Ventrolateral prefrontal cortex and self-initiated semantic elaboration during memory retrieval

Underdeveloped recollection during adolescence: Semantic elaboration and inhibition as underlying mechanisms

Recognition memory abilities undergo important developmental changes until adulthood, with earlier studies showing different trajectories for recollection and familiarity-based processes. However, previous work has primarily focused on childhood, and differences in memory retrieval, notably in recollection, between adolescents and adults, have been hard to confirm. To address this gap in the literature and to better characterize the development of recollection and familiarity during adolescence, we applied the process dissociation procedure to a word recognition memory task, after semantic and perceptual encoding of words, in adolescents (n = 30, 13-15 years of age) and young adults (n = 30, 20-22 years). Relative to young adults, adolescents' lower recognition memory performance was restricted to context recollection of semantically encoded items. This effect was predicted by individual differences in inhibitory control abilities. These findings highlight the distinct developmental trajectories of familiarity and context recollection over the course of adolescence, and suggest that semantic elaboration and inhibition are two key mechanisms toward the full maturation of recollection processes.

Examining the effects of transcranial direct current stimulation on human episodic memory with machine learning

We aimed to replicate a published effect of transcranial direct-current stimulation (tDCS)-induced recognition enhancement over the human ventrolateral prefrontal cortex (VLPFC) and analyse the data with machine learning. We investigated effects over an adjacent region, the dorsolateral prefrontal cortex (DLPFC). In total, we analyzed data from 97 participants after exclusions. We found weak or absent effects over the VLPFC and DLPFC. We conducted machine learning studies to examine the effects of semantic and phonetic features on memorization, which revealed no effect of VLPFC tDCS on the original dataset or the current data. The highest contributing factor to memory performance was individual differences in memory not explained by word features, tDCS group, or sample size, while semantic, phonetic, and orthographic word characteristics did not contribute significantly. To our knowledge, this is the first tDCS study to investigate cognitive effects with machine learning, and future studies may benefit from studying physiological as well as cognitive effects with data-driven approaches and computational models.

Orienting Attention to Short-Term Memory Representations via Sensory Modality and Semantic Category Retro-Cues

There is growing interest in characterizing the neural mechanisms underlying the interactions between attention and memory. Current theories posit that reflective attention to memory representations generally involves a fronto-parietal attentional control network. The present study aimed to test this idea by manipulating how a particular short-term memory (STM) representation is accessed, that is, based on its input sensory modality or semantic category, during functional magnetic resonance imaging (fMRI). Human participants performed a novel variant of the retro-cue paradigm, in which they were presented with both auditory and visual non-verbal stimuli followed by Modality, Semantic, or Uninformative retro-cues. Modality and, to a lesser extent, Semantic retro-cues facilitated response time relative to Uninformative retro-cues. The univariate and multivariate pattern analyses (MVPAs) of fMRI time-series revealed three key findings. First, the posterior parietal cortex (PPC), including portions of the intraparietal sulcus (IPS) and ventral angular gyrus (AG), had activation patterns that spatially overlapped for both modality-based and semantic-based reflective attention. Second, considering both the univariate and multivariate analyses, Semantic retro-cues were associated with a left-lateralized fronto-parietal network. Finally, the experimental design enabled us to examine how dividing attention cross-modally within STM modulates the brain regions involved in reflective attention. This analysis revealed that univariate activation within bilateral portions of the PPC increased when participants simultaneously attended both auditory and visual memory representations. Therefore, prefrontal and parietal regions are flexibly recruited during reflective attention, depending on the representational feature used to selectively access STM representations.

