Thalamic theta phase alignment predicts human memory formation and anterior thalamic cross-frequency coupling

Previously we reported electrophysiological evidence for a role for the anterior thalamic nucleus (ATN) in human memory formation (Sweeney-Reed et al., 2014). Theta-gamma cross-frequency coupling (CFC) predicted successful memory formation, with the involvement of gamma oscillations suggesting memory-relevant local processing in the ATN. The importance of the theta frequency range in memory processing is well-established, and phase alignment of oscillations is considered to be necessary for synaptic plasticity. We hypothesized that theta phase alignment in the ATN would be necessary for memory encoding. Further analysis of the electrophysiological data reveal that phase alignment in the theta rhythm was greater during successful compared with unsuccessful encoding, and that this alignment was correlated with the CFC. These findings support an active processing role for the ATN during memory formation.

DOI:http://dx.doi.org/10.7554/eLife.07578.001

The hippocampus is sensitive to the mismatch in novelty between items and their contexts

In previous functional magnetic resonance imaging (fMRI) studies of continuous recognition memory it was reported that new items elicit greater hippocampal activity than old (repeated) items (hippocampal 'novelty' effects). Rather than reflecting recency differences between new and old items, hippocampal novelty effects may instead reflect the novelty of the association between test items and the experimental context, or a mismatch in the novelty of the test item and the context. The present continuous recognition study assessed these possibilities by manipulating item-context associations on a trial-by-trial basis. Each trial comprised the presentation of an object-word (context-item) pair. Repeated items were paired either with the same context as on their first presentation, a different but previously presented context, or a new context. The task was to judge whether each item was old or new, regardless of the study status of the associated context. We found no evidence that hippocampal novelty effects reflected either item and context recency, or the novelty of the item-context association. Rather, enhanced hippocampal activity was elicited when the novelty of the item and its context mismatched. These findings support the possibility that hippocampal novelty effects reflect, at least in part, the disjunction in novelty between test items and their contexts.

Cortical reinstatement and the confidence and accuracy of source memory

Cortical reinstatement refers to the overlap between neural activity elicited during the encoding and the subsequent retrieval of an episode, and is held to reflect retrieved mnemonic content. Previous findings have demonstrated that reinstatement effects reflect the quality of retrieved episodic information as this is operationalized by the accuracy of source memory judgments. The present functional magnetic resonance imaging (fMRI) study investigated whether reinstatement-related activity also co-varies with the confidence of accurate source judgments. Participants studied pictures of objects along with their visual or spoken names. At test, they first discriminated between studied and unstudied pictures and then, for each picture judged as studied, they also judged whether it had been paired with a visual or auditory name, using a three-point confidence scale. Accuracy of source memory judgments- and hence the quality of the source-specifying information--was greater for high than for low confidence judgments. Modality-selective retrieval-related activity (reinstatement effects) also co-varied with the confidence of the corresponding source memory judgment. The findings indicate that the quality of the information supporting accurate judgments of source memory is indexed by the relative magnitude of content-selective, retrieval-related neural activity.

Pre-stimulus neural activity predicts successful encoding of inter-item associations

fMRI was employed to investigate the relationship between pre-stimulus neural activity and associative encoding of words and pictures in humans. While undergoing scanning, subjects studied randomly interleaved word or picture pairs. A pre-stimulus cue preceded the presentation of each study pair and signaled whether it would comprise words or pictures. Memory for the study pairs was later tested with an associative recognition test, which comprised word or picture pairs presented either in the same (intact) or a different (rearranged) pairing as at study, along with pairs of new items. The critical fMRI contrast was between study activity associated with pairs later correctly judged intact and pairs incorrectly judged as rearranged. A key question was whether material-selective pre-stimulus encoding effects could be identified which overlapped regions selectively activated by the respective study material. Picture-selective pre-stimulus effects were identified in bilateral fusiform and the intraparietal sulcus (IPS), whereas word-selective effects could not be identified. Material-invariant pre-stimulus subsequent memory effects were also identified in several neocortical regions as well as in the hippocampus. Whereas the loci of the neocortical effects suggest that they reflect the benefit to encoding that accrues from engagement of cognitive control processes, their magnitude was negatively correlated across subjects with associative recognition performance and positively related to false alarm rate. Conversely, the hippocampal effects also predicted unique variance in associative memory and were negatively related to hit rate. It is suggested that the neocortical pre-stimulus effects may reflect encoding processes that increase familiarity of single items, whereas the hippocampal pre-stimulus effects are proposed to reflect either the encoding of task-irrelevant features or the retrieval of task-relevant information associated with the pre-stimulus cues. Overall, the results provide evidence that pre-stimulus processes may be deleterious, rather than beneficial, to associative encoding.

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 ($2.00 vs. $0.02) was disclosed after study. At test, a source memory procedure was employed in which subjects discriminated between studied and unstudied objects and, for objects judged studied, indicated the identity of the coin paired with the object at study. Correct judgments earned a reward corresponding to the value of the coin, whereas incorrect judgments were penalized. No regions were identified where the magnitude of recollection effects was modulated by reward. Exclusive effects of source accuracy were evident in the hippocampus. Different striatal sub-regions demonstrated exclusive recollection effects, exclusive reward effects, and overlap between the 2 effects. The left angular gyrus and medial prefrontal cortex were additively responsive to source accuracy and the reward. The findings suggest that reward value and recollection success are conjointly but independently represented in at least 2 cortical regions and that striatal retrieval success effects cannot be accounted for in terms of a single construct, such as goal satisfaction.

