Effects of anodal transcranial direct current stimulation on motor evoked potentials variability in humans

Medial Prefrontal Cortex Stimulation Reduces Retrieval-Induced Forgetting via Fronto-parietal Beta Desynchronization

The act of recalling memories can paradoxically lead to the forgetting of other associated memories, a phenomenon known as retrieval-induced forgetting (RIF). Inhibitory control mechanisms, primarily mediated by the prefrontal cortex, are thought to contribute to RIF. In this study, we examined whether stimulating the medial prefrontal cortex (mPFC) with transcranial direct current stimulation modulates RIF and investigated the associated electrophysiological correlates. In a randomized study, 50 participants (27 males and 23 females) received either real or sham stimulation before performing retrieval practice on target memories. After retrieval practice, a final memory test to assess RIF was administered. We found that stimulation selectively increased the retrieval accuracy of competing memories, thereby decreasing RIF, while the retrieval accuracy of target memories remained unchanged. The reduction in RIF was associated with a more pronounced beta desynchronization within the left dorsolateral prefrontal cortex (left-DLPFC), in an early time window (<500 ms) after cue onset during retrieval practice. This led to a stronger beta desynchronization within the parietal cortex in a later time window, an established marker for successful memory retrieval. Together, our results establish the causal involvement of the mPFC in actively suppressing competing memories and demonstrate that while forgetting arises as a consequence of retrieving specific memories, these two processes are functionally independent. Our findings suggest that stimulation potentially disrupted inhibitory control processes, as evidenced by reduced RIF and stronger beta desynchronization in fronto-parietal brain regions during memory retrieval, although further research is needed to elucidate the specific mechanisms underlying this effect.

Cortical excitability and multifidus activation responses to transcranial direct current stimulation in patients with chronic low back pain during remission

Evidence indicates that patients with chronic low back pain (CLBP) have lumbar multifidus muscle (LM) activation deficit which might be caused by changes in cortical excitability. Anodal transcranial direct current stimulation (a-tDCS) can be used to restore cortical excitability. This study aimed to (1) determine the immediate effects of a-tDCS on the cortical excitability and LM activation and (2) explore the relationship between cortical excitability and LM activation. Thirteen participants with CLBP during remission and 11 healthy participants were recruited. Cortical excitability (peak-to-peak motor evoked potential amplitude; P2P and cortical silent period; CSP) and LM activation were measured at pre- and post-intervention. We found significant difference (P < 0.05) in P2P between groups. However, no significant differences (P > 0.05) in P2P, CSP and LM activation were found between pre- and post-intervention in CLBP. The CLBP group demonstrated significant correlation (P = 0.05) between P2P and LM activation. Although our finding demonstrates change in P2P in the CLBP group, one-session of a-tDCS cannot induce changes in cortical excitability and LM activation. However, moderate to strong correlation between P2P and LM activation suggests the involvement of cortical level in LM activation deficit. Therefore, non-significant changes could have been due to inadequate dose of a-tDCS.

A blended neurostimulation protocol to delineate cortico-muscular and spino-muscular dynamics following neuroplastic adaptation

In this paper we propose a novel neurostimulation protocol that provides an intervention-based assessment to distinguish the contributions of different motor control networks in the cortico-spinal system. Specifically, we use a combination of non-invasive brain stimulation and neuromuscular stimulation to probe neuromuscular system behavior with targeted impulse-response system identification. In this protocol, we use an in-house developed human-machine interface (HMI) for an isotonic wrist movement task, where the user controls a cursor on-screen. During the task, we generate unique motor evoked potentials based on triggered cortical or spinal level perturbations. Externally applied brain-level perturbations are triggered through TMS to cause wrist flexion/extension during the volitional task. The resultant contraction output and related reflex responses are measured by the HMI. These movements also include neuromodulation in the excitability of the brain-muscle pathway via transcranial direct current stimulation. Colloquially, spinal-level perturbations are triggered through skin-surface neuromuscular stimulation of the wrist muscles. The resultant brain-muscle and spinal-muscle pathways perturbed by the TMS and NMES, respectively, demonstrate temporal and spatial differences as manifested through the human-machine interface. This then provides a template to measure the specific neural outcomes of the movement tasks, and in decoding differences in the contribution of cortical- (long-latency) and spinal-level (short-latency) motor control. This protocol is part of the development of a diagnostic tool that can be used to better understand how interaction between cortical and spinal motor centers changes with learning, or injury such as that experienced following stroke.

