is graded according to the amount of information retrieved about a study episode.

An investigation of the effects of relative probability of old and new test items on the neural correlates of successful and unsuccessful source memory.

The present event-related fMRI study addressed the question whether retrieval-related neural activity in lateral parietal cortex is affected by the relative probability of test items. We varied the proportion of old to new items across two test blocks, with 25% of the items being old in one block and 75% being old in the other. Prior to each block, participants (N=18) completed one of two types of study judgment on each of 108 object images. They then performed a source memory test with four response options: studied in task 1, studied in task 2, old but unsure of the study task, and new. Retrieval-related activity in regions previously identified as recollection-sensitive, including the left inferior lateral parietal cortex and bilateral medial temporal cortex, was unaffected by old/new ratio. Generic retrieval success effects--retrieval-related effects common to recognized items attracting either a correct or an incorrect source judgment--were identified in several regions of left superior parietal cortex. These effects dissociated between a middle region of the intraparietal sulcus (IPS), where activity did not interact with ratio, and regions anterior and posterior to the middle IPS where activity was sensitive to old/new ratio. The findings are inconsistent with prior proposals that retrieval-related activity in and around the left middle IPS reflects the relative salience of old and new test items. Rather, they suggest that, as in the case of more inferior left parietal regions, retrieval-related activity in this region reflects processes directly linked to retrieval.

Comparison of the Neural Correlates of Retrieval Success in Tests of Cued Recall and Recognition Memory

The neural correlates of successful retrieval on tests of word stem recall and recognition memory were compared. In the recall test, subjects viewed word stems, half of which were associated with studied items and half with unstudied items, and for each stem attempted to recall a corresponding study word. In the recognition test, old/new judgments were made on old and new words. The neural correlates of successful retrieval were identified by contrasting activity elicited by correctly endorsed test items. Old > new effects common to the two tasks were found in medial and lateral parietal and right entorhinal cortex. Common new > old effects were identified in medial and left frontal cortex, and left anterior intra‐parietal sulcus. Greater old > new effects were evident for cued recall in inferior parietal regions abutting those demonstrating common effects, whereas larger new > old effects were found for recall in left frontal cortex and the anterior cingulate. New > old effects were also found for the recall task in right lateral anterior prefrontal cortex, where they were accompanied by old > new effects during recognition. It is concluded that successful recall and recognition are associated with enhanced activity in a common set of recollection‐sensitive parietal regions, and that the greater activation in these regions during recall reflects the greater dependence of that task on recollection. Larger new > old effects during recall are interpreted as reflections of the greater opportunity for iterative retrieval attempts when retrieval cues are partial rather than copy cues. Hum Brain Mapp, 2012.

Motivated Memories: Effects of Reward and Recollection in the Core Recollection Network and Beyond

fMRI was employed to assess whether the neural correlates of accurate source memory are modulated by the reward value of recollected information. Study items comprised pictures of objects, each paired with a depiction of 1 of 2 coins. The reward value of the coins (