The multifactorial nature of healthy brain ageing: Brain changes, functional decline and protective factors

Association of serum n-3 and n-6 docosapentaenoic acids with cognitive performance in elderly adults: National Health and Nutrition Examination Survey 2011-2014

Limited information exists on the influence of docosapentaenoic acid (DPA) on cognitive function. We investigated the association between serum n-3 and n-6 DPAs and cognitive performance in an elderly population from the National Health and Nutrition Examination Survey, 2011-2014. Restricted cubic spline and logistic regression analyses were utilized. A total of 1,366 older participants were included. Elevated proportions of DPA(n3) in total serum fatty acids were slightly associated with higher DSST scores (OR 0.61, 95% CI (0.38-0.97)), and higher proportions of DPA(n6) in total serum fatty acids were significantly associated with lower scores on different cognitive tests (CERAD (1.64, 1.02-2.65), AFT (2.31, 1.43- 3.75), DSST (3.21, 1.98-5.22) and global cognition (2.85, 1.74-4.66)). After multivariable adjustment, DPA(n3) exhibited no association with cognitive performance, whereas DPA(n6) remained correlated with AFT (1.98, 1.13-3.48), DSST (2.63, 1.43-4.82) and global cognition (2.15, 1.19-3.90). In stratified analyses, higher levels of DPA(n3) were associated with better performance in CERAD among participants aged ≥70, in DSST among those without diabetes and in global cognition among people with lower incomes. Increased DPA(n6) levels were associated with worse performance in AFT and DSST among those aged 60-70 and in all cognitive tests among those with better incomes. In conclusions, elevated levels of serum DPA(n3) may be beneficial for cognitive performance among elderly adults, especially in those over 70 years, with lower incomes and without diabetes. Serum n-6 DPA might be negatively associated with cognitive function, and this association is more pronounced among those who aged 60-70 with higher incomes.

Cognitive frailty in older adults: examining the impact of frailty criteria on neuropsychological profile, functional outcomes, activity levels, and quality of life

PURPOSE

Cognitive frailty (CF) is the co-existence of cognitive impairment and physical frailty without dementia, conferring greater risks of adverse clinical outcomes compared to either condition alone. However, the impact of physical frailty components on cognitive performance remains unclear. This study aims to evaluate CF by determining the neuropsychological profiles, functional outcomes, activity levels, and quality of life across the Fried Frailty Phenotype (FFP) and its components.

METHODS

Cross-sectional study involving 120 community-dwelling older adults without dementia, but with subjective cognitive complaints (SCC, defined as AD8 ≥ 1). Participants were stratified into three groups to assess CF: SCC-Robust, SCC-Prefrail, and SCC-Frail, and further categorized by individual FFP components. Cognitive performance was assessed by comparing neuropsychological test battery (NTB) Z-scores between CF and non-CF groups with Cohen's d for effect sizes. We performed linear regression to examine the relationships between both groups with NTB scores, Instrumental Activities of Daily Living (IADL), Frenchay Activities Index (FAI), and quality of life scores.

RESULTS

NTB scores showed no differences between individuals with CF when classified according to FFP criteria. Individuals with SCC-slow gait speed exhibited reduced processing speed (d = 0.62) and memory (d = 0.61); SCC-fatigue was associated with decreased working memory (d = 0.55). Regression analyses, adjusted for demographic and clinical variables, identified significant associations: slow gait speed with logical memory (- 0.42; 95% CI - 0.79 to - 0.038]) and symbol search (- 0.28; 95% CI - 0.56 to - 0.006]); fatigue with digit span backwards (- 0.66; 95% CI - 1.19 to - 0.14) and color trails 2 (- 0.67; 95% CI, - 1.15 to - 0.20). SCC-slow gait speed and SCC-fatigue were associated with reduced quality of life scores, but not with IADL and FAI scores.