Safety and feasibility of transcranial direct current stimulation stratified by corticospinal organization in children with hemiparesis

Children with hemiparesis (CWH) due to stroke early in life face lifelong impairments in motor function. Transcranial direct current stimulation (tDCS) may be a safe and feasible adjuvant therapy to augment rehabilitation. Given the variability in outcomes following tDCS, tailored protocols of tDCS are required. We evaluated the safety, feasibility, and preliminary effects of a single session of targeted anodal tDCS based on individual corticospinal tract organization on corticospinal excitability. Fourteen CWH (age = 13.8 ± 3.63) were stratified into two corticospinal organization subgroups based on transcranial magnetic stimulation (TMS)-confirmed motor evoked potentials (MEP): ipsilesional MEP presence (MEPIL+) or absence (MEPIL-). Subgroups were randomized to real anodal or sham tDCS (1.5 mA, 20 min) applied to the ipsilesional (MEPIL + group) or contralesional (MEPIL- group) hemisphere combined with hand training. Safety was assessed with questionnaires and motor function evaluation, and corticospinal excitability was assessed at baseline and every 15 min for 1 h after tDCS. No serious adverse events occurred and anticipated minor side effects were reported and were self-limiting. Six of 14 participants had consistent ipsilesional MEPs (MEPIL + group). Paretic hand MEP amplitude increased in 5/8 participants who received real anodal tDCS to either the ipsilesional or contralesional hemisphere (+80% change). Application of tDCS based on individual corticospinal organization was safe and feasible with expected effects on excitability, indicating the potential for tailored tDCS protocols for CWH. Additional research involving expanded experimental designs is needed to confirm these effects and to determine if this approach can be translated into a clinically relevant intervention.

The effects of aerobic exercise and transcranial direct current stimulation on cognitive function in older adults with and without cognitive impairment: A systematic review and meta-analysis

BACKGROUND

Aerobic exercise (AE) may slow age-related cognitive decline. However, such cognition-sparing effects are not uniform across cognitive domains and studies. Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation and is also emerging as a potential alternative to pharmaceutical therapies. Like AE, the effectiveness of tDCS is also inconsistent for reducing cognitive impairment in ageing. The unexplored possibility exists that pairing AE and tDCS could produce synergistic effects and reciprocally augment cognition-improving effects in older individuals with and without cognitive impairments. Previous research found such synergistic effects on cognition when cognitive training is paired with tDCS in older individuals with and without mild cognitive impairment (MCI) or dementia.

AIM

The purpose of this systematic review with meta-analysis was to explore if pairing AE with tDCS could augment singular effects of AE and tDCS on global cognition (GC), working memory (WM) and executive function (EF) in older individuals with or without MCI and dementia.

METHODS

Using a PRISMA-based systematic review, we compiled studies that examined the effects of AE alone, tDCS alone, and AE and tDCS combined on cognitive function in older individuals with and without mild cognitive impairment (MCI) or dementia. Using a PICOS approach, we systematically searched PubMed, Scopus and Web of Science searches up to December 2021, we focused on 'MoCA', 'MMSE', 'Mini-Cog' (measures) and 'cognition', 'cognitive function', 'cognitive', 'cognitive performance', 'executive function', 'executive process', 'attention', 'memory', 'memory performance' (outcome terms). We included only randomized controlled trials (RTC) in humans if available in English full text over the past 20 years, with participants' age over 60. We assessed the methodological quality of the included studies (RTC) by the Physiotherapy Evidence Database (PEDro) scale.

RESULTS

Overall, 68 studies were included in the meta-analyses. AE (ES = 0.56 [95% CI: 0.28-0.83], p = 0.01) and tDCS (ES = 0.69 [95% CI: 0.12-1.26], p = 0.02) improved GC in all three groups of older adults combined (healthy, MCI, demented). In healthy population, AE improved GC (ES = 0.46 [95% CI: 0.22-0.69], p = 0.01) and EF (ES = 0.27 [95% CI: 0.05-0.49], p = 0.02). AE improved GC in older adults with MCI (ES = 0.76 [95% CI: 0.21-1.32], p = 0.01). tDCS improved GC (ES = 0.69 [90% CI: 0.12-1.26], p = 0.02), all three cognitive function (GC, WM and EF) combined in older adults with dementia (ES = 1.12 [95% CI: 0.04-2.19], p = 0.04) and improved cognitive function in older adults overall (ES = 0.69 [95% CI: 0.20-1,18], p = 0.01).