The relationship between task-related and subsequent memory effects

The primary aim of this fMRI study was to assess the proposal that negative subsequent memory effects-greater activity for later forgotten relative to later remembered study items-are localized to regions demonstrating task-negative effects, and hence to potential components of the default mode network. Additionally, we assessed whether positive subsequent memory effects overlapped with regions demonstrating task-positive effects. Eighteen participants were scanned while they made easy or difficult relational judgments on visually presented word pairs. Easy and hard task blocks were interleaved with fixation-only rest periods. In the later unscanned test phase, associative recognition judgments were required on intact word pairs (studied pairs), rearranged pairs (pairs formed from words presented on different study trials) and new pairs. Subsequent memory effects were identified by contrasting the activity elicited by study pairs that went on to be correctly endorsed as intact versus incorrectly endorsed as rearranged. Task effects were identified by contrasting all study items and rest blocks. Both task-negative and task-positive effects were evident in widespread cortical regions and negative and positive subsequent memory effects were generally confined to task-negative and task-positive regions respectively. However, subsequent memory effects could be identified in only a fraction of task-sensitive voxels and, unlike task effects, were insensitive to the difficulty manipulation. The findings for the negative subsequent memory effects are consistent with recent proposals that the default mode network is functionally heterogeneous, and suggest that these effects are not accurately characterized as reflections of the modulation of the network as a whole.

Temporal dissociations within the core recollection network

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.

Event-related potentials and recognition memory for low- and high-frequency words

Event-related potentials (ERPs) were recorded while subjects made recognition judgments on high- and low-frequency words, half of which had previously been presented in an incidental study task. Compared to high-frequency items, low-frequency words were associated with superior recognition performance, and attracted a higher proportion of confident judgments. In the case of the low-frequency words only, the region of the ERPs post-500 msec evoked by correctly classified, previously studied (old) words was more positive-going than was the same region of the EWs to nonstudied (new) words. These "old/new" ERP differences were larger from electrodes over the left than over the right hemisphere. This old/new by frequency interaction held when EWs were formed only from words that attracted confident judgments. It is argued that these data are consistent with the ideas that (1) post-500 msec "old/ new" EW differences in recognition memory tasks reflect differences in old and new words' levels of relative familiarity, and (2) the recognition memory advantage for low-frequency words results, at least in part, from the higher level of relative familiarity engendered at test by previously studied low-frequency items. The data are interpreted as providing support for "two-process" models of recognition memory.

Comparison of the neural correlates of encoding item-item and item-context associations

fMRI was employed to investigate the role of the left inferior frontal gyrus (LIFG) in the encoding of item-item and item-context associations. On each of a series of study trials subjects viewed a picture that was presented either to the left or right of fixation, along with a subsequently presented word that appeared at fixation. Memory was tested in a subsequent memory test that took place outside of the scanner. On each test trial one of two forced choice judgments was required. For the associative test, subjects chose between the word paired with the picture at study and a word studied on a different trial. For the source test, the judgment was whether the picture had been presented on the left or right. Successful encoding of associative information was accompanied by subsequent memory effects in several cortical regions, including much of the LIFG. By contrast, successful source encoding was selectively associated with a subsequent memory effect in right fusiform cortex. The finding that the LIFG was enhanced during successful associative, but not source, encoding is interpreted in light of the proposal that subsequent memory effects are localized to cortical regions engaged by the on-line demands of the study task.

Effects of age on negative subsequent memory effects associated with the encoding of item and item-context information

It has consistently been reported that "negative" subsequent memory effects--lower study activity for later remembered than later forgotten items--are attenuated in older individuals. The present functional magnetic resonance imaging study investigated whether these findings extend to subsequent memory effects associated with successful encoding of item-context information. Older (n = 25) and young (n = 17) subjects were scanned while making 1 of 2 encoding judgments on a series of pictures. Memory was assessed for the study item and, for items judged old, the item's encoding task. Both memory judgments were made using confidence ratings, permitting item and source memory strength to be unconfounded and source confidence to be equated across age groups. Replicating prior findings, negative item effects in regions of the default mode network in young subjects were reversed in older subjects. Negative source effects, however, were invariant with respect to age and, in both age groups, the magnitude of the effects correlated with source memory performance. It is concluded that negative item effects do not reflect processes necessary for the successful encoding of item-context associations in older subjects. Negative source effects, in contrast, appear to reflect the engagement of processes that are equally important for successful episodic encoding in older and younger individuals.

Recollection, familiarity, and content-sensitivity in lateral parietal cortex: a high-resolution fMRI study

Numerous studies have identified brain regions where activity is consistently correlated with the retrieval (recollection) of qualitative episodic information. This 'core recollection network' can be contrasted with regions where activity differs according to the contents of retrieval. The present study used high-resolution fMRI to investigate whether these putatively-distinct retrieval processes engage common versus dissociable regions. Subjects studied words with two encoding tasks and then performed a memory test in which they distinguished between recollection and different levels of recognition confidence. The fMRI data from study and test revealed several overlapping regions where activity differed according to encoding task, suggesting that content was selectively reinstated during retrieval. The majority of recollection-related regions, though, did not exhibit reinstatement effects, providing support for a core recollection network. Importantly, lateral parietal cortex demonstrated a clear dissociation, whereby recollection effects were localized to angular gyrus and confidence effects were restricted to intraparietal sulcus. Moreover, the latter region exhibited a non-monotonic pattern, consistent with a neural signal reflecting item familiarity rather than a generic form of memory strength. Together, the findings show that episodic retrieval relies on both content-sensitive and core recollective processes, and these can be differentiated from familiarity-based recognition memory.

