The relevance of aging-related changes in brain function to rehabilitation in aging-related disease

Investigating the neural mechanisms of transcranial direct current stimulation effects on human cognition: current issues and potential solutions

Transcranial direct current stimulation (tDCS) has been studied extensively for its potential to enhance human cognitive functions in healthy individuals and to treat cognitive impairment in various clinical populations. However, little is known about how tDCS modulates the neural networks supporting cognition and the complex interplay with mediating factors that may explain the frequently observed variability of stimulation effects within and between studies. Moreover, research in this field has been characterized by substantial methodological variability, frequent lack of rigorous experimental control and small sample sizes, thereby limiting the generalizability of findings and translational potential of tDCS. The present manuscript aims to delineate how these important issues can be addressed within a neuroimaging context, to reveal the neural underpinnings, predictors and mediators of tDCS-induced behavioral modulation. We will focus on functional magnetic resonance imaging (fMRI), because it allows the investigation of tDCS effects with excellent spatial precision and sufficient temporal resolution across the entire brain. Moreover, high resolution structural imaging data can be acquired for precise localization of stimulation effects, verification of electrode positions on the scalp and realistic current modeling based on individual head and brain anatomy. However, the general principles outlined in this review will also be applicable to other imaging modalities. Following an introduction to the overall state-of-the-art in this field, we will discuss in more detail the underlying causes of variability in previous tDCS studies. Moreover, we will elaborate on design considerations for tDCS-fMRI studies, optimization of tDCS and imaging protocols and how to assure high-level experimental control. Two additional sections address the pressing need for more systematic investigation of tDCS effects across the healthy human lifespan and implications for tDCS studies in age-associated disease, and potential benefits of establishing large-scale, multidisciplinary consortia for more coordinated tDCS research in the future. We hope that this review will contribute to more coordinated, methodologically sound, transparent and reproducible research in this field. Ultimately, our aim is to facilitate a better understanding of the underlying mechanisms by which tDCS modulates human cognitive functions and more effective and individually tailored translational and clinical applications of this technique in the future.

Functional Connectivity and Microstructural Network Correlates of Interindividual Variability in Distinct Executive Functions of Healthy Older Adults

Executive functions, essential for daily life, are known to be impaired in older age. Some executive functions, including working memory updating and value-based decision-making, are specifically sensitive to age-related deterioration. While their neural correlates in young adults are well-described, a comprehensive delineation of the underlying brain substrates in older populations, relevant to identify targets for modulation against cognitive decline, is missing. Here, we assessed letter updating and Markov decision-making task performance to operationalize these trainable functions in 48 older adults. Resting-state functional magnetic resonance imaging was acquired to quantify functional connectivity (FC) in task-relevant frontoparietal and default mode networks. Microstructure in white matter pathways mediating executive functions was assessed with diffusion tensor imaging and quantified by tract-based fractional anisotropy (FA). Superior letter updating performance correlated with higher FC between dorsolateral prefrontal cortex and left frontoparietal and hippocampal areas, while superior Markov decision-making performance correlated with decreased FC between basal ganglia and right angular gyrus. Furthermore, better working memory updating performance was related to higher FA in the cingulum bundle and the superior longitudinal fasciculus. Stepwise linear regression showed that cingulum bundle FA added significant incremental contribution to the variance explained by fronto-angular FC alone. Our findings provide a characterization of distinct functional and structural connectivity correlates associated with performance of specific executive functions. Thereby, this study contributes to the understanding of the neural correlates of updating and decision-making functions in older adults, paving the way for targeted modulation of specific networks by modulatory techniques such as behavioral interventions and non-invasive brain stimulation.

Association Between Responsiveness to Transcranial Magnetic Stimulation and Interhemispheric Functional Connectivity of Sensorimotor Cortex in Older Adults