Divided attention and the encoding effects of retrieval

Retrieving from memory both reveals as well as modifies memory. It is important to understand how these encoding effects of retrieval differ from other forms of encoding. One possible difference relates to attention: divided attention is well known to disrupt memory encoding but typically has much less impact on memory retrieval. However, less is known about the relative attentional demands of the encoding consequences of retrieval. The current experiments examined retrieval-based encoding using free recall, a retrieval task purported to require substantial attentional resources. In three experiments, participants studied common category exemplars (Phase 1), restudied or freely recalled the exemplars (Phase 2), and then took a final free-recall test (Phase 3). Phase 2 occurred under full attention (FA) or divided attention (DA). In all three experiments, the negative effect of DA on final recall was significant in the restudy but not retrieval condition. The pattern persisted with short (Experiment 1) or long study lists (Experiment 2), requiring lesser or greater retrieval effort, and with multiple Phase 2 tests, permitting the development of more elaborate retrieval strategies (Experiment 3). The encoding effects of retrieval appear resilient to distraction, even using a memory task that is more effortful and easily disrupted by DA (i.e., free recall). In addition, these results are inconsistent with elaboration and effort accounts of retrieval-based learning.

Temporal Proximity Promotes Integration of Overlapping Events

Events with overlapping elements can be encoded as two separate representations or linked into an integrated representation, yet we know little about the conditions that promote one form of representation over the other. Here, we tested the hypothesis that the proximity of overlapping events would increase the probability of integration. Participants first established memories for house-object and face-object pairs; half of the pairs were learned 24 hr before an fMRI session, and the other half 30 min before the session. During scanning, participants encoded object-object pairs that overlapped with the initial pairs acquired on the same or prior day. Participants were also scanned as they made inference judgments about the relationships among overlapping pairs learned on the same or different day. Participants were more accurate and faster when inferring relationships among memories learned on the same day relative to those acquired across days, suggesting that temporal proximity promotes integration. Evidence for reactivation of existing memories-as measured by a visual content classifier-was equivalent during encoding of overlapping pairs from the two temporal conditions. In contrast, evidence for integration-as measured by a mnemonic strategy classifier from an independent study [Richter, F. R., Chanales, A. J. H., & Kuhl, B. A. Predicting the integration of overlapping memories by decoding mnemonic processing states during learning. Neuroimage, 124, 323-335, 2016]-was greater for same-day overlapping events, paralleling the behavioral results. During inference itself, activation patterns further differentiated when participants were making inferences about events acquired on the same day versus across days. These findings indicate that temporal proximity of events promotes integration and further influences the neural mechanisms engaged during inference.

Framing memories: How the retrieval query format shapes the neural bases of remembering

The way memory questions are framed influences the information that is searched, retrieved, and monitored during remembering. This fMRI study aimed at clarifying how the format of the retrieval query shapes the neural basis of source recollection. During encoding, participants made semantic (pleasantness) or perceptual (number of letters) judgments about words. Subsequently, in a source memory test, the retrieval query was manipulated such that for half of the items from each encoding task, the retrieval query emphasized the semantic source (i.e., semantic query format: "Is this word from the pleasantness task?"), whereas for the other half the retrieval query emphasized the alternate, perceptual source (i.e., perceptual query format: "Is this word from the letter task?"). The results showed that the semantic query format was associated with higher source recognition than the perceptual query format. This behavioral advantage was accompanied by increased activation in several regions associated to controlled semantic elaboration and monitoring of internally-generated features about the past event. In particular, for items semantically encoded, the semantic query, relative to the perceptual query, induced activation in medial prefrontal cortex (PFC), hippocampal, parahippocampal and middle temporal cortex. Conversely, for items perceptually encoded, the semantic query recruited the lateral PFC and occipital-fusiform areas. Interestingly, the semantic format also influenced the processing of new items, eliciting greater L lateral and medial PFC activation. In contrast, the perceptual query format (versus the semantic format) only prompted greater activation in R orbitofrontal cortex and the R inferior parietal lobe, for items encoded in a perceptual manner and for new items, respectively. The results highlight the role of the retrieval query format in source remembering, showing that the retrieval query that emphasizes the semantic source promotes the use of semantic strategies via medial and L lateral PFC activations. These frontal activations are accompanied by differential recruitment of more posterior regions, depending on the type of information that had been encoded.

