The dorsolateral prefrontal cortex plays a role in self-initiated elaborative cognitive processing during episodic memory encoding: rTMS evidence

Transcranial magnetic stimulation combined with endogenous human hippocampal and motor cortical activity enhances memory

The hippocampus is a fundamental cortical structure in the memory process of encoding, retaining, and recalling information. Transcranial Magnetic Stimulation (TMS) following a Paired Associative Stimulation (PAS) enhances nervous system excitability and promotes cortical plasticity mechanisms by synchronizing two stimuli in the same neural pathway. However, PAS has not been shown to improve memorization capacity yet. Here, we present an innovative protocol stemming from the PAS paradigm, which combines single-pulse TMS to the hippocampus with endogenous hippocampal activity during a working memory (WM) task. 96 volunteers were randomized across one experimental group and three parallel groups (motor cortex stimulation, sham stimulation, and no stimulation) in a single session. This combined-stimuli configuration resulted in an increased memorization capacity in the WM task, which was dependent on the stimulated brain location and subjects' basal memory performance. These results are potentially significant for clinical research on memory dysfunction and its related neurological disorders. Future research on paired associative or combined stimulation is required to unveil stimulation-derived neural mechanisms that enhance the ability to memorize.

Does Multisession Cathodal Transcranial Direct Current Stimulation of the Left Dorsolateral Prefrontal Cortex Prime the Effects of Cognitive Behavioral Therapy on Fear of Pain, Fear of Movement, and Disability in Patients with Nonspecific Low Back Pain? A Randomized Clinical Trial Study

Many studies have shown that low back pain (LBP) is associated with psychosomatic symptoms which may lead to brain changes. This study aimed to investigate the effect of the concurrent application of cognitive behavioral therapy (CBT) and transcranial direct electrical stimulation (tDCS) over the left dorsolateral prefrontal cortex (DLPFC) on fear of pain, fear of movement, and disability in patients with nonspecific LBP. This study was performed on 45 LBP patients (23 women, 22 men; mean age 33.00 ± 1.77 years) in three groups: experimental (2 mA cathodal tDCS (c-tDCS)), sham (c-tDCS turned off after 30 s), and control (only received CBT). In all groups, CBT was conducted for 20 min per session, with two sessions per week for four weeks. Fear of pain, fear of movement, and disability were evaluated using questionnaires at baseline, immediately after, and one month after completion of interventions. Results indicated that all three different types of intervention could significantly reduce fear and disability immediately after intervention (p > 0.05). However, improvement in the experimental group was significantly higher than in the other groups immediately after and at the one-month follow-up after interventions (p < 0.05). DLPFC c-tDCS can prime the immediate effects of CBT and also the lasting effects on the reduction in the fear of pain, fear of movement, and disability in LBP patients.

Effect of repetitive transcranial magnetic stimulation combined with transcranial direct current stimulation on post-stroke dysmnesia: A preliminary study

OBJECTIVE

This study examined the effect of repetitive transcranial magnetic stimulation (rTMS) combined with transcranial direct current stimulation (tDCS) as a bimodal neuromodulatory approach for post-stroke dysmnesia.

METHODS

Thirty-four patients with post-stroke dysmnesia were randomly allocated into a sham group treated with neither rTMS nor tDCS, a group treated with rTMS, and a group treated with a combination of rTMS and tDCS. All three groups received cognitive rehabilitation training for 4 weeks. The memory function of each group before and after the intervention was assessed using the Montreal Cognitive Assessment (MoCA) and Rivermead Behavioral Memory Test (RBMT) scales, as well as in terms of the Mismatch Negativity（MMN）and P300 of event-related potentials.

RESULTS

The sham, rTMS, and rTMS-tDCS groups all showed improvement in the total MoCA score after the intervention. Delayed recall, a MoCA item, scored better in the rTMS-tDCS group than in the rTMS and sham groups. Delayed processing, an RBMT item, scored better in the rTMS-tDCS combination group than in the rTMS and sham groups. MMN and P300 latency was significantly shorter in the rTMS-tDCS combination group.

