Dynamic changes of brain networks during standing balance control under visual conflict

Cortical adaptations in Tai Chi practitioners during sensory conflict: an EEG-based effective connectivity analysis of postural control

BACKGROUND

Tai Chi (TC) is recognized for enhancing balance and postural control. However, studies on its effects on the central nervous system are limited and often involve static experiments despite the dynamic nature of TC. This study addressed that gap by examining cortical network activity during dynamic, multisensory conflict balance tasks. We aimed to determine whether long-term TC practice leads to neuroplastic changes in brain connectivity that improve sensory integration for postural control.

METHODS

Fifty-two young adult participants (long-term TC practitioners = 22; non-practitioners = 30) performed balance tasks under sensory congruent and conflict conditions using a virtual reality headset with a rotating supporting surface. EEG was performed, and generalized partial directed coherence was used to assess directed functional connectivity in the mu rhythm (8-13 Hz) between predefined regions of interest (ROIs) in the cortex implicated in sensory and motor integration. Graph-theoretic measures (in-strength and out-strength) indexed the total incoming and outgoing connection strengths for each region. Statistical analysis used mixed-design ANOVAs (Group × Condition) to compare balance and connectivity measures.

RESULTS

TC practitioners demonstrated significantly better postural stability under both sensory conditions, with a reduced sway area. EEG analysis revealed that increased sensory conflict decreased the global efficiency of the visual integration network but increased that of the somatosensory integration network. Furthermore, TC practitioners demonstrated enhanced out-strength of the somatosensory cortex and lower out-strength of the right posterior parietal cortex (PPC) compared to non-practitioners.

CONCLUSIONS

Long-term TC practice is associated with quantifiable neuroplastic changes in mu-band cortical effective connectivity, specifically enhanced information outflow from somatosensory reduce parietal influence regions. Our findings demonstrate central mechanisms by which TC practice may improve balance, providing neuroengineering evidence for TC as a neuroplasticity-driven balance intervention.

Role of the Functional Gait Assessment in Validating Item Difficulty Hierarchy and Fall Risk for Idiopathic Normal Pressure Hydrocephalus

BACKGROUND

Falls along with gait-balance disturbances are important symptoms of idiopathic normal pressure hydrocephalus (iNPH).

OBJECTIVES

This study aimed to validate the Functional Gait Assessment (FGA) item-difficulty hierarchy, determine its optimal fall cutoff value, and identify key items for assessing fall risk in iNPH.

METHODS

One hundred eighty-eight iNPH patients underwent pre-intervention assessments using the FGA, timed up-and-go (TUG), 10-Meter Gait Speed (10MGS) tests, and their falls history in the past 6 months. Rasch analysis confirmed the validity of the FGA item-difficulty hierarchy and established the relationship between total scores and logit estimates. Logistic regression identified the optimal FGA cutoff value and key fall-related items.

RESULTS

The FGA's item-difficulty hierarchy was confirmed to range from item 5 ("gait and pivot turn") at -2.58 logits (easiest) to item 7 ("gait with narrow base of support") at 5.35 logits (most difficult). Total scores corresponded to logits from -10.39 (score 0) to 11.64 (score 30). The fall rate was 63.3%. The optimal FGA cutoff value was ≤15 points (area under curve [AUC]: 0.901, P < 0.001), outperforming TUG (AUC: 0.712) and 10MGS (AUC: 0.742). Key fall-related items (R2: 0.545, P < 0.001) were item 2 ("change in gait speed"), item 7 ("gait with narrow base of support"), item 1 ("gait on level surface"), item 10 ("stair steps"), and item 5 ("gait and pivot turn").

CONCLUSIONS

Our findings may facilitate early diagnosis and timely interventions, including tailored rehabilitation and fall prevention strategies, by leveraging the item difficulty and characteristics of the FGA for iNPH patients.

Cortical adaptations in regional activity and backbone network following short-term postural training with visual feedback for older adults

This study investigated cortical reorganization in older adults following short-term interactive balance training. Twenty participants aged 65-74 received training in stabilometer stance, visually aligning plate movement with a horizontal line on a monitor. Pre-test and post-test measured posture fluctuations and scalp EEG during stabilometer stance. Results showed a training-related decrease in root mean square (RMS) (p = 0.001) and an increase in mean frequency (p = 0.006) of posture fluctuations. Despite a decline in theta relative power in Fp1 (p = 0.027), stabilometer training led to a post-test increase in alpha relative power around electrodes of the ventral visual pathway (p = 0.002). Additionally, augmentations were noted in theta relative power in Tp8 (p = 0.033) and beta relative power in F7 (p = 0.039). Analysis of the minimum spanning tree (MST) of alpha inter-regional connectivity indicated a training-related decrease in leaf fraction (p = 0.011) and increase in average eccentricity (p = 0.041), respectively. Training-related changes in the RMS of posture fluctuation were positively correlated with changes in pooled alpha relative power in electrodes of the ventral visual pathway (r = 0.459, p = 0.042) and negatively correlated with changes in average eccentricity of the alpha MST network (r =  - 0.487, p = 0.029). In conclusion, short-term interactive training enhances balance by reorganizing regional and alpha-band network activities, which supports improved visual attention and prevents early visual processing idling during initial postural learning.