Introduction: Repetitive transcranial magnetic stimulation (rTMS) is a promising therapeutic technique, and is believed to accomplish its effect by influencing the stimulated and remotely connected areas. However, responsiveness to rTMS shows high interindividual variability, and this intersubject variability is particularly high in older adults. It remains unclear whether baseline resting-state functional connectivity (rsFC) contributes to this variability in older adults. The aims of this study are to (1) examine rTMS effects over the primary motor cortex (M1) in older adults, and (2) identify baseline network properties that may contribute to the interindividual variability. Methods: We tested response to intermittent theta burst stimulation (iTBS), an effective rTMS protocol, over M1 by using both electromyography and resting-state functional magnetic resonance imaging in older adults. Outcome measures included motor-evoked potential (MEP) elicited by single-pulse transcranial magnetic stimulation and rsFC before and after an iTBS session. Results: iTBS significantly increased MEP amplitudes and rsFC between the stimulation site, sensorimotor cortex, and supplementary motor area (SMA) in older adults. iTBS-induced changes in MEP amplitude were positively correlated with increases in interhemispheric rsFC after iTBS. Furthermore, older adults with lower baseline interhemispheric rsFC between sensorimotor cortex and SMA exhibited stronger MEP response after iTBS. Discussion: Findings of the study suggest that different levels of interhemispheric communication during resting state might contribute to the response heterogeneity to iTBS in older adults. Interhemispheric rsFC may have great potential serving as a useful marker for predicting iTBS responsiveness in older adults. ClinicalTrials.gov ID: 1707654427 Impact statement Factors contributing to interindividual variability of the responsive to repetitive transcranial magnetic stimulation (rTMS) in older adults remain poorly understood. In this study, we examined the effects of rTMS over the primary motor cortex in older adults, and found that response to rTMS is associated with prestimulation interhemispheric connectivity in the sensorimotor and premotor areas. Findings of the study have great potential to be translated into a connectivity-based strategy for identification of responders for rTMS in older adults.

Neuromodulation in post-stroke aphasia treatment

PURPOSE OF REVIEW

This paper aims to review non-invasive brain stimulation (NIBS) methods to augment speech and language therapy (SLT) for patients with post-stroke aphasia.

RECENT FINDINGS

In the past five years there have been more than 30 published studies assessing the effect of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) for improving aphasia in people who have had a stroke. Different approaches to NIBS treatment have been used in post-stroke aphasia treatment including different stimulation locations, stimulation intensity, number of treatment sessions, outcome measures, type of aphasia treatment, and time post-stroke.

SUMMARY

This review of NIBS for post-stroke aphasia shows that both tDCS and TMS can be beneficial for improving speech and language outcomes for patients with stroke. Prior to translating NIBS to clinical practice, further studies are needed to determine optimal tDCS and TMS parameters as well as the mechanisms underlying tDCS and TMS treatment outcomes.

Bone mineral density in patients with chronic heart failure: a meta-analysis

Objective

This study aimed to verify the existing relationship between bone mineral density (BMD) and chronic heart failure (CHF) by meta-analysis.

Methods

Databases, including PubMed, Web of Science, and Chinese National Knowledge Infrastructure, published in English or Chinese up to February 28, 2017, were searched for studies on the association between CHF and BMD. Two independent reviewers collected the relevant articles. The standard mean deviation (SMD) and 95% confidence interval were calculated for BMD with fixed- and random-effect models. Subgroup and sensitivity analyses were also conducted.

Results

A total of six studies (552 CHF and 243 non-CHF patients) were included. The results indicated that the patients with CHF had a lower total BMD compared with the non-CHF patients. Similar effects were also observed for femoral neck, arm, leg, and trunk BMD. However, no difference was observed in the lumbar spine BMD. The SMD of total BMD in New York Heart Association classes I or II (NYHA I or II) patients was −0.62, while that in NYHA III or IV patients was −0.87, and the SMD of femoral bone mineral density in NYHA I or II patients was −0.47, while that in NYHA III or IV patients was −1.07. Moreover, vitamin D and parathyroid hormone (PTH) were also found to be associated with CHF.

Conclusion

Patients with CHF had a lower total BMD and femoral neck, arm, leg, or trochanter BMD than patients with non-CHF. Vitamin D reduced, whereas PTH increased, with the severity of CHF. The clinical significance of the present findings remains uncertain and should be confirmed by future studies.

Inhibitory neuron-specific Cre-dependent red fluorescent labeling using VGAT BAC-based transgenic mouse lines with identified transgene integration sites