CONCLUSION

rTMS-tDCS bimodal stimulation was more effective than cognitive rehabilitation or rTMS alone in treating patients with post-stroke dysmnesia, offering new possibilities for enhancing cognitive function and treating post-stroke dysmnesia.

Transcranial Direct Current Stimulation Intervention in Alzheimer's Disease and Its Follow-Up

BACKGROUND

Alzheimer's disease (AD) stands as the prevailing type of dementia, marked by gradual memory loss and cognitive decline. Transcranial direct current stimulation (tDCS) is a non-invasive method used to regulate cortical brain function and has been explored as a potential treatment for cognitive impairment.

OBJECTIVE

This study aimed to compare the effects of daily home-based active or sham tDCS on cognitive function in patients with early-stage AD and its follow-up after one month.

METHODS

The study involved a randomized, blinded, and controlled-placebo design, with 18 participants enrolled. The primary outcome measures were general cognitive function, immediate, and delayed recall, and executive function. Participants included in the study were randomly assigned to the anodal and sham tDCS groups. Participants were assessed before and after the intervention and one month after the end of treatment. The home-based intervention was applied for 5 consecutive days, daily.

RESULTS

The results showed a significant interaction between the active and sham groups; in particular, improvements in MMSE scores, immediate memory and delayed recall were observed at one-month follow-up in the active group.

CONCLUSIONS

The positive effects of tDCS on cognitive function in AD patients observed suggest that tDCS may induce long-term neuroplastic changes, leading to sustained improvements in cognitive abilities.

The role of semantic clustering in the relationship between verbal memory and psychosocial functioning in schizophrenia and bipolar disorder: Possible distinct cognitive pathway compared to healthy controls

BACKGROUND

Verbal memory (VM) is impaired in schizophrenia (SZ) and bipolar disorder (BD), and predicts psychosocial functioning. However, there is a lack of research exploring the role of VM component processes, including semantic clustering, in these disorders. Semantic clustering might impact this association, as effective semantic memory strategies may reflect unimpaired executive control, leading to an adequate functioning. We aimed to investigate VM components in SZ and BD, and the role of semantic clustering in the relationship between VM and functioning.

METHODS

We included 495 participants (156 SZ, 172 BD, and 167 healthy controls (HC)) that underwent an assessment using the Hopkins Verbal Learning Test - Revised for VM and the Functioning Assessment Short Test for psychosocial functioning. We compared groups through ANOVAs and investigated the effect of semantic clustering in the relationship between VM total immediate free recall and functioning through linear regression models.

RESULTS

SZ had worse overall VM performance compared to BD, which performed worse than HCs. HCs used more semantic clustering than SZ and BD, but there were no differences between the two clinical groups. In HCs, semantic clustering impacted the relationship between VM performance and functioning, while no interaction was observed in SZ or BD.

LIMITATIONS

Cross-sectional design; no medication effects or other cognitive functions were assessed.

CONCLUSIONS

SZ and BD may use an alternative cognitive pathway in which the relationship between VM and functioning is independent of complex cognitive processes such as semantic clustering, supporting the cognitive remediation targeting of VM in these disorders.

Neural Correlates of Garment Fit and Purchase Intention in the Consumer Decision-Making Process and the Influence of Product Presentation

In today’s competitive e-commerce markets, it is crucial to promote product satisfaction and to quickly identify purchase intention in decision-making consumers. The present investigation examined the relationship between perceived garment fit and purchase intention, together with how product presentation methods (mannequin versus self-model) contribute to decision-making processes of clothing. Thirty-nine female volunteers were scanned using fMRI while performing an online shopping task. In Part 1, univariate analysis was conducted between garment fit and product presentation factors to assess their effects on purchase deliberation. In Part 2, univariate, multivariate pattern, and psychophysiological interaction analyses were carried out to examine the predictive ability of fit evaluation and product presentation on purchase intention. First, garment fit × product presentation interaction effects on purchase deliberation were observed in the frontopolar cortex, superior frontal gyrus, anterior cingulate cortex, and posterior cingulate cortex. Part 2 demonstrated neural signals of the dorsomedial prefrontal cortex, premotor cortex, supplementary motor area, superior parietal lobule, supramarginal gyrus, superior temporal sulcus, fusiform gyrus, and insula to distinguish subsequent purchase intentions. Overall, the findings denote directed exploration, visual and action processing as key neural processes in decision-making that uniquely reflect garment fit and product presentation type during purchase deliberation. Additionally, with respect to the effects of purchase intention on product evaluation, the evidence conveys that mental interactions with products and social cognition are fundamental processes that capture subsequent purchase intention at the product evaluation stage.

