The Facilitative Effect of Transcranial Direct Current Stimulation on Visuospatial Working Memory in Patients with Diabetic Polyneuropathy: A Pre-post Sham-Controlled Study

Effect of tele-yoga on aerobic capacity, respiratory muscle strength, and cognitive performance in patients with obstructive sleep apnea syndrome

BACKGROUND

Obstructive sleep apnea syndrome (OSAS) causes a decrease in aerobic capacity, respiratory muscle strength, and cognitive function, negatively affecting health-related quality of life.

OBJECTIVES

This study aimed to compare aerobic capacity, respiratory muscle strength, cognitive performance, functional capacity, sleep quality and daytime sleepiness in OSAS patients practicing and not practicing tele-yoga (TY).

METHODS

44 OSAS patients (40 M, 4F) were randomized into TY and control groups. TY group underwent live synchronous group-based TY sessions, 60 min/day, three days/week, for 12 weeks. Control group performed unsupervised thoracic expansion exercises at home 4 times daily for 12 weeks. The following were evaluated at baseline and at the end of the 6th and 12th weeks: inspiratory and expiratory respiratory muscle strength (MIP, MEP), cardiopulmonary exercise test, Corsi Block Tapping Test (CBTT) and Stroop TBAG test, six-minute walk test (6MWT), Pittsburgh Sleep Quality Index (PSQI) and Epworth Sleepiness Scale (ESS).

RESULTS

TY significantly improved MIP, and exercise test parameters (VE, HRmax,%HR, heart rate recovery in the first minute and RQmax), CBTT (forward) and Stroop TBAG test scores (parts of 3,4,5) compared to the controls (p<0.05). There were no significant changes 6MWT in the TY group compared to the control group (p > 0.05). Sleep duration (min), sleep efficiency, sleep quality of PSQI and ESS score improved significantly in the TY group compared to the control group (p < 0.05).

CONCLUSION

We suggest including TY intervention as a method of exercise in addition to CPAP treatments since it improves the health-related parameters of OSAS patients.

Cognitive function in diabetic persons with peripheral neuropathy: a systematic review and meta-analysis

OBJECTIVES

The aim of this study is to improve our knowledge of cognitive function in individuals with type 1 (T1DM) or type 2 (T2DM) diabetes mellitus and with peripheral diabetic neuropathy (DPN).

METHODS

A systematic review and meta-analysis was performed of publications included in PubMed, Scopus, PsycInfo and Web of Science databases until November 2021. The study protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO) with reference number: CRD42021229163.

RESULTS

A total of 832 articles were identified, 19 of which were selected. The presence of DPN was associated with global cognitive impairment in the T1DM persons in two studies (p=0.046; p=0.03) and T2DM persons in four (p<0.001; p<0.02; p=0.011; p=<0.05). Differences in specific dimensions - memory, attention, and psychomotor speed - were found in both kinds of diabetes. The meta-analysis showed that the individuals with T2DM and DPN presented a lower mean cognitive performance than those without DPN (-1.0448; 95% CI: -1.93%; -0.16%). Depression was associated with impaired cognitive function in these diabetic persons (p<0.01).

CONCLUSION

The review reveals the great variability in instruments and methodologies, while providing results that support the presence of both global and domain-specific cognitive impairment in diabetic persons with DPN.

Functional brain changes associated with cognitive trajectories determine specific tDCS-induced effects among older adults

The combination of transcranial direct current stimulation (tDCS) with functional magnetic resonance imaging (fMRI) can provide original data to investigate age-related brain changes. We examined neural activity modulations induced by two multifocal tDCS procedures based on two distinct montages fitting two N-back task-based fMRI patterns ("compensatory" and "maintenance") related to high working memory (WM) in a previous publication (Fernández-Cabello et al. Neurobiol Aging (2016);48:23-33). We included 24 participants classified as stable or decliners according to their 4-year WM trajectories following a retrospective longitudinal approach. Then, we studied longitudinal fMRI differences between groups (stable and decliners) and across multifocal tDCS montages ("compensatory" and "maintenance") applied using a single-blind sham-controlled cross-over design. Decliners evidenced over-activation of non-related WM areas after 4 years of follow-up. Focusing on tDCS effects, among the decliner group, the "compensatory"-tDCS montage reduced the activity over the posterior regions where these subjects showed longitudinal hyperactivation. These results reinforce the notion that tDCS effects are characterized by an activity reduction and might be more noticeable in compromised systems. Importantly, the data provide novel evidence that cognitive trajectories predict tDCS effects in older adults.