Retrieval orientation and the control of recollection: an fMRI study

This study used event-related fMRI to examine the impact of the adoption of different retrieval orientations on the neural correlates of recollection. In each of two study-test blocks, participants encoded a mixed list of words and pictures and then performed a recognition memory task with words as the test items. In one block, the requirement was to respond positively to test items corresponding to studied words and to reject both new items and items corresponding to the studied pictures. In the other block, positive responses were made to test items corresponding to pictures, and items corresponding to words were classified along with the new items. On the basis of previous ERP findings, we predicted that in the word task, recollection-related effects would be found for target information only. This prediction was fulfilled. In both tasks, targets elicited the characteristic pattern of recollection-related activity. By contrast, nontargets elicited this pattern in the picture task, but not in the word task. Importantly, the left angular gyrus was among the regions demonstrating this dissociation of nontarget recollection effects according to retrieval orientation. The findings for the angular gyrus parallel prior findings for the "left-parietal" ERP old/new effect and add to the evidence that the effect reflects recollection-related neural activity originating in left ventral parietal cortex. Thus, the results converge with the previous ERP findings to suggest that the processing of retrieval cues can be constrained to prevent the retrieval of goal-irrelevant information.

Cognitive rehabilitation changes memory-related brain activity in people with Alzheimer disease

BACKGROUND

People with Alzheimer disease (AD) are capable of new learning when cognitive support is provided, suggesting that there is plasticity even in a degenerating brain. However, it is unclear how a cognition-focused intervention operates on a neural level.

OBJECTIVE

The present study examined the effects of cognitive rehabilitation (CR) on memory-related brain activation in people with early-stage AD, as measured by functional magnetic resonance imaging (fMRI).

METHODS

A total of 19 participants either received 8 weeks of CR treatment (n = 7) or formed a control group (n = 12). We scanned participants pretreatment and posttreatment while they learned and recognized unfamiliar face-name pairs.

RESULTS

Following treatment, the CR group showed higher brain activation during recognition of face-name pairs in the left middle and inferior frontal gyri, the left insula, and 2s regions in the right medial parietal cortex. The control group showed decreased activation in these areas during recognition after the intervention period. Neither group showed an activation change during encoding. Behavioral performance on face-name learning did not improve for either group.

CONCLUSIONS

We suggest that CR may have operated on the process of recognition through partial restoration of function in frontal brain areas that are less compromised in early-stage AD and that physiological markers may be more sensitive indicators of brain plasticity than behavioral performance.

An fMRI investigation of posttraumatic flashbacks

Flashbacks are a defining feature of posttraumatic stress disorder (PTSD), but there have been few studies of their neural basis. We tested predictions from a dual representation model of PTSD that, compared with ordinary episodic memories of the same traumatic event, flashbacks would be associated with activity in dorsal visual stream and related areas rather than in the medial temporal lobe. Participants with PTSD, with depression but not PTSD, and healthy controls were scanned during a recognition task with personally relevant stimuli. The contrast of flashbacks versus ordinary episodic trauma memories in PTSD was associated with increased activation in sensory and motor areas including the insula, precentral gyrus, supplementary motor area, and mid-occipital cortex. The same contrast was associated with decreased activation in the midbrain, parahippocampal gyrus, and precuneus/posterior cingulate cortex. The results were discussed in terms of theories of PTSD and dual-process models of recognition.

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.

Neural correlates of the encoding of multimodal contextual features

Functional magnetic resonance imaging (fMRI) was employed to identify neural regions engaged during the encoding of contextual features belonging to different modalities. Subjects studied objects that were presented to the left or right of fixation. Each object was paired with its name, spoken in either a male or a female voice. The test requirement was to discriminate studied from unstudied pictures and, for each picture judged old, to retrieve its study location and the gender of the voice that spoke its name. Study trials associated with accurate rather than inaccurate location memory demonstrated enhanced activity in the fusiform and parahippocampal cortex and the hippocampus and reduced activity (a negative subsequent memory effect) in the medial occipital cortex. Successful encoding of voice information was associated with enhanced study activity in the right middle superior temporal sulcus and activity reduction in the right superior frontal cortex. These findings support the proposal that encoding of a contextual feature is associated with enhanced activity in regions engaged during its online processing. In addition, they indicate that negative subsequent memory effects can also demonstrate feature-selectivity. Relative to other classes of study trials, trials for which both contextual features were later retrieved demonstrated enhanced activity in the lateral occipital complex and reduced activity in the temporo-parietal junction. These findings suggest that multifeatural encoding was facilitated when the study item was processed efficiently and study processing was not interrupted by redirection of attention toward extraneous events.

Item memory, context memory and the hippocampus: fMRI evidence

Dual-process models of recognition memory distinguish between the retrieval of qualitative information about a prior event (recollection), and judgments of prior occurrence based on an acontextual sense of familiarity. fMRI studies investigating the neural correlates of memory encoding and retrieval conducted within the dual-process framework have frequently reported findings consistent with the view that the hippocampus selectively supports recollection, and has little or no role in familiarity-based recognition. An alternative interpretation of these findings has been proposed, however, in which it is argued that the hippocampus supports the encoding and retrieval of 'strong' memories, regardless of whether the memories are recollection- or familiarity-based. Here, we describe the findings of eight fMRI studies from our laboratory: one study of source memory encoding, four studies of the retrieval of contextual information, and three studies of continuous recognition. Together, the findings support the proposal that hippocampal activity co-varies with the amount of contextual information about a study episode that is encoded or retrieved, and not with the strength of an undifferentiated memory signal.

Hippocampal activity during recognition memory co-varies with the accuracy and confidence of source memory judgments

It has been proposed that the hippocampus selectively supports retrieval of contextual associations, but an alternative view holds that the hippocampus supports strong memories regardless of whether they contain contextual information. We employed a memory test that combined the 'Remember/Know' and source memory procedures, which allowed test items to be segregated both by memory strength (recognition accuracy) and, separately, by the quality of the contextual information that could be retrieved (indexed by the accuracy/confidence of a source memory judgment). As measured by fMRI, retrieval-related hippocampal activity tracked the quality of retrieved contextual information and not memory strength. These findings are consistent with the proposal that the hippocampus supports contextual recollection rather than recognition memory more generally.