Adaptive changes in balance control strategies under continuous exposure to visual-somatosensory conflicts

Human postural control system has the capacity to adapt to balance-challenging perturbations. However, the characteristics and mechanisms of postural adaptation to continuous perturbation under the sensory conflicting environments remain unclear. We aimed to investigate the functional role of oscillatory coupling drive to lower-limb muscles with changes in balance control during postural adaptation under multisensory congruent and incongruent environments. We combined a platform moving sinusoidally (0.24 Hz) along the anterior-posterior (AP) axis and a virtual scene moving sinusoidally (0.24 Hz) either along the AP or the medio-lateral (ML) axis to present a 3-min visual-somatosensory congruent condition (n = 10) or incongruent condition (n = 12), respectively. We analyzed the kinematic data and performed intermuscular coherence analysis of surface EMG data from bilateral lower limbs. We found that the inter-limb coherence was larger under the congruent condition and decreased over the 3-min perturbation, while inter-limb coherence remained low and showed no changes under the incongruent condition over the 3-min perturbation. These results suggest that exposure to the incongruent condition disrupted inter-limb intermuscular coupling. Besides, we found the bilateral intra-limb coherence decreased over 3-min congruent and incongruent perturbation, with the bilateral ankle joint angular velocity decreased and the coupling strength (0.2-0.3 Hz) between whole body sway and sinusoidal stimuli in AP decreased. These findings suggest that continuous exposure to sinusoidal perturbation in AP under congruent and incongruent conditions decreased bilateral intermuscular coupling, contributing to flexibility in the sagittal plane. Overall, we suggested the postural control system adapts context specifically to different sensory environments, with distinct characteristics of neuromuscular control strategies.NEW & NOTEWORTHY Lower limb muscle coordination plays a vital role when facing continuous perturbation by updating sensorimotor mappings. However, it is unclear how muscle coordination adapts to visual-somatosensory congruent and incongruent perturbations. Here, we found that muscle coordination showed context-specific adaptive changes to visual-somatosensory congruent or incongruent environment.

Sensory reweighting and self-motion perception for postural control under single-sensory and multisensory perturbations in older Tai Chi practitioners

Purpose

Impairment in perception and sensory reweighting could predispose older adults to falls. This exploratory study aimed to investigate the differences in sensory reweighting and self-motion perception for postural control under single-sensory and multisensory perturbations between older Tai Chi (TC) practitioners and healthy active older controls.

Methods

Twenty-four TC practitioners and 23 age-matched non-practitioners were recruited in this observational study. Participants stood on a force plate with or without a foam surface (baseline), followed by 36 s of visual rotation stimuli, vestibular rotation stimuli, or reduced somatosensory input (adaptation), and then continued standing for 44 s (reintegration). The center-of-pressure (COP) trajectories and self-motion perception were recorded. COP signals were analyzed using traditional sway, multiscale entropy, and wavelet analysis methods and the time-window-of-integration model to examine the postural balance performance and the flexibility and speed of sensory reweighting.

Results

Significant interaction effects of group with sensory perturbation and group with time window on COP parameters were observed (p < 0.05). Compared with non-practitioners, TC practitioners exhibited higher postural stability and complexity as the difficulty of standing tasks increased and smaller time windows to return to baseline levels as disturbance time evolved. Moreover, TC practitioners exhibited significantly greater weighting on unperturbed sensory systems, lower weighting on perturbed sensory systems for postural control, and higher self-motion perception ability under visual, vestibular, and visual-vestibular perturbations (p < 0.05).

Conclusion

Long-term TC practitioners exhibited superior postural stability and adaptability under challenging sensory perturbations, and smaller amplitudes and shorter durations of postural aftereffects over time during adaptation and reintegration. These improvements may be partly attributed to more rapid and flexible sensory reweighting and improved self-motion perception for postural control.