A parietal memory network revealed by multiple MRI methods

The manner by which the human brain learns and recognizes stimuli is a matter of ongoing investigation. Through examination of meta-analyses of task-based functional MRI and resting state functional connectivity MRI, we identified a novel network strongly related to learning and memory. Activity within this network at encoding predicts subsequent item memory, and at retrieval differs for recognized and unrecognized items. The direction of activity flips as a function of recent history: from deactivation for novel stimuli to activation for stimuli that are familiar due to recent exposure. We term this network the 'parietal memory network' (PMN) to reflect its broad involvement in human memory processing. We provide a preliminary framework for understanding the key functional properties of the network.

Inefficient Encoding as an Explanation for Age-Related Deficits in Recollection-Based Processing

A cardinal feature of aging is a decline in episodic memory (EM). Nevertheless, there is evidence that some older adults may be able to “compensate” for failures in recollection-based processing by recruiting brain regions and cognitive processes not normally recruited by the young. We review the evidence suggesting that age-related declines in EM performance and recollection-related brain activity (left-parietal EM effect; LPEM) are due to altered processing at encoding. We describe results from our laboratory on differences in encoding- and retrieval-related activity between young and older adults. We then show that, relative to the young, in older adults brain activity at encoding is reduced over a brain region believed to be crucial for successful semantic elaboration in a 400–1,400-ms interval (left inferior prefrontal cortex, LIPFC; Johnson, Nessler, & Friedman, 2013 ; Nessler, Friedman, Johnson, & Bersick, 2007 ; Nessler, Johnson, Bersick, & Friedman, 2006 ). This reduced brain activity is associated with diminished subsequent recognition-memory performance and the LPEM at retrieval. We provide evidence for this premise by demonstrating that disrupting encoding-related processes during this 400–1,400-ms interval in young adults affords causal support for the hypothesis that the reduction over LIPFC during encoding produces the hallmarks of an age-related EM deficit: normal semantic retrieval at encoding, reduced subsequent episodic recognition accuracy, free recall, and the LPEM. Finally, we show that the reduced LPEM in young adults is associated with “additional” brain activity over similar brain areas as those activated when older adults show deficient retrieval. Hence, rather than supporting the compensation hypothesis, these data are more consistent with the scaffolding hypothesis, in which the recruitment of additional cognitive processes is an adaptive response across the life span in the face of momentary increases in task demand due to poorly-encoded episodic memories.

Recognizing an Object from the Sum of Its Parts: An Intracranial Study on Alpha Rhythms

Little is known about the relation of alpha rhythms and object recognition. Alpha has been generally proposed to be associated with attention and memory and to be particularly important for the mediation of long-distance communication between neuronal populations. However, how these apply to object recognition is still unclear. This study aimed at describing the spatiotemporal dynamics of alpha rhythms while recognizing fragmented images of objects presented for the first time and presented again 24 hr later. Intracranial electroencephalography was performed in six epileptic patients undergoing presurgical evaluation. Time–frequency analysis revealed a strong alpha activity, mainly of the evoked type, propagating from posterior cerebral areas to anterior regions, which was similar whether the objects were recognized or not. Phase coherence analysis, however, showed clear phase synchronization specific for the moment of recognition. Twenty-four hr later, frontal regions displayed stronger alpha activity and more distributed phase synchronization than when images were presented for the first time. In conclusion, alpha amplitude seems to be related to nonspecific mechanism. Phase coherence analysis suggests a communicational role of alpha activity in object recognition, which may be important for the comparison between bottom–up representations and memory templates.

Different brain activity in response to emotional faces alone and augmented by contextual information

This study examined the extent to which emotional face stimuli differ from the neural reactivity associated with more ecological contextually augmented stimuli. Participants were scanned when they viewed contextually rich pictures depicting both emotional faces and context, and pictures of emotional faces presented alone. Emotional faces alone were more strongly associated with brain activity in paralimbic and social information processing regions, whereas emotional faces augmented by context were associated with increased and sustained activity in regions potentially representing increased complexity and subjective emotional experience. Furthermore, context effects were modulated by emotional intensity and valence. These findings suggest that cortical elaboration that is apparent in contextually augmented stimuli may be missed in studies of emotional faces alone, whereas emotional faces may more selectively recruit limbic reactivity.