Dissociation of Recollection-Related Neural Activity in Ventral Lateral Parietal Cortex

fMRI responses to recognition memory test items in two regions of ventral lateral parietal cortex-the angular gyrus and temporo-parietal junction (TPJ)-are enhanced when recognition is accompanied by recollection. According to the 'episodic buffer' hypothesis, ventral parietal recollection effects reflect processes involved in maintaining or representing recollected information. According to the 'attention to memory' hypothesis, however, the effects reflect attentional re-orienting to the products of recollection. The present experiment addressed the question whether these operations map on to the angular gyrus and TPJ, respectively. Subjects were scanned during a memory test that required a Remember/Know/New and a source memory judgment, allowing recollected items to be segregated by amount of contextual information recollected. Angular gyrus activity tracked amount of recollected information, whereas activity in the TPJ was enhanced for items endorsed as recollected, but was insensitive to amount of information recollected. Thus, the two regions likely support functionally dissociable processes.

Autobiographical memory in depression: an fMRI study

Depression is associated with three distinct alterations in memory functioning: mood-congruent recall, over-generality, and intrusive memories. These concern the autobiographical memory system, yet no previous studies have examined the neural correlates of autobiographical memory function in depression. In the present study we used functional magnetic resonance imaging (fMRI) to assess depressed and control participants during an autobiographical memory task. In their first visit to the laboratory, participants wrote a narrative account of a distressing event. Participants were scanned during the second visit while they viewed old items from their narrative and new words or phrases in a recognition memory task. Activity common to both groups during the successful identification of personal emotional memories was observed in regions previously associated with autobiographical memory retrieval. Reduced activity in the depressed group was observed in three regions of the prefrontal cortex associated with cognitive, emotional, and memory inhibition. These results are consistent with a failure by depressed individuals to inhibit task-irrelevant information during an autobiographical memory task.

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 word's 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.

Effects of age on the neural correlates of familiarity as indexed by ERPs

ERPs were recorded from samples of young (18-29 years) and older (63-77 years) participants while they performed a modified "remember-know" recognition memory test. ERP correlates of familiarity-driven recognition were obtained by contrasting the waveforms elicited by unrecollected test items accorded "confident old" and "confident new" judgments. Correlates of recollection were identified by contrasting the ERPs elicited by items accorded "remember" and confident old judgments. Behavioral analyses revealed lower estimates of both recollection and familiarity in older participants than in young participants. The putative ERP correlate of recollection-the "left parietal old-new effect"-was evident in both age groups, although it was slightly but significantly smaller in the older sample. By contrast, the putative ERP correlate of familiarity-the "midfrontal old-new effect"-could be identified in young participants only. This age-related difference in the sensitivity of ERPs to familiarity was also evident in subgroups of young and older participants, in whom familiarity-based recognition performance was equivalent. Thus, the inability to detect a reliable midfrontal old-new effect in older participants was not a consequence of an age-related decline in the strength of familiarity. These findings raise the possibility that familiarity-based recognition memory depends upon qualitatively different memory signals in older and young adults.

Effects of modality on the neural correlates of encoding processes supporting recollection and familiarity

Prior research has demonstrated that the neural correlates of successful encoding ("subsequent memory effects") partially overlap with neural regions selectively engaged by the on-line demands of the study task. The primary goal of the present experiment was to determine whether this overlap is associated solely with encoding processes supporting later recollection, or whether overlapping subsequent memory and study condition effects are also evident when later memory is familiarity-based. Subjects (N = 17) underwent fMRI scanning while studying a series of visually and auditorily presented words. Memory for the words was subsequently tested with a modified Remember/Know procedure. Auditorily selective subsequent familiarity effects were evident in bilateral temporal regions that also responded preferentially to auditory items. Although other interpretations are possible, these findings suggest that overlap between study condition-selective subsequent memory effects and regions selectively sensitive to study demands is not uniquely associated with later recollection. In addition, modality-independent subsequent memory effects were identified in several cortical regions. In every case, the effects were greatest for later recollected items, and smaller for items later recognized on the basis of familiarity. The implications of this quantitative dissociation for dual-process models of recognition memory are discussed.

Structural brain abnormalities common to posttraumatic stress disorder and depression

BACKGROUND

Posttraumatic stress disorder (PTSD) and major depression are reliably associated with reductions in brain volume in markedly similar areas. To our knowledge, no volumetric studies have directly contrasted these conditions. We investigated which, if any, grey matter reductions would be uniquely associated with each disorder. We also investigated more subtle independent effects: specifically, correlations between brain volume and self-report measures of psychopathology.

METHODS

We obtained structural magnetic resonance imaging scans from participants with PTSD, major depression and healthy controls exposed to trauma. Participants completed standardized self-report measures of anxiety and depression. We used voxel-based morphometry, applying the DARTEL algorithm within SPM5 to identify associated volumetric changes.

RESULTS

We enrolled 24 patients with PTSD, 29 with major depression and 29 controls in our study. The clinical groups had regions of markedly smaller volume compared with the control group, particularly in prefrontal areas, but did not differ from each other. Greater self-reported anxiety was inversely related to volume in several areas, particularly the inferior temporal cortex, among patients with PTSD, but was associated with some volume increases in patients with major depression. Greater self-reported depression showed similar but weaker effects, being inversely related to brain volume in patients with PTSD but positively related to volume in the cuneus and precuneus of those with major depression.

LIMITATIONS

To maintain the representativeness of the sample, patients with PTSD were not excluded if they had typical comorbid conditions, such as depression. Patients were not all medication-free, but we controlled for group differences in antidepressant use in the analyses.