Nonlinear dynamics of postural control system under visual-vestibular habituation balance practice: evidence from EEG, EMG and center of pressure signals

Human postural control system is inherently complex with nonlinear interaction among multiple subsystems. Accordingly, such postural control system has the flexibility in adaptation to complex environments. Previous studies applied complexity-based methods to analyze center of pressure (COP) to explore nonlinear dynamics of postural sway under changing environments, but direct evidence from central nervous system or muscular system is limited in the existing literature. Therefore, we assessed the fractal dimension of COP, surface electromyographic (sEMG) and electroencephalogram (EEG) signals under visual-vestibular habituation balance practice. We combined a rotating platform and a virtual reality headset to present visual-vestibular congruent or incongruent conditions. We asked participants to undergo repeated exposure to either congruent (n = 14) or incongruent condition (n = 13) five times while maintaining balance. We found repeated practice under both congruent and incongruent conditions increased the complexity of high-frequency (0.5-20 Hz) component of COP data and the complexity of sEMG data from tibialis anterior muscle. In contrast, repeated practice under conflicts decreased the complexity of low-frequency (<0.5 Hz) component of COP data and the complexity of EEG data of parietal and occipital lobes, while repeated practice under congruent environment decreased the complexity of EEG data of parietal and temporal lobes. These results suggested nonlinear dynamics of cortical activity differed after balance practice under congruent and incongruent environments. Also, we found a positive correlation (1) between the complexity of high-frequency component of COP and the complexity of sEMG signals from calf muscles, and (2) between the complexity of low-frequency component of COP and the complexity of EEG signals. These results suggested the low- or high-component of COP might be related to central or muscular adjustment of postural control, respectively.

Multisensory Conflict Impairs Cortico-Muscular Network Connectivity and Postural Stability: Insights from Partial Directed Coherence Analysis

Sensory conflict impacts postural control, yet its effect on cortico-muscular interaction remains underexplored. We aimed to investigate sensory conflict's influence on the cortico-muscular network and postural stability. We used a rotating platform and virtual reality to present subjects with congruent and incongruent sensory input, recorded EEG (electroencephalogram) and EMG (electromyogram) data, and constructed a directed connectivity network. The results suggest that, compared to sensory congruence, during sensory conflict: (1) connectivity among the sensorimotor, visual, and posterior parietal cortex generally decreases, (2) cortical control over the muscles is weakened, (3) feedback from muscles to the cortex is strengthened, and (4) the range of body sway increases and its complexity decreases. These results underline the intricate effects of sensory conflict on cortico-muscular networks. During the sensory conflict, the brain adaptively decreases the integration of conflicting information. Without this integrated information, cortical control over muscles may be lessened, whereas the muscle feedback may be enhanced in compensation.

The 'Postural Rhythm' of the Ground Reaction Force during Upright Stance and Its Conversion to Body Sway-The Effect of Vision, Support Surface and Adaptation to Repeated Trials

The ground reaction force (GRF) recorded by a platform when a person stands upright lies at the interface between the neural networks controlling stance and the body sway deduced from centre of pressure (CoP) displacement. It can be decomposed into vertical (VGRF) and horizontal (HGRF) vectors. Few studies have addressed the modulation of the GRFs by the sensory conditions and their relationship with body sway. We reconsidered the features of the GRFs oscillations in healthy young subjects (n = 24) standing for 90 s, with the aim of characterising the possible effects of vision, support surface and adaptation to repeated trials, and the correspondence between HGRF and CoP time-series. We compared the frequency spectra of these variables with eyes open or closed on solid support surface (EOS, ECS) and on foam (EOF, ECF). All stance trials were repeated in a sequence of eight. Conditions were randomised across different days. The oscillations of the VGRF, HGRF and CoP differed between each other, as per the dominant frequency of their spectra (around 4 Hz, 0.8 Hz and <0.4 Hz, respectively) featuring a low-pass filter effect from VGRF to HGRF to CoP. GRF frequencies hardly changed as a function of the experimental conditions, including adaptation. CoP frequencies diminished to <0.2 Hz when vision was available on hard support surface. Amplitudes of both GRFs and CoP oscillations decreased in the order ECF > EOF > ECS ≈ EOS. Adaptation had no effect except in ECF condition. Specific rhythms of the GRFs do not transfer to the CoP frequency, whereas the magnitude of the forces acting on the ground ultimately determines body sway. The discrepancies in the time-series of the HGRF and CoP oscillations confirm that the body's oscillation mode cannot be dictated by the inverted pendulum model in any experimental conditions. The findings emphasise the robustness of the VGRF "postural rhythm" and its correspondence with the cortical theta rhythm, shed new insight on current principles of balance control and on understanding of upright stance in healthy and elderly people as well as on injury prevention and rehabilitation.