CONCLUSION

Specific frailty components, notably slow gait speed and fatigue, influence cognitive function and quality of life. Our findings provide greater insights into characterizing CF. Further longitudinal studies are required to determine the cognitive and functional trajectories of CF.

How Can Early Stress Influence Later Alzheimer's Disease Risk? Possible Mediators and Underlying Mechanisms

Alzheimer's disease (AD) is a progressive, age-related neurodegenerative disorder to which genetic mutations and risk factors contribute. Evidence is increasing that environmental and lifestyle-related factors, such as exercise, nutrition, education, and exposure to (early-life) stress modify the onset, incidence, and progression of AD. Here, we discuss recent preclinical findings on putative substrates that can explain or contribute to the effects of stress early in life on the risk of developing AD. We focus in particular on stress hormones, neural networks, synapses, mitochondria, nutrient and lipid metabolism, adult neurogenesis, engram cell ensembles, and neuroinflammation. We discuss the idea that stress exposure early in life can alter these processes, either combined or in isolation, thereby reducing the capacity of the brain to resist deleterious consequences of, for example, amyloid-β accumulation, thereby accelerating cognitive decline and progression of Alzheimer-related changes in model systems of the disease. A better understanding of whether experiences early in life also modify trajectories of cognitive decline and pathology in AD and how the substrates discussed translate to humans may help develop novel preventive and/or therapeutic strategies to mitigate the consequences of stressors early in life and increase resilience to developing dementia.

The role of parvalbumin interneuron dysfunction across neurodegenerative dementias

Parvalbumin-positive (PV+) basket neurons are fast-spiking, non-adapting inhibitory interneurons whose oscillatory activity is essential for regulating cortical excitation/inhibition balance. Their dysfunction results in cortical hyperexcitability and gamma rhythm disruption, which have recently gained substantial traction as contributing factors as well as potential therapeutic targets for the treatment of Alzheimer's Disease (AD). Recent evidence indicates that PV+ cells are also impaired in Frontotemporal Dementia (FTD) and Dementia with Lewy bodies (DLB). However, no attempt has been made to integrate these findings into a coherent pathophysiological framework addressing the contribution of PV+ interneuron dysfunction to the generation of cortical hyperexcitability and gamma rhythm disruption in FTD and DLB. To fill this gap, we epitomized the most recent evidence on PV+ interneuron impairment in AD, FTD, and DLB, focusing on its contribution to the generation of cortical hyperexcitability and gamma oscillatory disruption and their interplay with misfolded protein accumulation, neuronal death, and clinical symptoms' onset. Our work deepens the current understanding concerning the role of PV+ interneuron dysfunction across neurodegenerative dementias, highlighting commonalities and differences among AD, FTD, and DLB, thus paving the way for identifying novel biomarkers and potential therapeutic targets for the treatment of these diseases.

You Are as Old as the Connectivity You Keep: Distinct Neurophysiological Mechanisms Underlying Age-Related Changes in Hand Dexterity and Strength

BACKGROUND

Aging can lead to a decline in motor control. While age-related motor impairments have been documented, the underlying changes in cortico-cortical interactions remain poorly understood.

METHODS

We took advantage of the high temporal resolution of dual-site transcranial magnetic stimulation (dsTMS) to investigate how communication between higher-order rostral premotor regions and the primary motor cortex (M1) influences motor control in young and elderly adults. We assessed the dynamics of connectivity from the inferior frontal gyrus (IFG) or pre-supplementary motor area (preSMA) to M1, by testing how conditioning of the IFG/preSMA affected the amplitude of motor evoked potentials (MEPs) induced by M1 stimulation at different temporal intervals. Moreover, we explored how age-related changes in premotor-M1 interactions relate to motor performance.

RESULTS

Our results show that both young and elderly adults had excitatory IFG-M1 and preSMA-M1 interactions, but the two groups' timing and strength differed. In young adults, IFG-M1 interactions were early and time-specific (8 ms), whereas in older individuals, they were delayed and more prolonged (12-16 ms). PreSMA-M1 interactions emerged early (6 ms) and peaked at 10-12 ms in young individuals but were attenuated in older individuals. Critically, a connectivity profile of the IFG-M1 circuit like that of the young cohort predicted better dexterity in older individuals, while preserved preSMA-M1 interactions predicted greater strength, suggesting that age-related motor decline is associated with specific changes in premotor-motor networks.