Frontoparietal theta tACS nonselectively enhances encoding, maintenance, and retrieval stages in visuospatial working memory

Neurobiological and cognitive evidence suggests that working memory is processed through three distinctive and well-characterized phases: encoding, maintenance, and retrieval. Several studies have reported that applying theta transcranial alternating current stimulation (tACS) to the right prefrontal and parietal cortices can significantly improve visual working memory performance. However, it remains unclear whether the facilitative effect of tACS on visual working memory is due to a domain-general or stage-specific process. In this study, we combined pre-task right frontoparietal theta tACS (6 Hz, 15 min) with a stage-specific change detection paradigm that provided retro-cues during various stages of working memory. This stage-specific tagging via the use of retro-cues enabled us to probe whether theta tACS would create a nonspecific/additive effect that is equal in magnitude across all cognitive stages or would create a stage-specific effect that is interactive with the retro-cue in a particular stage (e.g., maintenance, retrieval). We observed significant retro-cue and theta tACS effects on visual working memory performance, but no interaction between them. This finding suggests that the aforementioned two factors can facilitate visual working memory processing independently in an additive manner. Furthermore, low-performers benefited more from tACS, and their VWM deficit seemed to have originated from the second half of the memory retention stage, which possibly suggests faster memory decay as the key to poor VWM performance. Together, we conclude that frontoparietal theta tACS likely creates a domain-general boost in visual attention, which in turn benefits overall visual working memory processes that are not specific to the information maintenance or retrieval stages.

Predicting Working Memory Training Benefits From Transcranial Direct Current Stimulation Using Resting-State fMRI

The effects of transcranial direct current stimulation (tDCS) on working memory (WM) performance are promising but variable and contested. In particular, designs involving one session of tDCS are prone to variable outcomes with notable effects of individual differences. Some participants benefit, whereas others are impaired by the same tDCS protocol. In contrast, protocols including multiple sessions of tDCS more consistently report WM improvement across participants. The objective of the current project was to test whether differences in resting-state connectivity between stimulation site and two WM-relevant networks [default mode network (DMN) and central executive network (CEN)] could account for initial and longitudinal responses to tDCS. Healthy young adults completed 5 days of visual WM training during sham or anodal right frontal tDCS. The behavioral data showed that only the active tDCS group significantly improved over the visual WM training period. There were no significant correlations between initial response to tDCS and resting-state activity. DMN activity in the anterior cingulate cortex significantly correlated with WM training slope. These data underscore the importance of sampling in studies applying tDCS; homogeneity (e.g., of gender, special population, and WM capacity) may produce more consistent data in a single experiment with limited power, whereas heterogeneity is important in determining the mechanism(s) and potential for tDCS-linked protocols. This issue is a limitation in tDCS findings that continues to hamper its optimization and translational value.

Exploring the effects of Transcranial Direct Current Stimulation over the prefrontal cortex on working memory: A cluster analysis approach

BACKGROUND

The interest in the use of anodal transcranial direct current stimulation (tDCS) for the enhancement of cognitive functioning has increased significantly in recent years. However, the efficacy of this technique remains to be established.

OBJECTIVE

The current study explored the effects of anodal vs. sham tDCS over the left dorsolateral prefrontal cortex (DLPFC) during the performance of the digit span backwards task.

METHODS

30 healthy participants received 'offline' anodal tDCS (1.5 mA, 15 min) to the left DLPFC in an intra-individual, cross-over, sham-controlled experimental design. Memory span performance was assessed before (baseline), immediately after tDCS administration (T1) and 10 min post-T1 (T2). We applied cluster analysis in order to characterize individual responses to tDCS, and in order to identify naturally occurring subgroups that may be present.