Moving beyond the mean: Subgroups and dimensions of brain activity and cognitive performance across domains

Human neuroimaging during cognitive tasks has provided unique and important insights into the neurobiology of cognition. However, the vast majority of research relies on group aggregate or average statistical maps of activity, which do not fully capture the rich intersubject variability in brain function. In order to fully understand the neurobiology of cognitive processes, it is necessary to explore the range of variability in activation patterns across individuals. To better characterize individual variability, hierarchical clustering was performed separately on six fMRI tasks in 822 participants from the Human Connectome Project. Across all tasks, clusters ranged from a predominantly 'deactivating' pattern towards a more 'activating' pattern of brain activity, with significant differences in out-of-scanner cognitive test scores between clusters. Cluster stability was assessed via a resampling approach; a cluster probability matrix was generated, as the probability of any pair of participants clustering together when both were present in a random subsample. Rather than forming distinct clusters, participants fell along a spectrum or into pseudo-clusters without clear boundaries. A principal components analysis of the cluster probability matrix revealed three components explaining over 90% of the variance in clustering. Plotting participants in this lower-dimensional 'similarity space' revealed manifolds of variations along an S 'snake' shaped spectrum or a folded circle or 'tortilla' shape. The 'snake' shape was present in tasks where individual variability related to activity along covarying networks, while the 'tortilla' shape represented multiple networks which varied independently.

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.

Greater Individual Variability in Functional Brain Activity during Working Memory Performance in young people with Autism and Executive Function Impairment

BACKGROUND

Individuals with autism spectrum disorder (ASD) often present with executive functioning (EF) deficits, including spatial working memory (SWM) impairment, which impedes real-world functioning. The present study examined task-related brain activity, connectivity and individual variability in fMRI-measured neural response during an SWM task in older youth and young adults with autism and clinically significant EF impairment.

METHODS

Neuroimaging was analyzed in 29 individuals with ASD without intellectual disability who had clinically significant EF impairment on the Behavior Rating Inventory of Executive Function, and 20 typically developing controls (participant age range=16-34). An SWM N-Back task was performed during fMRI. SWM activity (2-Back vs. 0-Back) and task-related dorsolateral prefrontal cortex (DLPFC) connectivity was examined within and between groups. Variability of neural response during SWM was also examined.

RESULTS

During SWM performance both groups activated the expected networks, and no group differences in network activation or task-related DLPFC-connectivity were found. However, greater individual variability in the pattern of SWM activity was found in the ASD versus the typically developing control group.

CONCLUSIONS

While there were no group differences in SWM task-evoked activity or connectivity, fronto-parietal network engagement was found to be more variable/idiosyncratic in ASD. Our results suggest that the fronto-parietal network may be shifted or sub-optimally engaged during SWM performance in participants with ASD with clinically significant EF impairment, with implications for developing targeted interventions for this subgroup.

How Can Transcranial Magnetic Stimulation Be Used to Modulate Episodic Memory?: A Systematic Review and Meta-Analysis