CONCLUSIONS

Preserving youthful motor network connectivity in older individuals is related to maintaining motor performance and providing information for interventions targeting aging effects on behavior.

The Hidden Dangers of Sedentary Living: Insights into Molecular, Cellular, and Systemic Mechanisms

With the aging of the global population, neurodegenerative diseases are emerging as a major public health issue. The adoption of a less sedentary lifestyle has been shown to have a beneficial effect on cognitive decline, but the molecular mechanisms responsible are less clear. Here we provide a detailed analysis of the complex molecular, cellular, and systemic mechanisms underlying age-related cognitive decline and how lifestyle choices influence these processes. A review of the evidence from animal models, human studies, and postmortem analyses emphasizes the importance of integrating physical exercise with cognitive, multisensory, and motor stimulation as part of a multifaceted approach to mitigating cognitive decline. We highlight the potential of these non-pharmacological interventions to address key aging hallmarks, such as genomic instability, telomere attrition, and neuroinflammation, and underscore the need for comprehensive and personalized strategies to promote cognitive resilience and healthy aging.

Age-induced changes in affective prosody comprehension and its relationship with general cognitive ability and social support utilization among older adults

Aging can impact emotional recognition, affecting older adults' mental health and social function. This study examined how aging affects affective prosody comprehension (APC: understanding emotions through speech) across seven emotions (happiness, surprise, sadness, anger, fear, disgust, and neutrality) and its relationship with cognitive function (via the Montreal Cognitive Assessment) and social support (via the Social Support Rating Scale) in 199 cognitively normal older adults. We found that older adults had lower APC accuracy and more errors, often mistaking negative emotions for neutral or positive ones. APC accuracy was significantly associated with social support, and a partial least squares (PLS) cognitive component fully mediated the relationship between the APC component and social support utilization, explaining 61.7% of the total effect. These results suggest that declines in APC during aging are linked to social support utilization through cognitive function, offering insights for interventions to improve social and cognitive health in older adults.

Determinants of cerebral blood flow and arterial transit time in healthy older adults

Cerebral blood flow (CBF) and arterial transit time (ATT), markers of brain vascular health, worsen with age. The primary aim of this cross-sectional study was to identify modifiable determinants of CBF and ATT in healthy older adults (n = 78, aged 60-81 years). Associations between cardiorespiratory fitness and CBF or ATT were of particular interest because the impact of cardiorespiratory fitness is not clear within existing literature. Secondly, this study assessed whether CBF or ATT relate to cognitive function in older adults. Multiple post-labelling delay pseudo-continuous arterial spin labelling estimated resting CBF and ATT in grey matter. Results from multiple linear regressions found higher BMI was associated with lower global CBF (β = -0.35, P = 0.008) and a longer global ATT (β = 0.30, P = 0.017), global ATT lengthened with increasing age (β = 0.43, P = 0.004), and higher cardiorespiratory fitness was associated with longer ATT in parietal (β = 0.44, P = 0.004) and occipital (β = 0.45, P = 0.003) regions. Global or regional CBF or ATT were not associated with processing speed, working memory, or attention. In conclusion, preventing excessive weight gain may help attenuate age-related declines in brain vascular health. ATT may be more sensitive to age-related decline than CBF, and therefore useful for early detection and management of cerebrovascular impairment. Finally, cardiorespiratory fitness appears to have little effect on CBF but may induce longer ATT in specific regions.

Poor sleep quality, dementia status and their association with all-cause mortality among older US adults

BACKGROUND

Evidence points to associations between sleep quality, dementia, and mortality. We examined whether poor sleep quality mediated or moderated the association between dementia and mortality risk among older US adults and vice versa, and whether these associations differed by sex and by race.