Interplay of hippocampus and prefrontal cortex in memory

Recent studies on the hippocampus and the prefrontal cortex have considerably advanced our understanding of the distinct roles of these brain areas in the encoding and retrieval of memories, and of how they interact in the prolonged process by which new memories are consolidated into our permanent storehouse of knowledge. These studies have led to a new model of how the hippocampus forms and replays memories and how the prefrontal cortex engages representations of the meaningful contexts in which related memories occur, as well as how these areas interact during memory retrieval. Furthermore, they have provided new insights into how interactions between the hippocampus and prefrontal cortex support the assimilation of new memories into pre-existing networks of knowledge, called schemas, and how schemas are modified in this process as the foundation of memory consolidation.

Age-related changes in prefrontal and hippocampal contributions to relational encoding

Age-related declines in relational encoding are well documented. It remains unclear, however, whether such declines reflect dysfunction of (1) ventrolateral prefrontal cortex (VLPFC) and deficient generation of associations; and/or (2) hippocampal dysfunction and impoverished binding of associations. In order to separate VLPFC and hippocampal contributions to relational encoding, we manipulated the generative demands of the encoding task by varying the number of semantic associations between the to-be-encoded information (three words). Thus, trials with fewer semantic associations (lower-association trials) require more generative processing during encoding, relative to trials in which more semantic associations are provided for binding (higher-association trials). Parametric modulation analyses on successfully encoded items revealed that, unlike younger adults, older adults did not show an up-regulation of VLPFC activity during lower-association trials. In contrast, hippocampal activity in both older and younger adults was greater in higher- relative to lower-association trials. Moreover, recognition accuracy improved significantly in both groups with the provision of more semantic associations, indicating that both younger and older adults benefitted from this form of encoding support. Our findings suggest that left VLPFC dysfunction may underlie relational encoding deficits in older adults, but that when provided with associations to bind, hippocampal activity in older adults is comparable to young, consistent with their increased recognition accuracy under conditions of encoding support.

The cognitive aging of episodic memory: a view based on the event-related brain potential

A cardinal feature of older-adult cognition is a decline, relative to the young, in the encoding and retrieval of personally relevant events, i.e., episodic memory (EM). A consensus holds that familiarity, a relatively automatic feeling of knowing that can support recognition-memory judgments, is preserved with aging. By contrast, recollection, which requires the effortful, strategic recovery of contextual detail, declines as we age. Over the last decade, event-related brain potential (ERPs) have become increasingly important tools in the study of the aging of EM, because a few, well-researched EM effects have been associated with the cognitive processes thought to underlie successful EM performance. EM effects are operationalized by subtracting the ERPs elicited by correctly rejected, new items from those to correctly recognized, old items. Although highly controversial, the mid-frontal effect (a positive component between ∼300 and 500 ms, maximal at fronto-central scalp sites) is thought to reflect familiarity-based recognition. A positivity between ∼500 and 800 ms, maximal at left-parietal scalp, has been labeled the left-parietal EM effect. A wealth of evidence suggests that this brain activity reflects recollection-based retrieval. Here, I review the ERP evidence in support of the hypothesis that familiarity is maintained while recollection is compromised in older relative to young adults. I consider the possibility that the inconsistency in findings may be due to individual differences in performance, executive function, and quality of life indices, such as socio-economic status.

Performance on the Green Word Memory Test following Operation Enduring Freedom/Operation Iraqi Freedom-era military service: Test failure is related to evaluation context

This study investigates prior reports of high neuropsychological symptom validity test (SVT) failure rates in post-deployed Operation Enduring Freedom/Operation Iraqi Freedom (OEF/OIF) active and veteran military personnel, using a large, multi-site sample (N = 214) drawn from three levels of the Department of Defense/Department of Veterans Affairs (VA) Polytrauma System of Care. The sample failure rate and its relationship to research versus dual research/clinical context of evaluation were examined, in addition to secondary variables explored in prior studies. Results yielded an overall failure rate of 25%, lower than prior reports describing OEF/OIF active-duty and veteran military personnel. Findings also supported the hypothesis that SVT failure rates would differ by context (dual > research). Participants with traumatic brain injury (TBI) failed more frequently than those without TBI in the dual context but not in the research context. Secondary analyses revealed that failure rates increased in the presence of depression, posttraumatic stress disorder, and male sex but were unrelated to active versus veteran military status, service connection (SC) or percentage of SC, age, education, or ethnicity. Further research is required to elucidate the underpinnings of these findings in light of the limited literature and variability between OEF/OIF-related SVT studies, as well as the substantial diagnostic and treatment implications for VA.