CONCLUSION

We identified commonalities in areas of brain volume in patients with PTSD and those with major depression, suggesting that existing findings concerning reductions in prefrontal areas in particular may not be specific to PTSD but rather related to features of the disorder that are shared with other conditions, such as depression. More subtle differences between patients with PTSD and those with major depression were represented by distinct structural correlates of self-reported anxiety and depression.

Recollection-related hippocampal activity during continuous recognition: a high-resolution fMRI study

We used high-resolution functional magnetic resonance imaging to investigate whether successful recollection during continuous recognition is associated with relative enhancement of hippocampal activity, consistent with prior findings from experiments employing separate study and test phases. While being scanned, subjects discriminated between new and repeated pictures. Each picture, which was repeated once after an interval of between 10 and 30 items, was surrounded by a frame that was colored gray, blue, or orange. When an item repeated, its frame color determined the correct response. Repeated items surrounded by a gray frame always required an "old" judgment. A repeated item surrounded by a blue or an orange frame required a different response depending whether it was represented in the same (Target) or a different (Nontarget) color from the first presentation. Consistent with the results from previous continuous recognition experiments, robust new > old effects were found in bilateral hippocampus. In addition, an across-subjects correlational analysis identified a cluster of voxels in right hippocampus where recollection-related activity (operationalized by the contrast between correctly vs. incorrectly judged Nontargets) was positively correlated with recollection performance. Thus, successful recollection during continuous recognition is associated with a relative enhancement of hippocampal activity.

Decrements in hippocampal activity with item repetition during continuous recognition: an fMRI study

fMRI (1.5 mm isotropic voxels) was employed to investigate the relationship between hippocampal activity and memory strength in a continuous recognition task. While being scanned, subjects were presented with colored photographs that each appeared on four occasions. The requirements were to make one response when an item was presented for the first or the third time and to make a different response when an item appeared for the second or the fourth time. Consistent with prior findings, items presented for the first time elicited greater hippocampal and parahippocampal activity than repeated items. The activity elicited by repeated items declined linearly as a function of number of presentations ("graded" new > old effects). No medial-temporal lobe regions could be identified where activity elicited by repeated items exceeded that for new items or where activity elicited by repeated items increased with number of presentations. These findings are inconsistent with the proposal that retrieval-related hippocampal activity is positively correlated with memory strength. We also identified graded new > old effects in several cortical regions outside the medial-temporal lobe, including the left retrosplenial/posterior cingulate cortex and the right lateral occipito-temporal cortex. By contrast, graded old > new effects were evident in bilateral mid-intraparietal sulcus and precuneus.

Association between flashbacks and structural brain abnormalities in posttraumatic stress disorder

OBJECTIVE

Posttraumatic stress disorder (PTSD) is reliably associated with reduced brain volume relative to healthy controls, in areas similar to those found in depression. We investigated whether in a PTSD sample brain volumes in these areas were related to reporting specific symptoms of PTSD or to overall symptom severity.

METHOD

Structural MRI scans were obtained from 28 participants diagnosed with PTSD according to DSM-IV-TR. Participants reported the extent of individual PTSD symptoms using the Posttraumatic Diagnostic Scale. Voxel-based morphometry applying the Dartel algorithm implemented within SPM5 was used to identify volumetric changes, related to PTSD total, symptom cluster, and individual symptom scores.

RESULTS

Brain volume was unrelated to overall PTSD severity, but greater reexperiencing scores predicted reduced volumes in the middle temporal and inferior occipital cortices. Increased reports of flashbacks predicted reduced volume in the insula/parietal operculum and in the inferior temporal gyrus.

CONCLUSION

The data illustrate the value of analyses at the symptom level within a patient population to supplement group comparisons of patients and healthy controls. Areas identified were consistent with a neurobiological account of flashbacks implicating specific abnormalities in the ventral visual stream.

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.

The effects of age, memory performance, and callosal integrity on the neural correlates of successful associative encoding

This functional magnetic resonance imaging study investigated the relationship between the neural correlates of associative memory encoding, callosal integrity, and memory performance in older adults. Thirty-six older and 18 young subjects were scanned while making relational judgments on word pairs. Neural correlates of successful encoding (subsequent memory effects) were identified by contrasting the activity elicited by study pairs that were correctly identified as having been studied together with the activity elicited by pairs wrongly judged to have come from different study trials. Subsequent memory effects common to the 2 age groups were identified in several regions, including left inferior frontal gyrus and bilateral hippocampus. Negative effects (greater activity for forgotten than for remembered items) in default network regions in young subjects were reversed in the older group, and the amount of reversal correlated negatively with memory performance. Additionally, older subjects' subsequent memory effects in right frontal cortex correlated positively with anterior callosal integrity and negatively with memory performance. It is suggested that recruitment of right frontal cortex during verbal memory encoding may reflect the engagement of processes that compensate only partially for age-related neural degradation.

Neural correlates of encoding within- and across-domain inter-item associations

The neural correlates of the encoding of associations between pairs of words, pairs of pictures, and word-picture pairs were compared. The aims were to determine, first, whether the neural correlates of associative encoding vary according to study material and, second, whether encoding of across- versus within-material item pairs is associated with dissociable patterns of hippocampal and perirhinal activity, as predicted by the "domain dichotomy" hypothesis of medial temporal lobe function. While undergoing fMRI scanning, subjects (n = 24) were presented with the three classes of study pairs, judging which of the denoted objects fit into the other. Outside of the scanner, subjects then undertook an associative recognition task, discriminating between intact study pairs, rearranged pairs comprising items that had been presented on different study trials, and unstudied item pairs. The neural correlates of successful associative encoding--subsequent associative memory effects--were operationalized as the difference in activity between study pairs correctly judged intact versus pairs incorrectly judged rearranged on the subsequent memory test. Pair type-independent subsequent memory effects were evident in the left inferior frontal gyrus (IFG) and the hippocampus. Picture-picture pairs elicited material-selective effects in regions of fusiform cortex that were also activated to a greater extent on picture trials than on word trials, whereas word-word pairs elicited material-selective subsequent memory effects in left lateral temporal cortex. Contrary to the domain-dichotomy hypothesis, neither hippocampal nor perirhinal subsequent memory effects differed depending on whether they were elicited by within- versus across-material study pairs. It is proposed that the left IFG plays a domain-general role in associative encoding, that associative encoding can also be facilitated by enhanced processing in material-selective cortical regions, and that the hippocampus and perirhinal cortex contribute equally to the formation of inter-item associations, regardless of whether the items belong to the same or to different processing domains.