METHODS

The study investigated bi-directional associations between sleep quality, dementia and mortality in older US adults using data from the Health and Retirement Study (N = 6,991, mean age = 78.1y, follow-up: 2006-2020, number of deaths = 4,938). It tested interactions and mediating effects, using Cox proportional hazards models and four-way decomposition models.

RESULTS

Poor sleep quality was associated with increased mortality risk, particularly among male and White older adults. However, the association was reversed in the fully adjusted model, with a 7% decrease in risk per tertile. Probable dementia was associated with a two-fold increase in mortality risk, with a stronger association found among White adults. The association was markedly attenuated in the fully adjusted models. Sleep quality-stratified models showed a stronger positive association between dementia and mortality among individuals with better sleep quality. Both mediation and interaction were involved in explaining the total effects under study, though statistically significant total effects were mainly composed of controlled direct effects.

CONCLUSIONS

Poor sleep quality is directly related to mortality risk before lifestyle and health-related factors are adjusted. Dementia is linked to mortality risk, especially in individuals with better sleep quality, males, and White older adults. Future research should explore the underlying mechanisms.

Effects of saturated and monounsaturated fatty acids on cognitive impairment: evidence from Mendelian randomization study

BACKGROUND

Prior observational studies have suggested correlations between saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) with cognitive function. However, causal relationships remains unclear.

METHODS

We assessed the causal impact of two SFAs (palmitic acid [PA] and stearic acid [SA]) and two MUFAs (oleic acid [OA] and palmitoleic acid [POA]) on cognitive function-related traits, and dementia-related traits by univariable Mendelian randomization (UVMR) and multivariable Mendelian randomization (MVMR) analyses.

RESULTS

UVMR indicated β of 0.060 (P = 4.05E-06) for cognitive performance score and 0.066 (P = 4.21E-04) for fluid intelligence per standard deviation (SD) increase in OA level. MVMR indicated: (i) β of -0.608 (P = 8.37E-05) for fluid intelligence score per SD increase in POA; (ii) β of 0.074 (P = 0.018) for fluid intelligence score per SD increase in OA; (iii) β of 0.029 (P = 0.033) for number of incorrect matches in round per SD increase in PA; and (iv) β of 0.039 (P = 0.032) for number of incorrect matches in round per SD increase in SA. In addition, a secondary MVMR analysis after excluding the effect of polyunsaturated fatty acids suggested that: (i) β of -0.043 (P = 1.97E-02) for cognitive performance score per SD increase in PA and (ii) β of -0.079 (P = 1.79E-03) for cognitive performance score per SD increase in SA.

CONCLUSIONS

Overall, UVMR and MVMR suggest that OA may be beneficial for cognitive function, while POA, PA, and SA may have detrimental effects on cognitive function.

Does resistance training in older adults lead to structural brain changes associated with a lower risk of Alzheimer's dementia? A narrative review

Dementia, particularly Alzheimer's Disease (AD), has links to several modifiable risk factors, especially physical inactivity. When considering the relationship between physcial activity and dementia risk, cognitive benefits are generally attributed to aerobic exercise, with resistance exercise (RE) receiving less attention. This review aims to address this gap by evaluating the impact of RE on brain structures and cognitive deficits associated with AD. Drawing insights from randomized controlled trials (RCTs) utilizing structural neuroimaging, the specific influence of RE on AD-affected brain structures and their correlation with cognitive function are discussed. Preliminary findings suggest that RE induces structural brain changes in older adults that could reduce the risk of AD or mitigate AD progression. Importantly, the impacts of RE appear to follow a dose-response effect, reversing pathological structural changes and improving associated cognitive functions if performed at least twice per week for at least six months, with greatest effects in those already experiencing some element of cognitive decline. While more research is eagerly awaited, this review contributes insights into the potential benefits of RE for cognitive health in the context of AD-related changes in brain structure and function.