Temporally specific divided attention tasks in young adults reveal the temporal dynamics of episodic encoding failures in elderly adults

Nessler, Johnson, Bersick, and Friedman (D. Nessler, R. Johnson, Jr., M. Bersick, & D. Friedman, 2006, On why the elderly have normal semantic retrieval but deficient episodic encoding: A study of left inferior frontal ERP activity, NeuroImage, Vol. 30, pp. 299-312) found that, compared with young adults, older adults show decreased event-related brain potential (ERP) activity over posterior left inferior prefrontal cortex (pLIPFC) in a 400- to 1,400-ms interval during episodic encoding. This altered brain activity was associated with significantly decreased recognition performance and reduced recollection-related brain activity at retrieval (D. Nessler, D. Friedman, R. Johnson, Jr., & M. Bersick, 2007, Does repetition engender the same retrieval processes in young and older adults? NeuroReport, Vol. 18, pp. 1837-1840). To test the hypothesis that older adults' well-documented episodic retrieval deficit is related to reduced pLIPFC activity at encoding, we used a novel divided attention task in healthy young adults that was specifically timed to disrupt encoding in either the 1st or 2nd half of a 300- to 1,400-ms interval. The results showed that diverting resources for 550 ms during either half of this interval reproduced the 4 characteristic aspects of the older participants' retrieval performance: normal semantic retrieval during encoding, reduced subsequent episodic recognition and recall, reduced recollection-related ERP activity, and the presence of "compensatory" brain activity. We conclude that part of older adults' episodic memory deficit is attributable to altered pLIPFC activity during encoding due to reduced levels of available processing resources. Moreover, the findings also provide insights into the nature and timing of the putative "compensatory" processes posited to be used by older adults in an attempt to compensate for age-related decline in cognitive function. These results support the scaffolding account of compensation, in which the recruitment of additional cognitive processes is an adaptive response across the life span.

Induced gamma-band response to fragmented images: An intracranial EEG study

Induced gamma-band response (iGBR) has been linked to coherent perception of images and is thought to represent the synchronisation of neuronal populations mediating binding of elements composing the image and the comparisons with memory for proper recognition. This study uses fragmented images with intracranial electroencephalography to investigate the precise spatio-temporal dynamic of iGBR elicited by the recognition of objects presented for the first time and 24h later. Results show an increased iGBR at recognition in regions involved in bottom-up processes such as the cuneus and the lateral occipital complex. Top-down facilitation involved the lingual gyrus, the precuneus and the superior parietal lobule when images were presented for the first time. Twenty-four hours later, top-down facilitation was mediated by frontal areas involved in retrieval from episodic memory. This study showed that the classically reported iGBR is related to object recognition and that top-down processes vary according to task demand.

Relational and Item-Specific Encoding (RISE): task development and psychometric characteristics

OBJECTIVES

The Relational and Item-Specific Encoding task (RISE) was designed to assess contributions of specific encoding and retrieval processes to episodic memory in schizophrenia. This manuscript describes how a cognitive neuroscience functional imaging paradigm was translated for clinical research.

METHODS

The RISE manipulates encoding by requiring participants to decide whether stimuli are "living/nonliving" (item-specific) or whether one stimulus fits inside the other (relational) and estimates familiarity (F) and recollection (R) by examining receiver operator characteristics (ROC) and assessing item and associative recognition. Two studies examined psychometric characteristics and tested the hypothesis that patients have differential deficits in relational vs item-specific encoding and disproportionate impairments in recollection vs familiarity.