A systematic review and meta-analysis were conducted to synthesize the existing literature on how transcranial magnetic stimulation (TMS) has been used to modulate episodic memory. Given the numerous parameters of TMS protocols and experimental design characteristics that can be manipulated, a mechanistic understanding of how changes in the combination of parameters (e.g., frequency, timing, intensity, targeted brain region, memory task) modulate episodic memory is needed. To address this, we reviewed 59 studies and conducted a meta-analysis on 245 effect sizes from 37 articles on healthy younger adults (N = 1,061). Analyses revealed generally more beneficial effects of 1-Hz rTMS vs. other frequencies on episodic memory. Moderation analyses revealed complex interactions as online 20-Hz rTMS protocols led to negative effects, while offline 20-Hz rTMS led to enhancing effects. There was also an interaction between stimulation intensity and frequency as 20-Hz rTMS had more negative effects when applied below- vs. at-motor threshold. Conversely, 1-Hz rTMS had more beneficial effects than other frequencies when applied below- vs. at- or above-motor threshold. No reliable aggregate or hypothesized interactions were found when assessing stimulation site (frontal vs. parietal cortex, left vs. right hemisphere), stimulated memory process (during encoding vs. retrieval), the type of retrieval (associative/recollection vs. item/familiarity), or the type of control comparison (active vs. sham or no TMS) on episodic memory. However, there is insufficient data to make strong inference based on the lack of aggregate or two-way interactions between these factors, or to assess more complex (e.g., 3-way) interactions. We reviewed the effects on other populations (healthy older adults and clinical populations), but systematic comparison of parameters was also prevented due to insufficient data. A database of parameters and effects sizes is available as an open source repository so that data from studies can be continuously accumulated in order to facilitate future meta-analysis. In conclusion, modulating episodic memory relies on complex interactions among the numerous moderator variables that can be manipulated. Therefore, rigorous, systematic comparisons need to be further investigated as the body of literature grows in order to fully understand the combination of parameters that lead to enhancing, detrimental or null effects on episodic memory.

Effects of 6-month at-home transcranial direct current stimulation on cognition and cerebral glucose metabolism in Alzheimer's disease

BACKGROUND

Although single or multiple sessions of transcranial direct current stimulation (tDCS) on the prefrontal cortex over a few weeks improved cognition in patients with Alzheimer's disease (AD), effects of repeated tDCS over longer period and underlying neural correlates remain to be elucidated.

OBJECTIVE

This study investigated changes in cognitive performances and regional cerebral metabolic rate for glucose (rCMRglc) after administration of prefrontal tDCS over 6 months in early AD patients.

METHODS

Patients with early AD were randomized to receive either active (n = 11) or sham tDCS (n = 7) over the dorsolateral prefrontal cortex (DLPFC) at home every day for 6 months (anode F3/cathode F4, 2 mA for 30 min). All patients underwent neuropsychological tests and brain ¹⁸F-fluoro-2-deoxyglucose positron emission tomography (FDG-PET) scans at baseline and 6-month follow-up. Changes in cognitive performances and rCMRglc were compared between the two groups.

RESULTS

Compared to sham tDCS, active tDCS improved global cognition measured with Mini-Mental State Examination (p for interaction = 0.02) and language function assessed by Boston Naming Test (p for interaction = 0.04), but not delayed recall performance. In addition, active tDCS prevented decreases in executive function at a marginal level (p for interaction < 0.10). rCMRglc in the left middle/inferior temporal gyrus was preserved in the active group, but decreased in the sham group (p for interaction < 0.001).

CONCLUSIONS

Daily tDCS over the DLPFC for 6 months may improve or stabilize cognition and rCMRglc in AD patients, suggesting the therapeutic potential of repeated at-home tDCS.

Supplementation With Whey Peptide Rich in β-Lactolin Improves Cognitive Performance in Healthy Older Adults: A Randomized, Double-Blind, Placebo-Controlled Study

Epidemiological reports showed that consumptions of fermented dairy products are beneficial for cognitive decline in elderly. Our previous preclinical studies have demonstrated that intakes of whey peptide rich in the β-lactolin [β-lactopeptide of glycine-thereonine-tryptophan-tyrosine (GTWY)] improve memory and attention by regulating monoamine system, and clinical study using neuropsychological test suggested that consumptions with GTWY-rich whey peptide enhance cognitive performance associated with the frontal cortex activity. However, corresponding interventional studies in humans are limited. Objectives: to evaluate the effects of the whey peptide on cognitive functions in healthy older adults using a randomized, double-blinded, placebo-controlled trial design. 114 healthy subjects aged 50–75 were supplemented with the whey peptide or placebo for 12 weeks, and changes in cognitive function were assessed using neuropsychological tests at weeks 0, 6, and 12 of the intervention. Neuropsychological tests included assessments for memory functions (subtests from Wechsler memory scale-revised, standard verbal paired-associate learning test, and recognition memory test for faces), assessments for attention (cancelation and detection tests), and assessments for general cognitive functions (repeatable battery for assessments of neuropsychological status). Cerebral blood flow was also assessed using near-infrared spectroscopy (NIRS) after 6 weeks of intervention. This study was registered on the 19 November, 2017 in the database of the University Hospital Medical Information Network (UMIN) prior to enrollment of subjects (Registration No. UMIN000030461: https://www.umin.ac.jp/ctr/index-j.htm). In the whey peptide group, visual paired-associates I and visual cancelation tests were significantly improved compared with those in the placebo group at weeks 6 and 12 of the intervention, respectively. Visuospatial and constructional scores of the repeatable battery for assessments of neuropsychological status and standard verbal paired-associate learning tests (S-PA) also tended to be improved by the intervention at week 12. Daily intakes of GTWY-rich whey peptide show beneficial effects on cognitive performance, especially associative learning memory and control of attention, in healthy older adults and might prevent age-related cognitive declines.