Prediction Model for Cognitive Impairment among Disabled Older Adults: A Development and Validation Study

Disabled older adults exhibited a higher risk for cognitive impairment. Early identification is crucial in alleviating the disease burden. This study aims to develop and validate a prediction model for identifying cognitive impairment among disabled older adults. A total of 2138, 501, and 746 participants were included in the development set and two external validation sets. Logistic regression, support vector machine, random forest, and XGBoost were introduced to develop the prediction model. A nomogram was further established to demonstrate the prediction model directly and vividly. Logistic regression exhibited better predictive performance on the test set with an area under the curve of 0.875. It maintained a high level of precision (0.808), specification (0.788), sensitivity (0.770), and F1-score (0.788) compared with the machine learning models. We further simplified and established a nomogram based on the logistic regression, comprising five variables: age, daily living activities, instrumental activity of daily living, hearing impairment, and visual impairment. The areas under the curve of the nomogram were 0.871, 0.825, and 0.863 in the internal and two external validation sets, respectively. This nomogram effectively identifies the risk of cognitive impairment in disabled older adults.

Comparing the Efficacy of Two Cognitive Screening Tools in Identifying Gray and White Matter Brain Damage among Older Adults

Background

Ageing is associated with structural changes in brain regions and functional decline in cognitive domains. Noninvasive tools for identifying structural damage in the brains of older adults are relevant for early treatment.

Aims

This study aims to evaluate and compare the accuracy of the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA©) in identifying gray and white matter brain damage in older individuals with varying degrees of cognitive impairment.

Methods

Ninety older adults (62 women) with an average age of 69 ± 7 years were enrolled and categorized as having no cognitive impairment (NCI), mild cognitive impairment (MCI), or moderate cognitive impairment (MoCI). Magnetic resonance imaging (MRI) was utilized to assess the number, volume, and distribution of brain damage. The Fazekas and Scheltens scales were applied to the brain MRIs, and inferential statistics were employed to compare variables among the groups.

Results

Cognitive impairment was observed in 56.7% of the participants (95% confidence interval (CI): 46.4-66.4%), with thirty-six older adults (40%) classified as MCI and 15 (17%) as MoCI. Cognitive impairment and medial temporal lobe (MTL) atrophy were found to be associated (p=0.001), exhibiting higher mean volume scales of the MTL atrophied area in the MoCI group (p < 0.001). The MMSE accurately revealed MTL atrophy based on the Scheltens (p < 0.05) and Fazekas (p < 0.05) scales. At the same time, the MoCA accurately identified periventricular white matter (PWM) abnormalities according to the Fazekas scale (p < 0.05).

Conclusions

The MMSE and MoCA screening tools effectively identified gray and white matter brain damage in older adults with varying degrees of cognitive impairment. Lower MMSE scores are associated with MTL atrophy and lesions, and lower MoCA scores are related to PWM lesions. The concurrent use of MMSE and MoCA is recommended for assessing structural changes in distinct brain regions.

Prevalence and Characteristics of Covert/Minimal Hepatic Encephalopathy in Patients With Liver Cirrhosis: A Systematic Review and Meta-Analysis

INTRODUCTION

Covert/minimal hepatic encephalopathy (C/MHE) is the mildest form of hepatic encephalopathy (HE), but it is closely related to the quality of life and prognosis of patients with cirrhosis. Currently, the epidemiological data of C/MHE have not been well described.

METHODS

We searched the PubMed, Embase, and Cochrane Library databases for relevant articles. We performed a random-effects meta-analysis of proportions to estimate the pooled prevalence of C/MHE in patients with cirrhosis. We also examined potential risk factors for C/MHE by comparing characteristics of patients with and without C/MHE.