CONCLUSION

Our systematic review with meta-analysis provided evidence that beyond the cardiovascular and fitness benefits of AE, pairing AE with tDCS may have the potential to slow symptom progression of cognitive decline in MCI and dementia. Future studies will examine the hypothesis of this present review that a potentiating effect would incrementally improve cognition with increasing severity of cognitive impairment.

Comparing amplitudes of transcranial direct current stimulation (tDCS) to the sensorimotor cortex during swallowing

PURPOSE

Transcranial direct current stimulation (tDCS) can alter cortical excitability, making it a useful tool for promoting neuroplasticity in dysphagia rehabilitation. Clinical trials show functional improvements in swallowing following anodal tDCS despite varying dosing parameters and outcomes. The aim of the current study was to determine the most effective amplitude criterion (e.g., 0 mA [sham/control], 1 mA, 2 mA) of anodal tDCS for upregulating the swallowing sensorimotor cortex.

METHOD

As a novel paradigm, tDCS, functional near-infrared spectroscopy (fNIRS), and surface electromyography (sEMG) were simultaneously administered while participants completed a swallowing task. This allowed for measurement of the cortical hemodynamic response and submental muscle contraction before, during, and after tDCS. At the conclusion of the study, participants were asked to rate their level of discomfort associated with tDCS using a visual analog scale.

RESULTS

There was no significant difference in the hemodynamic response by time or amplitude. However, post-hoc analyses indicated that in the post-stimulation period, changes to the hemodynamic response in the left (stimulated) hemisphere were significantly different for the groups receiving 1 mA and 2 mA of tDCS compared to baseline. Participants receiving 1 mA of tDCS demonstrated reduced hemodynamic response. There was no significant difference in submental muscle contraction during or after tDCS regardless of amplitude. Anodal tDCS was well tolerated in healthy adults with no difference among participant discomfort scores across tDCS amplitude.

CONCLUSIONS

During a swallowing task, healthy volunteers receiving 1 mA of anodal tDCS demonstrated a suppressed hemodynamic response during and after stimulation whereas those receiving 2 mA of anodal tDCS had an increase in the hemodynamic response. tDCS remains a promising tool in dysphagia rehabilitation, but dosing parameters require further clarification.

Anodal Transcranial Direct Current Stimulation Could Modulate Cortical Excitability and the Central Cholinergic System in Akinetic Rigid-Type Parkinson's Disease: Pilot Study

Background

Transcranial direct current stimulation (tDCS) is a non-invasive technique that has been widely studied as an alternative treatment for Parkinson's disease (PD). However, its clinical benefit remains unclear. In this study, we aimed to investigate the effect of tDCS on the central cholinergic system and cortical excitability in mainly akinetic rigid-type patients with PD.

Methods

In total, 18 patients with PD were prospectively enrolled and underwent 5 sessions of anodal tDCS on the M1 area, which is on the contralateral side of the dominant hand. We excluded patients with PD who had evident resting tremor of the hand to reduce the artifact of electrophysiologic findings. We compared clinical scales reflecting motor, cognitive, and mood symptoms between pre- and post-tDCS. Additionally, we investigated the changes in electrophysiologic parameters, such as short latency afferent inhibition (SAI) (%), which reflects the central cholinergic system.

Results

The United Parkinson's Disease Rating Scale Part 3 (UPDRS-III), the Korean-Montreal Cognitive Assessment (MoCA-K), and Beck Depression Inventory (BDI) scores were significantly improved after anodal tDCS (p < 0.01, p < 0.01, and p < 0.01). Moreover, motor evoked potential amplitude ratio (MEPAR) (%) and integrated SAI showed significant improvement after tDCS (p < 0.01 and p < 0.01). The mean values of the change in integrated SAI (%) were significantly correlated with the changes in UPDRS-III scores; however, the MoCA-K and BDI scores did not show differences.