Prestimulus hippocampal activity predicts later recollection

Functional magnetic resonance imaging (fMRI) was used to address the question whether medial temporal lobe (MTL) activity prior to a stimulus event is predictive of whether the event will be successfully encoded in an incidental study task. Participants were scanned while making pleasantness judgments on words presented either in written or spoken form. A cue presented at a variable interval before the onset of each word signaled the modality of the upcoming item. Following the study phase, a surprise recognition memory test was administered that required items to be endorsed as "Remembered," "Known," or "New." Activity in the MTL, including the hippocampus, differed during the cue-item interval according to whether the item was later endorsed as Remembered rather than judged as Known or New. Thus, the level of hippocampal activity prior to the onset of an event predicts whether the event will be successfully encoded into episodic memory.

Dissociation of the neural correlates of visual and auditory contextual encoding

The present study contrasted the neural correlates of encoding item-context associations according to whether the contextual information was visual or auditory. Subjects (N=20) underwent fMRI scanning while studying a series of visually presented pictures, each of which co-occurred with either a visually or an auditorily presented name. The task requirement was to judge whether the name corresponded to the presented object. In a subsequent memory test subjects judged whether test pictures were studied or unstudied and, for items judged as studied, indicated the presentation modality of the associated name. Dissociable cortical regions demonstrating increased activity for visual vs. auditory trials (and vice versa) were identified. A subset of these modality-selective regions also showed modality-selective subsequent source memory effects, that is, enhanced responses on trials associated with correct modality judgments relative to those for which modality or item memory later failed. These findings constitute direct evidence for the proposal that successful encoding of a contextual feature is associated with enhanced activity in the cortical regions engaged during the on-line processing of that feature. In addition, successful encoding of visual objects within auditory contexts was associated with more extensive engagement of the hippocampus and adjacent medial temporal cortex than was the encoding of such objects within visual contexts. This raises the possibility that the encoding of across-modality item-context associations places more demands on the hippocampus than does the encoding of within-modality associations.

Dissociation of the electrophysiological correlates of familiarity strength and item repetition

Event-related potentials (ERPs) were employed to investigate the relationship between the familiarity strength of recognition memory test items (pictures of animate and inanimate objects) and a putative ERP correlate of familiarity, the mid-frontal 'old/new' effect. A modified Remember/Know task was used in which subjects endorsed items as 'remembered' if any detail of the study presentation could be retrieved and, if not, judged the old/new status of the item using a 4-point confidence scale ('confident old' to 'confident new'). Studied test items elicited a mid-frontal old/new effect that varied according to the rated familiarity of the eliciting item. Thus, prior findings that the mid-frontal effect is graded according to familiarity strength are not attributable to the confounding influence of study status, as has been suggested. ERPs elicited by studied and unstudied items that were rated equally familiar differed in the same latency range as that occupied by the mid-frontal old/new effect. Furthermore, the scalp topography of this repetition effect differed significantly from the topography of the mid-frontal effect. The findings suggest that ERPs elicited by recognition memory test items are modulated during the 300-500 ms latency range both by the familiarity strength of the item and, separately, by an implicit memory process that acts independently of the processes supporting familiarity-driven recognition judgments.

Neural bases of autobiographical support for episodic recollection of faces

Incidental retrieval of autobiographical knowledge can provide rich contextual support for episodic recollection of a recent event. We examined the neural bases of these two processes by performing fMRI scanning during a recognition memory test for faces that were unfamiliar, famous, or personally known. The presence of pre-experimental knowledge of a face was incidental to the task, but nonetheless resulted in improved performance. Two distinct networks of activation were associated with correct recollection of a face's prior presentation (recollection hits vs. correct rejections) on one hand, and with pre-experimental knowledge about it (famous or personally known vs. unfamiliar faces) on the other. The former included mid/posterior cingulate cortex, precuneus, and ventral striatum. The latter included bilateral hippocampus, retrosplenial, and ventromedial prefrontal cortices. Anterior and medial thalamic activations showed an interaction between both effects, driven by increased activation for recollection of unfamiliar faces. When recollecting the presentation of a famous or personally known face, hippocampal activation increased with participants' ratings of how well they felt they knew the person shown. Ventromedial prefrontal cortex showed significantly greater activation for personally known than famous faces. Our results indicate a dissociation between the involvement of retrosplenial vs. mid/posterior cingulate and precuneus in memory tasks. They also indicate that, during recognition memory experiments, the hippocampus supports incidental retrieval of pre-experimental knowledge about the stimuli presented. This type of knowledge likely underlies the additional recollection found for prior presentation of well known stimuli compared with novel ones and may link hippocampal activation at encoding to subsequent memory performance more generally.

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.