RESULTS

Study 1, using visual objects, provided support for the encoding hypotheses and revealed good internal consistency and alternate forms reliability, with small differences between test forms. ROC analysis revealed R and F deficits, with F deficits most prominent following relational encoding. Study 2 used word stimuli, which lowered item recognition, but patients had difficulty understanding task demands, and words were less desirable for non-English speaking clinical trials, leading to the decision to proceed with the original task.

CONCLUSIONS

The RISE is a valid and reliable measure of item-specific and relational memory that is well tolerated, with good psychometric characteristics and equivalent forms to facilitate treatment studies. Results indicate that episodic memory in schizophrenia is most preserved under conditions promoting item-specific encoding that is supported by familiarity-based recognition and is most impaired under relational encoding and recollection-based retrieval conditions.

Differential neural activity in the recognition of old versus new events: an activation likelihood estimation meta-analysis

This study presents a meta-analysis comparing hit and correct rejection (CR) conditions across 48 fMRI studies. Old/new (hit > CR) effects associated most consistently with (1) components of the default-mode network, including the left angular gyrus, bilateral precuneus, and bilateral posterior cingulate regions, which may support the mental re-experiencing of an old event, or ecphory; (2) components of the cognitive-control network, involving the left dorsolateral and dorsomedial prefrontal cortex and bilateral intraparietal sulcus regions, which may mediate memory and non-memory control functions; and (3) the caudate nucleus, a key part of the brain's reward system that may support the satisfaction tied to target-detection. Direct comparisons of old/new effects between item versus source retrieval and "remember" versus "know" retrieval yielded three main sets of findings. First, default-mode network regions showed greater old/new effects in conditions associated with richer ecphoric processing. Second, cognitive-control network regions showed greater old/new effects in conditions associated with a greater demand for strategic-retrieval processing. Third, the caudate nucleus showed greater old/new effects in conditions tied to greater confidence in target-detection. New/old (CR > hit) effects most strongly associated with the bilateral medial temporal lobe, possibly reflecting greater encoding-related activity for new than for old items, and the right posterior middle temporal regions, possibly reflecting repetition-related neural priming for old items. In conclusion, neural activity distinguishing old from new events comprises an ensemble of multiple memory-specific activities, including encoding, retrieval, and priming, as well as multiple types of more general cognitive activities, including default-mode, cognitive-control, and reward processing.

The role of left ventrolateral prefrontal cortex during episodic decisions: semantic elaboration or resolution of episodic interference?

Context memory retrieval tasks often implicate the left ventrolateral pFC (LVPFC) during functional imaging. Although this region has been linked to controlled semantic processing of materials, it may also play a more general role in selecting among competing episodic representations during demanding retrieval tasks. Thus, the LVPFC response during context memory retrieval may reflect either semantic processing of memoranda or adjudication of interfering episodic memories evoked by memoranda. To distinguish between these hypotheses, we contrasted context and item memory retrieval tasks for meaningful and nonmeaningful memoranda using fMRI. Increased LVPFC activation during context compared with item memory only occurred for meaningful memory probes. In contrast, even demanding context retrieval for nonmeaningful materials failed to engage LVPFC. These data demonstrate that the activation previously seen during episodic tasks likely reflects semantic processing of the probes during episodic retrieval attempt, not the selection among competing elicited episodic representations. Posterior middle temporal gyrus and the body/head of the caudate demonstrated the same selective response as LVPFC, although resting state functional connectivity analyses suggested that these two regions likely shared separate functional relationships with the LVPFC.

The hierarchical organization of semantic memory: executive function in the processing of superordinate concepts

Research on the processing of objects at different hierarchical levels has suggested that understanding superordinate concepts (e.g. fruit), relative to basic level concepts (e.g. apple), requires greater semantic control demands. Yet, it is unclear which factors underlie this difference in executive processing. We built on previous research showing that superordinate concepts have less shared features among their members and therefore may involve higher semantic control requirements. To test this hypothesis, we developed an fMRI study in which we orthogonally manipulated feature sharedness (more shared vs. less shared) and concept level (superordinate vs. basic) in a sentence verification task. Sentences involving less shared features, relative to more shared features, significantly engaged the L lateral PFC. Importantly, sentences that included superordinate concepts, relative to those with basic level concepts, also revealed a stronger response in L lateral PFC, along with posterior temporal gyrus activation. There was also a significant interaction between feature sharedness and concept level in several PFC regions and L posterior temporal areas. The results suggest that relative to basic level concepts, processing superordinate concepts requires extra semantic control in L lateral PFC to coordinate information that is less shared by other members of the category level. These findings demonstrate that feature sharedness impacts the neural basis of semantic knowledge, and is a critical dimension in the processing of superordinate concepts.