Conclusions

Anodal tDCS could influence the central cholinergic system, such as frontal cortical excitability and depression in PD. This mechanism could underlie the clinical benefit of tDCS in patients with PD.

Altered Brain Function and Causal Connectivity Induced by Repetitive Transcranial Magnetic Stimulation Treatment for Major Depressive Disorder

Objective

Repetitive transcranial magnetic stimulation (rTMS) can effectively improve depression symptoms in patients with major depressive disorder (MDD); however, its mechanism of action remains obscure. This study explored the neuralimaging mechanisms of rTMS in improving depression symptoms in patients with MDD.

Methods

In this study, MDD patients with first-episode, drug-naive (n = 29) and healthy controls (n = 33) were enrolled. Depression symptoms before and after rTMS treatment were assessed using the Hamilton Depression Rating Scale (HAMD-17). Resting-state functional magnetic resonance imaging (rs-fMRI) data were collected both before and after the treatment. Changes in the brain function after the treatment were compared using the following two indices: the amplitude of the low-frequency fluctuation (ALFF) and regional homogeneity (ReHo), which are sensitive for evaluating spontaneous neuronal activity. The brain region with synchronous changes was selected as the seed point, and the differences in the causal connectivity between the seed point and whole brain before and after rTMS treatment were investigated via Granger causality analysis (GCA).

Results

Before treatment, patients with MDD had significantly lower ALFF in the left superior frontal gyrus (p < 0.01), higher ALFF in the left middle frontal gyrus and left precuneus (p < 0.01), and lower ReHo in the left middle frontal and left middle occipital gyri (p < 0.01) than the values observed in healthy controls. After the rTMS treatment, the ALFF was significantly increased in the left superior frontal gyrus (p < 0.01) and decreased in the left middle frontal gyrus and left precuneus (p < 0.01). Furthermore, ReHo was significantly increased in the left middle frontal and left middle occipital gyri (p < 0.01) in patients with MDD. Before treatment, GCA using the left middle frontal gyrus (the brain region with synchronous changes) as the seed point revealed a weak bidirectional causal connectivity between the middle and superior frontal gyri as well as a weak causal connectivity from the inferior temporal to the middle frontal gyri. After treatment, these causal connectivities were strengthened. Moreover, the causal connectivity from the inferior temporal gyrus to the middle frontal gyri negatively correlated with the total HAMD-17 score (r = −0.443, p = 0.021).

Conclusion

rTMS treatment not only improves the local neural activity in the middle frontal gyrus, superior frontal gyrus, and precuneus but also strengthens the bidirectional causal connectivity between the middle and superior frontal gyri and the causal connectivity from the inferior temporal to the middle frontal gyri. Changes in these neuroimaging indices may represent the neural mechanisms underlying rTMS treatment in MDD.

Clinical Trial Registration

This study was registered in the Chinese Clinical Trial Registry (Registration number: ChiCTR1800019761).

Improved Pre-attentive Processing With Occipital rTMS Treatment in Major Depressive Disorder Patients Revealed by MMN

OBJECTIVES

To investigate the improvement effect of occipital repetitive transcranial magnetic stimulation (rTMS) combined with escitalopram oxalate tablets on pre-attentive processing in patients with first-episode, medication-naive depression.

METHODS

Patients who were hospitalized between January and December 2019 were selected. They were randomly allocated to real occipital rTMS stimulation group with 27 cases receiving intermittent theta-burst (iTBS) and sham stimulation group with 24 cases over 20 days. The rTMS treatment target is located at the Oz point of the occipital region. Both groups took escitalopram oxalate tablets, and the average daily drug dose was 15.294 ± 5.041 mg. Hamilton Depression Rating Scale (HAMD) was used to assess the symptoms of depression before and after treatment, and mismatch negativity (MMN) was used to assess the improvement of pre-attentive processing before and after treatment.

RESULTS

After 20 days of treatment, the total score of HAMD (13.495 ± 3.700) in both groups was significantly lower than that before treatment [21.910 ± 3.841, F(1, 49) = 46, 3.690, p < 0.001]. After treatment, the latency of MMN in the real stimulation group (182.204 ± 31.878 ms) was significantly lower than that in the sham stimulation group (219.896 ± 42.634 ms, p < 0.001), and the amplitude of MMN in the real stimulation group (-7.107 ± 3.374 ms) was significantly higher than that in the sham stimulation group (-2.773 ± 3.7 32 ms, p < 0.001).