Inhibitory neurons are crucial for shaping and regulating the dynamics of the entire network, and disturbances in these neurons contribute to brain disorders. Despite the recent progress in genetic labeling techniques, the heterogeneity of inhibitory neurons requires the development of highly characterized tools that allow accurate, convenient, and versatile visualization of inhibitory neurons in the mouse brain. Here, we report a novel genetic technique to visualize the vast majority and/or sparse subsets of inhibitory neurons in the mouse brain without using techniques that require advanced skills. We developed several lines of Cre-dependent tdTomato reporter mice based on the vesicular GABA transporter (VGAT)-BAC, named VGAT-stop-tdTomato mice. The most useful line (line #54) was selected for further analysis based on two characteristics: the inhibitory neuron-specificity of tdTomato expression and the transgene integration site, which confers efficient breeding and fewer adverse effects resulting from transgene integration-related genomic disruption. Robust and inhibitory neuron-specific expression of tdTomato was observed in a wide range of developmental and cellular contexts. By breeding the VGAT-stop-tdTomato mouse (line #54) with a novel Cre driver mouse line, Galntl4-CreER, sparse labeling of inhibitory neurons was achieved following tamoxifen administration. Furthermore, another interesting line (line #58) was generated through the unexpected integration of the transgene into the X-chromosome and will be used to map X-chromosome inactivation of inhibitory neurons. Taken together, our studies provide new, well-characterized tools with which multiple aspects of inhibitory neurons can be studied in the mouse.

The progeroid gene BubR1 regulates axon myelination and motor function

Myelination, the process by which oligodendrocytes form the myelin sheath around axons, is key to axonal signal transduction and related motor function in the central nervous system (CNS). Aging is characterized by degenerative changes in the myelin sheath, although the molecular underpinnings of normal and aberrant myelination remain incompletely understood. Here we report that axon myelination and related motor function are dependent on BubR1, a mitotic checkpoint protein that has been linked to progeroid phenotypes when expressed at low levels and healthy lifespan when overabundant. We found that oligodendrocyte progenitor cell proliferation and oligodendrocyte density is markedly reduced in mutant mice with low amounts of BubR1 (BubR1^H/H mice), causing axonal hypomyelination in both brain and spinal cord. Expression of essential myelin-related genes such as MBP and PLP1 was significantly reduced in these tissues. Consistent with defective myelination, BubR1^H/H mice exhibited various motor deficits, including impaired motor strength, coordination, and balance, irregular gait patterns and reduced locomotor activity. Collectively, these data suggest that BubR1 is a key determinant of oligodendrocyte production and function and provide a molecular entry point to understand age-related degenerative changes in axon myelination.

Influence of Linguistic and Nonlinguistic Variables on Generalization and Maintenance Following Phonomotor Treatment for Aphasia

PURPOSE

Although phonomotor treatment shows promise as an effective intervention for anomia in people with aphasia, responses to this treatment are not consistent across individuals. To better understand this variability, we examined the influence of 5 participant characteristics-age, time postonset, aphasia severity, naming impairment, and error profile-on generalization and maintenance of confrontation naming and discourse abilities following phonomotor treatment.

METHOD

Using retrospective data from 26 participants with aphasia who completed a 6-week phonomotor treatment program, we examined the relationships between participant characteristics of interest and change scores on confrontation naming and discourse tasks, measured pretreatment, immediately following treatment, and 3 months following treatment.

RESULTS

Although the participant characteristics of aphasia severity and error profile appeared to predict generalization to improved confrontation naming of untrained items and discourse performance, a post hoc analysis revealed that no one characteristic predicted generalization across participants at 3 months posttreatment.

CONCLUSIONS

Response to phonomotor treatment does not appear to be influenced by aphasia and anomia severity level, error profile, participant age, or time postonset. Other factors, however, may influence response to intensive aphasia treatment and are worthy of continued exploration.

Advances in neurocognitive rehabilitation research from 1992 to 2017: The ascension of neural plasticity

OBJECTIVE

The last 25 years have seen profound changes in neurocognitive rehabilitation that continue to motivate its evolution. Although the concept of nervous system plasticity was discussed by William James (1890), the foundation for experience-based plasticity had not reached the critical empirical mass to seriously impact rehabilitation research until after 1992. The objective of this review is to describe how the emergence of neural plasticity has changed neurocognitive rehabilitation research.

METHOD

The important developments included (a) introduction of a widely available tool that could measure brain plasticity (i.e., functional MRI); (b) development of new structural imaging techniques that could define limits of and opportunities for neural plasticity; (c) deployment of noninvasive brain stimulation to leverage neural plasticity for rehabilitation; (d) growth of a literature indicating that exercise has positively impacts neural plasticity, especially for older persons; and (e) enhancement of neural plasticity by creating interventions that generalize beyond the boundaries of treatment activities. Given the massive literature, each of these areas is developed by example.

RESULTS

The expanding influence of neural plasticity has provided new models and tools for neurocognitive rehabilitation in neural injuries and disorders, as well as methods for measuring neural plasticity and predicting its limits and opportunities. Early clinical trials have provided very encouraging results.