RESULTS

Finally, a total of 101 studies were included. The prevalence of C/MHE was 40.9% (95% confidence interval, 38.3%-43.5%) among patients with cirrhosis worldwide. The pooled C/MHE prevalence was 39.9% (95% confidence interval 36.7%-43.1%) based on studies using the psychometric HE score as a diagnostic tool. Meta-regression models showed that geographic region, sample size, mean age, sex ratio, and Child-Pugh classification were influencing factors for the heterogeneity of C/MHE prevalence. The presence of C/MHE was found to be associated with various factors including age, level of education, alcoholic etiology, Child-Pugh classification, MELD score, history of overt HE, presence of other complications, and laboratory tests related to impaired liver function.

DISCUSSION

This study reports detailed data on the prevalence of C/MHE as well as clinical features associated with C/MHE, suggesting that C/MHE is one of the most common complications of liver cirrhosis.

Unexpected cognitive similarities between older adults and young people: Scores variability and cognitive performances

BACKGROUND

Increased interindividual variability in cognitive performance during aging has been proposed as an indicator of cognitive reserve.

OBJECTIVE

To determine if interindividual variability performance in episodic memory (PAL), working memory (SWM), reaction time (RTI), and sustained attention (RVP) could differentiate clusters of differential cognitive performance in healthy young and older adults and search for cognitive tests that most contribute to these differential performances.

METHODS

We employed hierarchical cluster and canonical discriminant function analyses of cognitive scores using the Cambridge Neuropsychological Test Automated Battery (CANTAB) to identify cognitive variability in older and young adults using the coefficient of variability of cognitive performances between and within groups. We also analyzed potential influences of age, education, and physical activity.

RESULTS

Cluster analysis distinguished groups with differential cognitive performance and correlation analysis revealed coefficient of variability and cognitive performance associations. The greater the coefficient of variability the poorer was cognitive performance in RTI but not in PAL and SWM. Older adults showed diverse trajectories of cognitive decline, and better education or higher percentage of physically active individuals exhibited better cognitive performance in both older and young adults.

CONCLUSION

PAL and SWM are the most sensitive tests to investigate the wide age range encompassing older and young adults. In older adults' intragroup analysis PAL showed greater discriminatory capacity, indicating its potential for clinical applications late in life. Our data underscore the importance of studying variability as a tool for early detection of subtle cognitive declines and for interpreting results that deviate from normality.

The Mediating Role of Cortical Atrophy on the Relationship between the Resilience Index and Cognitive Function: Findings from the Healthy Brain Initiative

Background

Lifestyle factors are linked to differences in brain aging and risk for Alzheimer's disease, underscored by concepts like 'cognitive reserve' and 'brain maintenance'. The Resilience Index (RI), a composite of 6 factors (cognitive reserve, physical and cognitive activities, social engagement, diet, and mindfulness) provides such a holistic measure.

Objective

This study aims to examine the association of RI scores with cognitive function and assess the mediating role of cortical atrophy.

Methods

Baseline data from 113 participants (aged 45+, 68% female) from the Healthy Brain Initiative were included. Life course resilience was estimated with the RI, cognitive performance with Cognivue®, and brain health using a machine learning derived Cortical Atrophy Score (CAS). Mediation analysis probed the relationship between RI, cognitive outcomes, and cortical atrophy.

Results

In age and sex adjusted models, the RI was significantly associated with CAS (β= -0.25, p = 0.006) and Cognivue® scores (β= 0.32, p < 0.001). The RI-Cognivue® association was partially mediated by CAS (β= 0.07; 95% CI [0.02, 0.14]).

Conclusions

Findings revealed that the collective effect of early and late-life lifestyle resilience factors on cognition are partially explained by their association with less brain atrophy. These findings underscore the value of comprehensive lifestyle assessments in understanding the risk and progression of cognitive decline and Alzheimer's disease in an aging population.