Neural mechanisms of semantic interference and false recognition in short-term memory

Decades of research using the Deese-Roediger-McDermott (DRM) paradigm have demonstrated that episodic memory is vulnerable to semantic distortion, and neuroimaging investigations of this phenomenon have shown dissociations between the neural mechanisms subserving true and false retrieval from long-term memory. Recently, false short-term memories have also been demonstrated, with false recognition of items related in meaning to memoranda encoded less than 5s earlier. Semantic interference is also evident in short-term memory, such that correct rejection of related lures is slowed relative to correct rejection of unrelated lures. The present research constitutes the first fMRI investigation of false recognition and semantic interference in short-term memory using a short-term DRM paradigm in which participants retained 4 semantic associates over a short 4-s filled retention interval. Results showed increased activation in the left mid-ventrolateral prefrontal cortex (BA45) associated with semantic interference, and significant correlations between these increases and behavioral measures of interference across subjects. Furthermore, increases in dorsolateral PFC occurred when related lures were correctly rejected versus falsely remembered. Compared with false recognition, true recognition was associated with increases in left fusiform gyrus, a finding consistent with the notion that increased perceptual processing may distinguish true from false recognition over both short and long retention intervals. Findings are discussed in relation to current models of interference resolution in short-term memory, and suggest that false short-term recognition occurs as a consequence of the failure of frontally mediated cognitive control processes which adjudicate semantic familiarity in support of accurate mnemonic retrieval.

The role of the dorsolateral prefrontal cortex in retrieval from long-term memory depends on strategies: a repetitive transcranial magnetic stimulation study

The ability to associate a name to a face is a crucially relevant task in daily life. In this study, we investigated the neuronal basis of face-name retrieval in young subjects using repetitive transcranial magnetic stimulation (rTMS) over the left or right dorsolateral prefrontal cortex (DLPFC). The experimental task was composed of two study phases: an encoding phase and a retrieval phase. During the encoding phase, subjects saw a face (familiar or unfamiliar) followed by a name. During the retrieval phase, they saw the face together with two names and had to choose the name that was correctly associated with the face. rTMS was delivered only during retrieval. In addition, we evaluated the use of memory strategies during the task. Accordingly, subjects were subdivided into two groups: strategy users (SU) and no-strategy users (NSU). No rTMS effects were present for familiar face-name pairs, probably due to a ceiling effect. However, for unfamiliar face-name pairs, the different use of memory strategies resulted in different rTMS effects. The SU group showed a selective interference effect after right DLPFC stimulation, whereas the NSU group showed an effect after left DLPFC stimulation. Importantly, the overall performance of the two groups was comparable. We suggest that during memory retrieval the left DLPFC might be recruited when the subject does not apply deliberately a retrieval strategy whereas there is a shift to the right DLPFC if cognitive control processes that are engaged by strategies are needed to guide episodic retrieval.

The effects of valence and arousal on the neural activity leading to subsequent memory

This study examined how valence and arousal affect the processes linked to subsequent memory for emotional information. While undergoing an fMRI scan, participants viewed neutral pictures and emotional pictures varying by valence and arousal. After the scan, participants performed a recognition test. Subsequent memory for negative or high arousal information was associated with occipital and temporal activity, whereas memory for positive or low arousal information was associated with frontal activity. Regression analyses confirmed that for negative or high arousal items, temporal lobe activity was the strongest predictor of later memory whereas for positive or low arousal items, frontal activity corresponded most strongly with later memory. These results suggest that the types of encoding processes relating to memory (e.g., sensory vs. elaborative processing) can differ based on the affective qualities of emotional information.