RESULTS

Analysis of all the subjects showed that anodal tDCS failed to improve memory span performance. Cluster analysis revealed the presence of a subgroup of 'responders' that significantly improved their performance after anodal (vs. sham) tDCS in T1 (47%) and T2 (46%). However, there was no significant improvement in performance after anodal tDCS compared to the best baseline performance.

CONCLUSION

Our findings suggest that tDCS does not improve memory span performance and highlights the need for better ways to optimize methodological approaches in order to account for inter-individual variability and accurately assess the evidential value of tDCS-linked cognitive outcomes.

Slow periodic activity in the longitudinal hippocampal slice can self-propagate non-synaptically by a mechanism consistent with ephaptic coupling

KEY POINTS

Slow periodic activity can propagate with speeds around 0.1 m s-1 and be modulated by weak electric fields. Slow periodic activity in the longitudinal hippocampal slice can propagate without chemical synaptic transmission or gap junctions, but can generate electric fields which in turn activate neighbouring cells. Applying local extracellular electric fields with amplitude in the range of endogenous fields is sufficient to modulate or block the propagation of this activity both in the in silico and in the in vitro models. Results support the hypothesis that endogenous electric fields, previously thought to be too small to trigger neural activity, play a significant role in the self-propagation of slow periodic activity in the hippocampus. Experiments indicate that a neural network can give rise to sustained self-propagating waves by ephaptic coupling, suggesting a novel propagation mechanism for neural activity under normal physiological conditions.

ABSTRACT

Slow oscillations are a standard feature observed in the cortex and the hippocampus during slow wave sleep. Slow oscillations are characterized by low-frequency periodic activity (<1 Hz) and are thought to be related to memory consolidation. These waves are assumed to be a reflection of the underlying neural activity, but it is not known if they can, by themselves, be self-sustained and propagate. Previous studies have shown that slow periodic activity can be reproduced in the in vitro preparation to mimic in vivo slow oscillations. Slow periodic activity can propagate with speeds around 0.1 m s-1 and be modulated by weak electric fields. In the present study, we show that slow periodic activity in the longitudinal hippocampal slice is a self-regenerating wave which can propagate with and without chemical or electrical synaptic transmission at the same speeds. We also show that applying local extracellular electric fields can modulate or even block the propagation of this wave in both in silico and in vitro models. Our results support the notion that ephaptic coupling plays a significant role in the propagation of the slow hippocampal periodic activity. Moreover, these results indicate that a neural network can give rise to sustained self-propagating waves by ephaptic coupling, suggesting a novel propagation mechanism for neural activity under normal physiological conditions.

Transcranial direct current stimulation over the right DLPFC selectively modulates subprocesses in working memory

Background

Working memory, as a complex system, consists of two independent components: manipulation and maintenance process, which are defined as executive control and storage process. Previous studies mainly focused on the overall effect of transcranial direct current stimulation (tDCS) on working memory. However, little has been known about the segregative effects of tDCS on the sub-processes within working memory.

Method

Transcranial direct current stimulation, as one of the non-invasive brain stimulation techniques, is being widely used to modulate the cortical activation of local brain areas. This study modified a spatial n-back experiment with anodal and cathodal tDCS exertion on the right dorsolateral prefrontal cortex (DLPFC), aiming to investigate the effects of tDCS on the two sub-processes of working memory: manipulation (updating) and maintenance. Meanwhile, considering the separability of tDCS effects, we further reconfirmed the causal relationship between the right DLPFC and the sub-processes of working memory with different tDCS conditions.

Results

The present study showed that cathodal tDCS on the right DLPFC selectively improved the performance of the modified 2-back task in the difficult condition, whereas anodal tDCS significantly reduced the performance of subjects and showed an speeding-up tendency of response time. More precisely, the results of discriminability index and criterion showed that only cathodal tDCS enhanced the performance of maintenance in the difficult condition. Neither of the two tDCS conditions affected the performance of manipulation (updating).