Urgent needs of caregiving in ageing populations with Alzheimer's disease and other chronic conditions: Support our loved ones

The ageing process begins at birth. It is a life-long process, and its exact origins are still unknown. Several hypotheses attempt to describe the normal ageing process, including hormonal imbalance, formation of reactive oxygen species, DNA methylation & DNA damage accumulation, loss of proteostasis, epigenetic alterations, mitochondrial dysfunction, senescence, inflammation, and stem cell depletion. With increased lifespan in elderly individuals, the prevalence of age-related diseases including, cancer, diabetes, obesity, hypertension, Alzheimer's, Alzheimer's disease and related dementias, Parkinson's, and other mental illnesses are increased. These increased age-related illnesses, put tremendous pressure & burden on caregivers, family members, and friends who are living with patients with age-related diseases. As medical needs evolve, the caregiver is expected to experience an increase in duties and challenges, which may result in stress on themselves, and impact their own family life. In the current article, we assess the biological mechanisms of ageing and its effect on body systems, exploring lifestyle and ageing, with a specific focus on age-related disorders. We also discussed the history of caregiving and specific challenges faced by caregivers in the presence of multiple comorbidities. We also assessed innovative approaches to funding caregiving, and efforts to improve the medical system to better organize chronic care efforts, while improving the skill and efficiency of both informal and formal caregivers. We also discussed the role of caregiving in end-of-life care. Our critical analysis strongly suggests that there is an urgent need for caregiving in aged populations and support from local, state, and federal agencies.

The Role of Serotonin in Fear Learning and Memory: A Systematic Review of Human Studies

Fear is characterized by distinct behavioral and physiological responses that are essential for the survival of the human species. Fear conditioning (FC) serves as a valuable model for studying the acquisition, extinction, and expression of fear. The serotonin (5-hydroxytryptamine, 5-HT) system is known to play a significant role in emotional and motivational aspects of human behavior, including fear learning and expression. Accumulating evidence from both animal and human studies suggests that brain regions involved in FC, such as the amygdala, hippocampus, and prefrontal cortex, possess a high density of 5-HT receptors, implicating the crucial involvement of serotonin in aversive learning. Additionally, studies exploring serotonin gene polymorphisms have indicated their potential influence on FC. Therefore, the objective of this work was to review the existing evidence linking 5-HT with fear learning and memory in humans. Through a comprehensive screening of the PubMed and Web of Science databases, 29 relevant studies were included in the final review. These studies investigated the relationship between serotonin and fear learning using drug manipulations or by studying 5-HT-related gene polymorphisms. The results suggest that elevated levels of 5-HT enhance aversive learning, indicating that the modulation of serotonin 5-HT2A receptors regulates the expression of fear responses in humans. Understanding the role of this neurochemical messenger in associative aversive learning can provide insights into psychiatric disorders such as anxiety and post-traumatic stress disorder (PTSD), among others.

Neurophysiological Markers of Premotor-Motor Network Plasticity Predict Motor Performance in Young and Older Adults

Aging is commonly associated with a decline in motor control and neural plasticity. Tuning cortico-cortical interactions between premotor and motor areas is essential for controlling fine manual movements. However, whether plasticity in premotor-motor circuits predicts hand motor abilities in young and elderly humans remains unclear. Here, we administered transcranial magnetic stimulation (TMS) over the ventral premotor cortex (PMv) and primary motor cortex (M1) using the cortico-cortical paired-associative stimulation (ccPAS) protocol to manipulate the strength of PMv-to-M1 connectivity in 14 young and 14 elderly healthy adults. We assessed changes in motor-evoked potentials (MEPs) during ccPAS as an index of PMv-M1 network plasticity. We tested whether the magnitude of MEP changes might predict interindividual differences in performance in two motor tasks that rely on premotor-motor circuits, i.e., the nine-hole pegboard test and a choice reaction task. Results show lower motor performance and decreased PMv-M1 network plasticity in elderly adults. Critically, the slope of MEP changes during ccPAS accurately predicted performance at the two tasks across age groups, with larger slopes (i.e., MEP increase) predicting better motor performance at baseline in both young and elderly participants. These findings suggest that physiological indices of PMv-M1 plasticity could provide a neurophysiological marker of fine motor control across age-groups.