CONCLUSION

Occipital rTMS treatment can enhance the early therapeutic effect and effectively improve the pre-attentive processing of patients with depression and provide a scientific basis for the new target of rTMS therapy in clinical patients with depression.

Genetic Polymorphisms Do Not Predict Interindividual Variability to Cathodal Transcranial Direct Current Stimulation of the Primary Motor Cortex

Introduction: High variability between individuals (i.e., interindividual variability) in response to transcranial direct current stimulation (tDCS) has become a commonly reported issue in the tDCS literature in recent years. Inherent genetic differences between individuals have been proposed as a contributing factor to observed response variability. This study investigated whether tDCS interindividual variability was genetically mediated. Methods: A large sample size of 61 healthy males received cathodal tDCS (c-tDCS) and sham-tDCS of the primary motor cortex at 1 mA and 10 min via 6 × 4 cm active and 7 × 5 cm return electrodes. Corticospinal excitability (CSE) was assessed via 25 single-pulse transcranial magnetic stimulation motor-evoked potentials (MEPs). Intracortical inhibition was assessed via twenty-five 3 msec interstimulus interval (ISI) paired-pulse MEPs, known as short-interval intracortical inhibition (SICI). Intracortical facilitation (ICF) was assessed via twenty-five 10 msec ISI paired-pulse MEPs. Gene variants encoding for excitatory and inhibitory neuroreceptors were determined via saliva samples. Predetermined thresholds and statistical cluster analyses were used to subgroup individuals. Results: Two distinct subgroups were identified, "responders" reducing CSE following c-tDCS and "nonresponders" showing no reduction or even increase in CSE. Differences in CSE between responders and nonresponders following c-tDCS were not explained by changes in SICI or ICF. Conclusions: No significant relationships were reported between gene variants and interindividual variability to c-tDCS, suggesting that the chosen gene variants did not influence the activity of the neuroreceptors involved in eliciting changes in CSE in responders following c-tDCS. In this largest c-tDCS study of its kind, novel insights were reported into the contribution genetic factors may play in observed interindividual variability to c-tDCS. Impact statement This study adds insight into the issue of interindividual variability to c-tDCS. It highlights not all individuals respond to c-tDCS similarly when exposed to the same stimulus parameters. This disparity in response to c-tDCS between individuals does not appear to be genetically mediated. For c-tDCS to progress to large-scale clinical application, reliability, predictability and reproducibility are essential. Systematically investigating factors contributing to interindividual variability take steps towards this progress the c-tDCS field towards the potential development of screening tools to determine clinical suitability to c-tDCS to ensure its application in those who may benefit the most.

Comparative study of motor cortical excitability changes following anodal tDCS or high-frequency tRNS in relation to stimulation duration

BACKGROUND

In this study, we investigate the capacity of two different non-invasive brain stimulation (NIBS) techniques (anodal transcranial direct current stimulation (anodal tDCS) and high-frequency transcranial random noise stimulation (hf-tRNS)) regarding the relationship between stimulation duration and their efficacy in inducing long-lasting changes in motor cortical excitability.

METHODS

Fifteen healthy subjects attended six experimental sessions (90 experiments in total) and underwent both anodal tDCS of 7, 13, and 20 min duration, as well as high-frequency 1mA-tRNS of 7, 13, and 20 min stimulation duration. Sessions were performed in a randomized order and subjects were blinded to the applied methods.

RESULTS

For anodal tDCS, no significant stable increases of motor cortical excitability were observed for either stimulation duration. In contrast, for hf -tRNS a stimulation duration of 7 min resulted in a significant increase of motor cortical excitability lasting from 20 to 60 min poststimulation. While an intermediate duration of 13 min hf-tRNS failed to induce lasting changes in motor cortical excitability, a longer stimulation duration of 20 min hf-tRNS led only to significant increases at 50 min poststimulation which did not outlast until 60 min poststimulation.

CONCLUSION

Hf-tRNS for a duration of 7 min induced robust increases of motor cortical excitability, suggesting an indirect proportional relationship between stimulation duration and efficacy. While hf-tRNS appeared superior to anodal tDCS in this study, further systematic and randomized experiments are necessary to evaluate the generalizability of our observations and to address current intensity as a further modifiable contributor to the variability of transcranial brain stimulation.