Right dorsolateral prefrontal cortex is engaged during post-retrieval processing of both episodic and semantic information

Post-retrieval processes are engaged when the outcome of a retrieval attempt must be monitored or evaluated. Functional neuroimaging studies have implicated right dorsolateral prefrontal cortex (DLPFC) as playing a role in post-retrieval processing. The present study used fMRI to investigate whether retrieval-related neural activity in DLPFC is associated specifically with monitoring the episodic content of a retrieval attempt. During study, subjects were cued to make one of two semantic judgments on serially presented pictures. One study phase was followed by a source memory task, in which subjects responded 'new' to unstudied pictures, and signaled the semantic judgment made on each studied picture. A separate study phase was followed by a task in which the studied items were subjected to a judgment about their semantic attributes. Both tasks required that retrieved information be evaluated prior to response selection, but only the source memory task required evaluation of retrieved episodic information. In both tasks, activity in a common region of right DLPFC was greater for studied than for unstudied items, and the magnitude of this effect did not differ between the tasks. Together with the results of a parallel event-related potential study [Hayama, H. R., Johnson, J. D., & Rugg, M. D. (2008). The relationship between the right frontal old/new ERP effect and post-retrieval monitoring: Specific or non-specific? Neuropsychologia, 46(5), 1211-1223, doi:S0028-3932(07)00390-9], the present findings indicate that putative right DLPFC correlates of post-retrieval processing are not associated exclusively with monitoring or evaluating episodic content. Rather, the effects likely reflect processing associated with monitoring or decision-making in multiple cognitive domains.

Effects of advanced aging on the neural correlates of successful recognition memory

Functional neuroimaging studies have reported that the neural correlates of retrieval success (old>new effects) are larger and more widespread in older than in young adults. In the present study we investigated whether this pattern of age-related 'over-recruitment' continues into advanced age. Using functional magnetic resonance imaging (fMRI), retrieval-related activity from two groups (N=18 per group) of older adults aged 84-96 years ('old-old') and 64-77 years ('young-old') was contrasted. Subjects studied a series of pictures, half of which were presented once, and half twice. At test, subjects indicated whether each presented picture was old or new. Recognition performance of the old-old subjects for twice-studied items was equivalent to that of the young-old subjects for once-studied items. Old>new effects common to the two groups were identified in several cortical regions, including medial and lateral parietal and prefrontal cortex. There were no regions where these effects were of greater magnitude in the old-old group, and thus no evidence of over-recruitment in this group relative to the young-old individuals. In one region of medial parietal cortex, effects were greater (and only significant) in the young-old group. The failure to find evidence of over-recruitment in the old-old subjects relative to the young-old group, despite their markedly poorer cognitive performance, suggests that age-related over-recruitment effects plateau in advanced age. The findings for the medial parietal cortex underscore the sensitivity of this cortical region to increasing age.

ERP correlates of the incidental retrieval of emotional information: effects of study-test delay

Prior studies indicate that, in tests of recognition memory, ERPs elicited by correctly recognized test items differ according to whether the items were encoded in an emotionally arousing or an emotionally neutral study context. These prior studies employed only a relatively brief (ca. 10 min) retention interval, however. The present study contrasted the ERP correlates of incidental emotional retrieval as a function of study-test delay. Pictures of emotionally neutral objects were encoded in association with either emotionally negative or emotionally neutral scenes. In a repeated measures design (N=19), half of the objects were subjected to a recognition memory test 10 min after completion of the study phase, whereas the remainder were tested 24 h later. After the short delay, ERPs elicited by objects paired with emotional vs. neutral backgrounds differed from around 200 ms post-stimulus, the objects paired with the emotional scenes eliciting the more positive-going waveforms. After 24 h, differences between the ERPs elicited by the two classes of object were still apparent from around 200 ms post-stimulus. Strikingly, these effects differed from those obtained 10 min after study in both their polarity and scalp distribution. The early onset of these ERP effects suggests that they may reflect a form of memory independent of the conscious recollection of the associated study contexts. The qualitative differences in the effects at the two retention intervals raise the possibility that the encoded objects were subjected to consolidation processes that differed according to the emotional attributes of their study contexts.

The relationship between aging, performance, and the neural correlates of successful memory encoding

The present functional magnetic resonance imaging (fMRI) study investigated whether age-related differences in the neural correlates of successful memory encoding are modulated by memory performance. Young (mean age 22 years; N = 16) and older (mean age 69 years; N = 32) subjects were scanned while making animacy decisions on visually presented words. Memory for the words was later assessed in a recognition test, allowing fMRI activity elicited by study words to be contrasted according to subsequent memory performance. Young and older adults exhibited equivalent subsequent memory effects (enhanced activity for later remembered items) in an extensive network that included left inferior prefrontal cortex and anterior hippocampus. In posterior cingulate cortex, reversed subsequent memory effects (greater activity for later forgotten items) were of greater magnitude in young subjects. A voxel-of-interest analysis conducted on left and right prefrontal subsequent memory effects revealed that the effects were distributed more bilaterally in older than in young subjects, replicating previous findings. This age-related difference was confined to older subjects with relatively poor recognition performance, who were also the only group to demonstrate statistically significant right prefrontal subsequent memory effects. The findings suggest that relative preservation of memory performance with increasing age does not depend upon right prefrontal "over-recruitment."

Effects of age on the neural correlates of retrieval cue processing are modulated by task demands

The electrophysiological correlates of retrieval orientation--the differential processing of retrieval cues according to the nature of the sought-for information--were investigated in healthy young (18-20 years old) and older (63-77 years old) adults. In one pair of study-test cycles, subjects studied either words or pictures presented in one of two visually distinct contexts, and then performed a yes/no recognition task with words as test items. In another pair of study-test cycles, subjects again made recognition judgments, but were required, in addition, to signal the study context for each item judged "old." Young subjects' event-related potentials (ERPs) for new (unstudied) test items were more negative-going when the study material was pictures rather than words, and this effect varied little between the two retrieval tasks. Replicating a previous report [Morcom, A. M., & Rugg, M. D. Effects of age on retrieval cue processing as revealed by ERPs. Neuropsychologia, 42, 1525-1542, 2004], the effects of study material on the ERPs of the older subjects were attenuated and statistically nonsignificant in the recognition task. In the source retrieval task, however, material effects in the older group were comparable in both onset latency and magnitude with those of the young subjects. Thus, the failure of older adults to demonstrate differential cue processing in tests of recognition memory likely reflects the adoption of a specific retrieval strategy rather than the incapacity to process retrieval cues in a goal-directed manner.