CONCLUSION

Now that neural plasticity has gained a firm foothold, it will continue to influence the evolution of neurocognitive rehabilitation research for the next 25 years and advance rehabilitation for neural injuries and disease. (PsycINFO Database Record

tDCS-Induced Modulation of GABA Levels and Resting-State Functional Connectivity in Older Adults

Transcranial direct current stimulation (tDCS) modulates human behavior, neuronal patterns, and metabolite concentrations, with exciting potential for neurorehabilitation. However, the understanding of tDCS-induced alterations on the neuronal level is incomplete, and conclusions from young adults, in whom the majority of studies have been conducted, cannot be easily transferred to older populations. Here, we investigated tDCS-induced effects in older adults (N = 48; age range, 50-79 years) using magnetic resonance spectroscopy to quantify GABA levels as well as resting-state functional magnetic resonance imaging to assess sensorimotor network strength and interhemispheric connectivity. In a randomized, counterbalanced, crossover design, we applied anodal tDCS (atDCS), cathodal tDCS (ctDCS), and sham tDCS (stDCS) over the left sensorimotor region. We observed a significant reduction of GABA levels after atDCS compared with stDCS, reflecting the preserved neuromodulatory effect of atDCS in older adults. Moreover, resting-state functional coupling was decreased during atDCS compared with stDCS, most likely indicating augmented efficiency in brain network functioning. Increased levels of interhemispheric connectivity with age were diminished by atDCS, suggesting stimulation-induced functional decoupling. Further, the magnitude of atDCS-induced local plasticity was related to baseline functional network strength. Our findings provide novel insight into the neuronal correlates underlying tDCS-induced neuronal plasticity in older adults and thus might help to develop tDCS interventions tailored to the aging brain.SIGNIFICANCE STATEMENT Transcranial direct current stimulation (tDCS) modulates human behavior, neuronal patterns, and metabolite concentrations, with exciting potential for neurorehabilitation. However, the understanding of tDCS-induced alterations on the neuronal level is incomplete, and conclusions from young adults cannot be easily transferred to older populations. We used a systematic multimodal imaging approach to investigate the neurophysiological effects of tDCS in older adults and found stimulation-induced effects on GABA levels, reflecting augmented local plasticity and functional connectivity, suggesting modulation of network efficiency. Our findings may help to reconcile some of the recent reports on the variability of tDCS-induced effects, not only implicating age as a crucial modulating factor, but detailing its specific impact on the functionality of neural networks.

Non-Invasive Brain Stimulation to Enhance Post-Stroke Recovery

Brain plasticity after stroke remains poorly understood. Patients may improve spontaneously within the first 3 months and then more slowly in the coming year. The first day, decreased edema and reperfusion of the ischemic penumbra may possibly account for these phenomena, but the improvement during the next weeks suggests plasticity phenomena and cortical reorganization of the brain ischemic areas and of more remote areas. Indeed, the injured ischemic motor cortex has a reduced cortical excitability at the acute phase and a suspension of the topographic representation of affected muscles, whereas the contralateral motor cortex has an increased excitability and an enlarged somatomotor representation; furthermore, contralateral cortex exerts a transcallosal interhemispheric inhibition on the ischemic cortex. This results from the imbalance of the physiological reciprocal interhemispheric inhibition of each hemisphere on the other, contributing to worsening of neurological deficit. Cortical excitability is measurable through transcranial magnetic stimulation (TMS) and prognosis has been established according to the presence of motor evoked potentials (MEP) at the acute phase of stroke, which is predictive of better recovery. Conversely, the lack of response to early stimulation is associated with a poor functional outcome. Non-invasive stimulation techniques such as repetitive TMS (rTMS) or transcranial direct current stimulation (tDCS) have the potential to modulate brain cortical excitability with long lasting effects. In the setting of cerebrovascular disease, around 1000 stroke subjects have been included in placebo-controlled trials so far, most often with an objective of promoting motor recovery of the upper limb. High frequency repetitive stimulation (>3 Hz) rTMS, aiming to increase excitability of the ischemic cortex, or low frequency repetitive stimulation (≤1 Hz), aiming to reduce excitability of the contralateral homonymous cortex, or combined therapies, have shown various effects on the functional disability score and neurological scales of treated patients and on the duration of the treatment. We review here the patients’ characteristics and parameters of stimulation that could predict a good response, as well as safety issues. At last, we review what we have learnt from experimental studies and discuss potential directions to conduct future studies.