Age-associated increase in mnemonic strategy use is linked to prefrontal cortex development

Memory functioning undergoes dynamic changes between childhood and adulthood. Spontaneous use of elaborative strategies, which can enhance the recall of information, expands with age and contributes to age-associated improvement in memory functioning. Findings from lesion and neuroimaging studies suggest that the ability to use elaborative strategies is dependent upon intact functioning of the prefrontal cortex (PFC), particularly the dorsolateral PFC region. Because the PFC undergoes protracted maturation, we examined whether age difference in the structure of the PFC is correlated with age-associated increase in strategy use. Here, we investigated the relationship between PFC volume and spontaneous strategy use in a sample of 120 participants aged 5-25 years. We assessed semantic clustering during recall with a standardized word-list recall task (California Verbal Learning Task children's version, CVLT-C) and computed PFC regional volumes from participants' structural brain images. We observed an age-associated increase in the use of semantic clustering and an age-associated decrease in volumes of the PFC. Further, we found that smaller PFC volume was linked to increased use of semantic clustering. Importantly, the volume of the right dorsolateral PFC partially explained the relation between age and the use of semantic clustering. These findings suggest that PFC maturation supports the development of strategy use and lends further support for the notion that brain-behavior relations change across development.

Strategy for Semantic Association Memory (SESAME) training: Effects on brain functioning in schizophrenia

Self-initiation of semantic encoding strategies is impoverished in schizophrenia and contributes to memory impairments. Recently, we observed that following a brief training, schizophrenia patients had the potential to increase the self-initiation of these strategies. In this study, we investigated the neural correlates underlying such memory improvements. Fifteen schizophrenia patients with deficits in self-initiation of semantic encoding strategies were enrolled in a Strategy for Semantic Association Memory (SESAME) training. Patients underwent a memory task in an fMRI scanner. Memory performance and brain activity during the task were measured pre- and post- training, and changes following training were assessed. We also investigated if structural preservation measured by the cortical thickness of the left dorsolateral prefrontal cortex (DLPFC) predicted memory improvement post-training. Memory training led to significant improvements in memory performance that were associated with increased activity in the left DLPFC, during a task in which patients needed to self-initiate semantic encoding strategies. Furthermore, patients with more cortical reserve in their left DLPFC showed greater memory improvement. Our findings provide evidence of neural malleability in the left DLPFC in schizophrenia using cognitive strategies training. Moreover, the brain-behavioural changes observed in schizophrenia provide hope that memory performance can be improved with a brief intervention.

Differing Time of Onset of Concurrent TMS-fMRI during Associative Memory Encoding: A Measure of Dynamic Connectivity