Conclusion

These findings provide evidence that cathodal tDCS of the right DLPFC selectively affects maintenance capacity. Besides, cathodal tDCS also serves as an interference suppressor to reduce the irrelevant interference, thereby indirectly improving the working memory capacity. Moreover, the right DLPFC is not the unique brain regions for working memory manipulation (updating).

Neuroprotective effect of phosphocreatine on oxidative stress and mitochondrial dysfunction induced apoptosis in vitro and in vivo: Involvement of dual PI3K/Akt and Nrf2/HO-1 pathways

Methylglyoxal (MGO), an active metabolite of glucose, is observed in high levels in the tissues and blood of diabetic patients. Phosphocreatine (PCr), a high-energy phosphate compound, exhibits a range of pharmacological actions but little is well known of its neuroprotective action. The aim of the present study was to investigate the neuroprotective effects and the possible mechanisms of PCr. Diabetes is closely associated with neurodegenerative diseases, leading not only to the peripheral nervous system (PNS) and but also to central nervous system (CNS) damage. Therefore, we established two rat models of diabetes in vivo induced by MGO and streptozocin (STZ) respectively, while utilized differentiated PC-12 cells in vitro. Treatment of PC-12 cells with PCr markedly attenuated MGO-induced change of viability, apoptosis, accompanied by decreased levels of caspase-3, casapse-9 and Bcl-2/Bax protein ratio. Determination of cellular respiratory function was performed with intact PC-12 cells and homogenized hippocampal neuron tissue of rat. Reactive oxygen species (ROS) generation was assessed by membrane permeable fluorescent probe DCFH-DA. The expressions of Akt, Nrf2 and HO-1 were examined by Western blot. PCr pretreatment significantly reduced oxidative stress-induced high LDH, MDA level, and ROS production of PC-12 cells. PCr pretreatment also significantly decreased mitochondrial dysfunction in vitro and in vivo. In addition, PCr pretreatment increased the expression of p-Akt, Nrf2 and HO-1, and reduced the apoptosis. Moreover, the expression of Cleaved caspase3 was partially increased and the p-Akt, Nrf2 and HO-1 was partially reduced by a PI3K inhibitor (LY294002). While, compared with LY294002 groups, pre-treatment with PCr at the concentrations of 20 mM significantly reduced the expression of Cleaved caspase3 and increased the expression of p-Akt, Nrf2 and HO-1. Molecular docking assay showed that PCr possessed powerful affinity towards to Akt with lower binding energy. In conclusion, the neuroprotective effects of PCr in vitro and in vivo rely on normalizing mitochondrial function and reducing oxidative stress via Akt mediated Nrf2/HO-1 pathway, suggesting that PCr may be a novel therapeutic candidate for the treatment of diabetes-associated neurodegenerative diseases.

Frontoparietal tDCS Benefits Visual Working Memory in Older Adults With Low Working Memory Capacity

Working memory (WM) permits maintenance of information over brief delays and is an essential executive function. Unfortunately, WM is subject to age-related decline. Some evidence supports the use of transcranial direct current stimulation (tDCS) to improve visual WM. A gap in knowledge is an understanding of the mechanism characterizing these tDCS linked effects. To address this gap, we compared the effects of two tDCS montages designed on visual working memory (VWM) performance. The bifrontal montage was designed to stimulate the heightened bilateral frontal activity observed in aging adults. The unilateral frontoparietal montage was designed to stimulate activation patterns observed in young adults. Participants completed three sessions (bilateral frontal, right frontoparietal, sham) of anodal tDCS (20 min, 2 mA). During stimulation, participants performed a visual long-term memory (LTM) control task and a visual WM task. There was no effect of tDCS on the LTM task. Participants receiving right unilateral tDCS showed a WM benefit. This pattern was most robust in older adults with low WM capacity. To address the concern that the key difference between the two tDCS montages could be tDCS over the posterior parietal cortex (PPC), we included new analyses from a previous study applying tDCS targeting the PPC paired with a recognition VWM task. No significant main effects were found. A subsequent experiment in young adults found no significant effect of either tDCS montage on either task. These data indicate that tDCS montage, age and WM capacity should be considered when designing tDCS protocols. We interpret these findings as suggestive that protocols designed to restore more youthful patterns of brain activity are superior to those that compensate for age-related changes.