Effects of anodal transcranial direct current stimulation on motor evoked potentials variability in humans

Motor evoked potentials (MEPs) obtained from transcranial magnetic stimulation (TMS) allow corticospinal excitability (CSE) to be measured in the human primary motor cortex (M1). CSE responses to transcranial direct current stimulation (tDCS) protocols are highly variable. Here, we tested the reproducibility and reliability of individual MEPs following a common anodal tDCS protocol. In this study, 32 healthy subjects received anodal tDCS stimulation over the left M1 for three durations (tDCS‐T5, tDCS‐T10, and tDCS‐T20 min) on separate days in a crossover‐randomized order. After the resting motor threshold (RMT) was determined for the contralateral first dorsal interosseous muscle, 15 single pulses 4–8 sec apart at an intensity of 120% RMT were delivered to the left M1 to determine the baseline MEP amplitude at T₀, T₅, T₁₀, T₂₀, T₃₀, T₄₀, T₅₀, and T₆₀ min after stimulation for each durations. During TMS delivery, 3D images of the participant's cortex and hot spot were visualized for obtaining MEPs from same position. Our findings revealed that there was a significant MEPs improvement at T₀ (P = 0.01) after 10 min of anodal stimulation. After the 20‐min stimulation duration, MEPs differed specifically at T_0, T_5, T₃₀ min (P < 0.05). This indicates that tDCS is a promising tool to improve MEPs. Our observed variability in response to the tDCS protocol is consistent with other noninvasive brain stimulation studies.

Effects of transcranial direct current stimulation on cortex modulation by stimulation of the primary motor cortex and parietal cortex in humans

AIM OF THE STUDY

Transcranial magnetic stimulation (TMS) is used to measure corticospinal excitability (CSE) from the primary motor cortex (M1) in humans through motor-evoked potentials (MEPs). The variability of CSE responses to transcranial direct current stimulation (tDCS) protocols is high and needs to be reproduced in the healthy population. The M1 and posterior parietal cortex (PPC) are anatomically and functionally connected and could play a role in understanding the variability in CSE responses. We tested the individual MEPs following a common cathodal (ctDCS) protocol over the M1 and PPC.

MATERIALS AND METHODS

Twenty-eight healthy subjects were randomized for a ctDCS stimulation over the left M1 and PPC for 20 min on a separate days. The first dorsal interosseous muscle (FDI) contralateral stimulation of the left M1 was used as the resting motor threshold (RMT), while 15 single pulses 4-8 s apart at an intensity of 120% RMT were used to determine the baseline MEP amplitude and at T0, 5, 10, 20, 30, 40, 50, and 60 min after ctDCS stimulation in both sessions.

RESULTS

A 20 min duration of ctDCS stimulation significantly deceased the CSE only at T0 (p = 0.046 at M1, p = 0.010 at PPC).

CONCLUSION

Our results suggested that PPC stimulation can modulate M1 excitability and PPC-M1 connectivity, but a significant effect is only observed immediately post ctDCS. The tDCS showed variability in response to the tDCS protocol is consistent with other non-invasive brain stimulation studies.

Neurodiagnostics in Sports: Investigating the Athlete's Brain to Augment Performance and Sport-Specific Skills

Enhancing performance levels of athletes during training and competition is a desired goal in sports. Quantifying training success is typically accompanied by performance diagnostics including the assessment of sports-relevant behavioral and physiological parameters. Even though optimal brain processing is a key factor for augmented motor performance and skill learning, neurodiagnostics is typically not implemented in performance diagnostics of athletes. We propose, that neurodiagnostics via non-invasive brain imaging techniques such as functional near-infrared spectroscopy (fNIRS) will offer novel perspectives to quantify training-induced neuroplasticity and its relation to motor behavior. A better understanding of such a brain-behavior relationship during the execution of sport-specific movements might help to guide training processes and to optimize training outcomes. Furthermore, targeted non-invasive brain stimulation such as transcranial direct current stimulation (tDCS) might help to further enhance training outcomes by modulating brain areas that show training-induced neuroplasticity. However, we strongly suggest that ethical aspects in the use of non-invasive brain stimulation during training and/or competition need to be addressed before neuromodulation can be considered as a performance enhancer in sports.