There has been a distinct shift in neuroimaging from localization of function into a more network based approach focused on connectivity. While fMRI has proven very fruitful for this, the hemodynamic signal is inherently slow which limits the temporal resolution of fMRI-only connectivity measures. The brain, however, works on a time scale of milliseconds. This study utilized concurrent transcranial magnetic stimulation (TMS)-fMRI in a novel way to obtain measures of dynamic connectivity by measuring changes in fMRI signal amplitude in regions distal to the site of stimulation following differing TMS onset times. Seventeen healthy subjects completed an associative memory encoding task known to involve the DLPFC, viewing pairs of objects which could be semantically related or unrelated. Three pulses of 10 Hz repetitive TMS were applied over the left DLPFC starting either at 200, 600, or 1000 ms after stimulus onset. Associations for related pairs were better remembered than unrelated pairs in a post-scan cued recall test. Differences in neural activity were assessed across different TMS onsets, separately for related and unrelated pairs. Time specific TMS effects were observed in several regions, including those associated with higher-level processing (lateral frontal, anterior cingulate), visual areas (occipital), and regions involved in semantic processing (e.g., left mid-temporal and medial frontal). Activity in the frontal cortex was decreased at 200 ms post-stimulus for unrelated pairs, and 1000 ms post-stimulus for related pairs. This suggests differences in the timing across conditions in which the DLFPC interacts with other PFC regions, consistent with the notion that the DLPFC is facilitating extended semantic processing for related items. This study demonstrates that time-varying TMS onset inside the MRI can be used to reliably measure fast dynamic connectivity with a temporal resolution in the hundreds of milliseconds.

Early onset marijuana use is associated with learning inefficiencies

OBJECTIVE

Verbal memory difficulties are the most widely reported and persistent cognitive deficit associated with early onset marijuana use. Yet, it is not known what memory stages are most impaired in those with early marijuana use.

METHOD

Forty-eight young adults, aged 18-25, who used marijuana at least once per week and 48 matched nonusing controls (CON) completed the California Verbal Learning Test, Second Edition (CVLT-II). Marijuana users were stratified by age of initial use: early onset users (EMJ), who started using marijuana at or before age 16 (n = 27), and late onset marijuana user group (LMJ), who started using marijuana after age 16 (n = 21). Outcome variables included trial immediate recall, total learning, clustering strategies (semantic clustering, serial clustering, ratio of semantic to serial clustering, and total number of strategies used), delayed recall, and percent retention.

RESULTS

Learning improved with repetition, with no group effect on the learning slope. EMJ learned fewer words overall than LMJ or CON. There was no difference between LMJ and CON in total number of words learned. Reduced overall learning mediated the effect on reduced delayed recall among EMJ, but not CON or LMJ. Learning improved with greater use of semantic versus serial encoding, but this did not vary between groups. EMJ was not related to delayed recall after adjusting for encoding.

CONCLUSIONS

Young adults reporting early onset marijuana use had learning weaknesses, which accounted for the association between early onset marijuana use and delayed recall. No amnestic effect of marijuana use was observed. (PsycINFO Database Record

Reduced Benefit of Memory Elaboration in Older Adults with Subjective Memory Decline

BACKGROUND

Cognitive interventions for neurodegenerative diseases, such as Alzheimer's disease (AD), are best targeted at the preclinical stages, and subjective memory decline (SMD) without objective memory impairment on standard tests in older adults may represent a very early preclinical stage. Elaborated encoding effectively enhances memory performance for healthy older adults (HOAs), but has not been examined in people with SMD.

OBJECTIVE

To examine elaborated encoding in people with SMD, compared with HOAs.

METHODS

Participants were 32 HOAs and 22 people with SMD, defined using the Memory Complaint Questionnaire. Participants completed a verbal paired associate learning (PAL) task with delayed recall under elaborated and non-elaborated encoding conditions, as well as the California Verbal Learning Test-II.

RESULTS

On the PAL learning trials, with age controlled, a significant interaction of group X encoding condition emerged, F(1, 51) =  6.47, MSE = 6.54, p = 0.014, ηp² = 0.11. Simple main effects revealed no differences between groups in the non-elaborated condition, but in the elaborated condition HOAs recalled more pairs than SMD, although both groups benefited from elaboration. At delayed recall, HOA recalled more pairs than SMD, F(1, 51) =  4.59, p = 0.037, ηp²= 0.08, and both groups benefited from elaboration, F(1, 52) =  19.25, p <  0.001, ηp² = 0.27.

CONCLUSION

People with SMD benefit from elaborated encoding, although not to the same extent as HOAs. This objective difference in complex learning and memory suggests neural changes in SMD that may represent preclinical AD. Elaborated encoding is a promising technique to help maintain memory and decrease anxiety in this at-risk population.