No Effects of Stimulating the Left Ventrolateral Prefrontal Cortex with tDCS on Verbal Working Memory Updating

The effects of transcranial direct current stimulation (tDCS) on dorsolateral prefrontal cortex functions, such as working memory (WM), have been examined in a number of studies. However, much less is known about the behavioral effects of tDCS over other important WM-related brain regions, such as the ventrolateral prefrontal cortex (VLPFC). In a counterbalanced within-subjects design with 33 young healthy participants, we examined whether online and offline single-session tDCS over VLPFC affects WM updating performance as measured by a digit 3-back task. We compared three conditions: anodal, cathodal and sham. We observed no significant tDCS effects on participants' accuracy or reaction times during or after the stimulation. Neither did we find any differences between anodal and cathodal stimulation. Largely similar results were obtained when comparing subgroups of high- and low-performing participants. Possible reasons for the lack of effects, including individual differences in responsiveness to tDCS, features of montage, task and sample characteristics, and the role of VLPFC in WM, are discussed.

Clinical utility and tolerability of transcranial direct current stimulation in mild cognitive impairment

BACKGROUND

Neuromodulatory interventions like transcranial direct current stimulation (tDCS) is emerging as a potential therapeutic strategy to promote cognitive function in healthy and pathological aging. There is need for more studies evaluating the utility and tolerability of tDCS in Mild cognitive impairment (MCI). Since MCI is considered as the prodromal stage of dementia, it has emerged as the most important target for intervention in dementia.

OBJECTIVE AND METHOD

This study investigated the feasibility, tolerability and clinical utility of tDCS in patients with MCI. In this observational study that included 11 patients with MCI, tDCS with an intensity of 2mA and duration of 20minutes per day was administered for 5 consecutive days with anode over left dorsolateral prefrontal cortex (DLPFC) and cathode over right supra orbital region. Treatment outcome was measured using picture memory impairment test (PMIT) immediately and also 1 month after the 5th session of tDCS RESULTS: All the patients tolerated tDCS sessions without any significant adverse effects. Stimulation of left DLPFC with tDCS was noted to significantly improve the immediate and delayed recall performance of the patients in PMIT after five days of stimulation and most of the benefits were persistent at one month follow up.

CONCLUSION

This study findings suggests that tDCS is safe and potentially beneficial in combating cognitive deficits in patients with MCI and provides a framework for further studies with better methodology (randomized and sham controlled trial) to investigate the same.

Decision-making in primary onset middle-age type 2 diabetes mellitus: a BOLD-fMRI study

Although type 2 diabetes mellitus (T2DM) is a well-recognized risk factor for dementia, the neural mechanisms that underlying cognitive impairment in T2DM remain unclear. We used functional magnetic resonance imaging (fMRI) during a computerized version of the Iowa Gambling Task to investigate the neural basis of decision making at the initial onset stage of T2DM. Eighteen newly diagnosed middle-aged T2DM patients, with no previous diabetic treatment history, and 18 matched controls were recruited. Results indicated that T2DM patients made more disadvantageous decisions than controls. Compared to healthy subjects, T2DM patients showed decreased activation in the ventral medial prefrontal cortex (VMPFC), orbitofrontal cortex (OFC) and anterior cingulate cortex, and increased activity in the dorsolateral prefrontal cortex, posterior cingulate cortex, insula and occipital lobes. IGT performance positively correlated with changes in brain activation in the VMPFC and OFC in both groups. Moreover, poor glycemic control was associated with decision-making function both in behavioral and brain activity in the VMPFC and OFC in patients. Conclusively, T2DM patients may suffer from weaknesses in their prefrontal cortex functions that lead to poorer decision-making under ambiguity, at least as assessed by the IGT.