Multiple repetitions reveal functionally and anatomically distinct patterns of hippocampal activity during continuous recognition memory

We used a continuous recognition procedure that included multiple presentations of test items, along with high-resolution functional magnetic resonance imaging (fMRI), to investigate the relationship between item novelty and recognition-related activity in the medial temporal lobe (MTL). In several regions of hippocampus and parahippocampal cortex, activity elicited by new items exceeded that for old items, whereas no MTL regions exhibited greater activity for old items. Critically, anatomically distinct regions of MTL were engaged by item novelty in two different ways, as evidenced by statistically dissociable profiles of activity. In bilateral medial hippocampus and left posterior parahippocampal cortex, activity followed a categorical profile in which it was greater for new than old items but did not differ further with additional presentations of old items. By contrast, effects in adjacent regions of right lateral hippocampus and left parahippocampal cortex were graded, whereby activity declined linearly with respect to each successive item presentation. These findings suggest that the relationship between hippocampal (and parahippocampal) activity and continuous psychological dimensions, such as item novelty, cannot be captured by a unitary function.

Neural correlates of successful encoding of semantically and phonologically mediated inter-item associations

This experiment investigated whether the neural correlates of inter-item associative encoding vary according to study task. At study, pairs of unrelated words were subjected to either semantic or phonological relational judgments. Test items comprised studied word pairs (intact), pairs comprised of words belonging to different study pairs (rearranged), and novel pairs. The test requirement was to discriminate between these different classes of test item. fMRI was employed to contrast the neural activity elicited by studied pairs that were correctly endorsed as intact on the later associative recognition test, as opposed to pairs for which associative information was unavailable. In contrast to prior findings for the encoding of single items, there was no evidence that the loci of subsequent associative memory effects varied according to study task. Instead, in both tasks, pairs that were later correctly endorsed as intact elicited enhanced activity in mid- and ventral regions of the left ventrolateral prefrontal cortex (VLPFC). These findings were accompanied by extensive task-invariant reversed subsequent memory effects in medial and lateral parietal and frontal cortices. The findings suggest that the left VLPFC may play a domain-general role in the encoding of item-item associations, and in addition highlight the importance of elucidating the functional significance of reversed subsequent memory effects.

Effects of study task on the neural correlates of source encoding

The present study investigated whether the neural correlates of source memory vary according to study task. Subjects studied visually presented words in one of two background contexts. In each test, subjects made old/new recognition and source memory judgments. In one study test cycle, study words were subjected to animacy judgments, whereas in another cycle the study task required syllable judgments. Functional magnetic resonance imaging (fMRI) was employed to contrast the neural activity elicited by study words that attracted accurate source judgments on the later memory test, as opposed to words for which source judgments were incorrect or for which source information was unavailable. In both tasks, relative to words for which source memory failed, study words that were later assigned to the correct source elicited enhanced activity in ventral extrastriate cortex. In addition to these common effects of subsequent source memory, additional effects were observed that were selective for each study task. The present findings add weight to the proposal that neural activity supporting successful episodic memory encoding is a reflection of both the online processing engaged by an episode as it is experienced, and the demands imposed by the later retrieval task.

Fractionation of the component processes underlying successful episodic encoding: a combined fMRI and divided-attention study

Considerable evidence suggests that attentional resources are necessary for the encoding of episodic memories, but the nature of the relationship between attention and neural correlates of encoding is unclear. Here we address this question using functional magnetic resonance imaging and a divided-attention paradigm in which competition for different types of attentional resources was manipulated. Fifteen volunteers were scanned while making animacy judgments to visually presented words and concurrently performing one of three tasks on auditorily presented words: male/female voice discrimination (control task), 1-back voice comparison (1-back task), or indoor/outdoor judgment (semantic task). The 1-back and semantic tasks were designed to compete for task-generic and task-specific attentional resources, respectively. Using the "remember/know" procedure, memory for the study words was assessed after 15 min. In the control condition, subsequent memory effects associated with later recollection were identified in the left dorsal inferior frontal gyrus and in the left hippocampus. These effects were differentially attenuated in the two more difficult divided-attention conditions. The effects of divided attention seem, therefore, to reflect impairments due to limitations at both task-generic and task-specific levels. Additionally, each of the two more difficult divided-attention conditions was associated with subsequent memory effects in regions distinct from those showing effects in the control condition. These findings suggest the engagement of alternative encoding processes to those engaged in the control task. The overall pattern of findings suggests that divided attention can impact later memory in different ways, and accordingly, that different attentional resources, including task-generic and task-specific resources, make distinct contributions to successful episodic encoding.

Memory retrieval and the parietal cortex: a review of evidence from a dual-process perspective

Although regions of the parietal cortex have been consistently implicated in episodic memory retrieval, the functional roles of these regions remain poorly understood. The present review presents a meta-analysis of findings from event-related fMRI studies reporting the loci of retrieval effects associated with familiarity- and recollection-related recognition judgments. The results of this analysis support previous suggestions that retrieval-related activity in lateral parietal cortex dissociates between superior regions, where activity likely reflects the task relevance of different classes of recognition test items, and more inferior regions where retrieval-related activity appears closely linked to successful recollection. It is proposed that inferior lateral parietal cortex forms part of a neural network supporting the 'episodic buffer' [Baddeley, A. D. (2000). The episodic buffer: A new component of working memory? Trends in Cognitive Sciences, 4, 417-423].