Diffusion tensor imaging studies in late-life depression: systematic review and meta-analysis

OBJECTIVES

Late-life depression (LLD) is the association with more cerebrovascular susceptibilities and white matter damage that can be assessed with diffusion tensor imaging (DTI). To better understand the white matter pathological alterations in LLD, we conducted a systematic review and meta-analysis.

METHODS

We searched MEDLINE, EMBASE, PsycINFO, PubMed, and Google Scholar databases for DTI studies comparing patients with LLD and healthy controls. For each study, details regarding participants, imaging methods, and results were extracted. Fractional anisotropy, an index of white matter integrity, was the dependent variable for group comparison. Effect sizes indicating the degree of group difference were estimated by random-effects meta-analysis.

RESULTS

A total of 15 eligible studies were included in the qualitative systematic review, nine of which were suitable for quantitative meta-analyses for the dorsolateral prefrontal cortex (DLPFC), corpus callosum, cingulum, and uncinate fasciculus (UF). Compared with the healthy control group, the LLD group showed lower fractional anisotropy in the DLPFC and UF with a large and a medium effect size, respectively, although heterogeneity and publication bias were found in the DLPFC.

CONCLUSION

Diffusion tensor imaging studies of LLD consistently showed reduced anisotropy in the DLPFC and UF of patients with LLD. These damaged regions are located with the frontostriatal and limbic networks. Thus, our findings showed that the disruption of frontal and frontal-to-limbic white matter tracts contributes to the pathogenesis of LLD.

The formation of source memory under distraction

Background

It is vital to select and process relevant information while restraining irrelevant information for successful retrieval. When multiple streams of information are concurrently present, the ability to overcome distraction is very crucial for processing relevant information. Despite its significance, the neural mechanism of successful memory formation under distraction remains unclear, especially with memory for associations. The present fMRI study investigated the effect of distraction due to irrelevant stimuli in source memory.

Methods

In the MR scanner, participants studied an item and perceptual context with no distractor, a letter-distractor, or a word-distractor. Following the study phase, a source recognition test was administered in which participants were instructed to judge the study status of the test items and context of studied items. Participants’ encoding activity was back-sorted by later source recognition to find the influence of distractors in subsequent memory effects.

Results

Source memory with distractors recruited greater encoding activity in the left dorsolateral prefrontal cortex, and the bilateral inferior temporal gyrus/fusiform cortex, along with the left posterior hippocampus. However, enhanced activity in the left anterior ventrolateral prefrontal cortex and the left parahippocampal cortex predicted successful source memory regardless of the presence of a distractor.

Conclusions

These findings of subsequent memory effects suggest that strong binding of the item-context associations, as well as resistance to interference, may have greater premium in the formation of successful source memory of pictures under distraction. Further, attentional selection to the relevant target seems to play a major role in contextual binding under distraction by enhancing the viability of memory representations from interference effects of distractors.

Electronic supplementary material

The online version of this article (doi:10.1186/1744-9081-10-40) contains supplementary material, which is available to authorized users.

Shaping memory accuracy by left prefrontal transcranial direct current stimulation

Human memory is dynamic and flexible but is also susceptible to distortions arising from adaptive as well as pathological processes. Both accurate and false memory formation require executive control that is critically mediated by the left prefrontal cortex (PFC). Transcranial direct current stimulation (tDCS) enables noninvasive modulation of cortical activity and associated behavior. The present study reports that tDCS applied to the left dorsolateral PFC (dlPFC) shaped accuracy of episodic memory via polaritiy-specific modulation of false recognition. When applied during encoding of pictures, anodal tDCS increased whereas cathodal stimulation reduced the number of false alarms to lure pictures in subsequent recognition memory testing. These data suggest that the enhancement of excitability in the dlPFC by anodal tDCS can be associated with blurred detail memory. In contrast, activity-reducing cathodal tDCS apparently acted as a noise filter inhibiting the development of imprecise memory traces and reducing the false memory rate. Consistently, the largest effect was found in the most active condition (i.e., for stimuli cued to be remembered). This first evidence for a polarity-specific, activity-dependent effect of tDCS on false memory opens new vistas for the understanding and potential treatment of disturbed memory control.