Repeated transcranial direct current stimulation improves cognitive dysfunction and synaptic plasticity deficit in the prefrontal cortex of streptozotocin-induced diabetic rats

BACKGROUND

Cognitive dysfunction is commonly observed in diabetic patients. We have previously reported that anodal transcranial direct current stimulation (tDCS) over the dorsolateral prefrontal cortex can facilitate visuospatial working memory in diabetic patients with concomitant diabetic peripheral neuropathy and mild cognitive impairment, but the underlying mechanisms remain unclear.

OBJECTIVE

We investigated the cellular mechanisms underlying the effect of tDCS on cognitive decline in streptozotocin (STZ)-induced diabetic rats.

METHODS

STZ-induced diabetic rats were subjected to either repeated anodal tDCS or sham stimulation over the medial prefrontal cortex (mPFC). Spatial working memory performance in delayed nonmatch-to-place T maze task (DNMT), the induction of long-term potentiation (LTP) in the mPFC, and dendritic morphology of Golgi-stained pyramidal neurons in the mPFC were assessed.

RESULTS

Repeated applications of prefrontal anodal tDCS improved spatial working memory performance in DNMT and restored the impaired mPFC LTP of diabetic rats. The mPFC of tDCS-treated diabetic rats exhibited higher levels of brain-derived neurotrophic factor (BDNF) protein and N-Methyl-d-aspartate receptor (NMDAR) subunit mRNA and protein compared to sham stimulation group. Furthermore, anodal tDCS significantly increased dendritic spine density on the apical dendrites of mPFC layer V pyramidal cells in diabetic rats, whereas the complexity of basal and apical dendritic trees was unaltered.

CONCLUSIONS

Our findings suggest that repeated anodal tDCS may improve spatial working memory performance in streptozotocin-induced diabetic rats through augmentation of synaptic plasticity that requires BDNF secretion and transcription/translation of NMDARs in the mPFC, and support the therapeutic potential of tDCS for cognitive decline in diabetes mellitus patients.

Elucidating and Modulating the Neural Correlates of Visuospatial Working Memory via Noninvasive Brain Stimulation

Visuospatial working memory refers to the short-term memory mechanism that enables humans to remember visual information across visual blackout periods such as eyeblinks or eye movements. In recent years, neuroscientific studies have made great progress in uncovering the brain regions that support visuospatial working memory. In this review, we focus on the role of the posterior parietal cortex in forming and maintaining visual information, and use it as an example to highlight how noninvasive brain-stimulation techniques, particularly transcranial magnetic, direct current, and alternating current stimulation, can shed light on this topic because of their unique strengths in modulating brain activities.

Individual Differences and State-Dependent Responses in Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) has been extensively used to examine whether neural activities can be selectively increased or decreased with manipulations of current polarity. Recently, the field has reevaluated the traditional anodal-increase and cathodal-decrease assumption due to the growing number of mixed findings that report the effects of the opposite directions. Therefore, the directionality of tDCS polarities and how it affects each individual still remain unclear. In this study, we used a visual working memory (VWM) paradigm and systematically manipulated tDCS polarities, types of different independent baseline measures, and task difficulty to investigate how these factors interact to determine the outcome effect of tDCS. We observed that only low-performers, as defined by their no-tDCS corsi block tapping (CBT) performance, persistently showed a decrement in VWM performance after anodal stimulation, whereas no tDCS effect was found when participants were divided by their performance in digit span. In addition, only the optimal level of task difficulty revealed any significant tDCS effect. All these findings were consistent across different blocks, suggesting that the tDCS effect was stable across a short period of time. Lastly, there was a high degree of intra-individual consistency in one’s responsiveness to tDCS, namely that participants who showed positive or negative effect to anodal stimulation are also more likely to show the same direction of effects for cathodal stimulation. Together, these findings imply that tDCS effect is interactive and state dependent: task difficulty and consistent individual differences modulate one’s responsiveness to tDCS, while researchers’ choices of independent behavioral baseline measures can also critically affect how the effect of tDCS is evaluated. These factors together are likely the key contributors to the wide range of “noises” in tDCS effects between individuals, between stimulation protocols, and between different studies in the literature. Future studies using tDCS, and possibly tACS, should take such state-dependent condition in tDCS responsiveness into account.