Why we need to better understand the cortical neurophysiology of impaired postural responses with age, disease, or injury

Touch may reduce cognitive load during assisted typing by individuals with developmental disabilities

Many techniques have attempted to provide physical support to ease the execution of a typing task by individuals with developmental disabilities (DD). These techniques have been controversial due to concerns that the support provider's touch can influence the typed content. The most common interpretation of assisted typing as an ideomotor phenomenon has been qualified recently by studies showing that users with DD make identifiable contributions to the process. This paper suggests a neurophysiological pathway by which touch could lower the cognitive load of seated typing by people with DD. The required sensorimotor processes (stabilizing posture and planning and executing manual reaching movements) and cognitive operations (generating and transcribing linguistic material) place concurrent demands on cognitive resources, particularly executive function (EF). A range of developmental disabilities are characterized by deficits in sensorimotor and EF capacity. As light touch has been shown to facilitate postural coordination, it is proposed that a facilitator's touch could assist the seated typist with sensorimotor and EF deficits by reducing their sensorimotor workload and thereby freeing up shared cognitive resources for the linguistic elements of the task. This is the first theoretical framework for understanding how a facilitator's touch may assist individuals with DD to contribute linguistic content during touch-assisted typing.

Midfrontal theta dynamics index the monitoring of postural stability

Stepping is a common strategy to recover postural stability and maintain upright balance. Postural perturbations have been linked to neuroelectrical markers such as the N1 potential and theta frequency dynamics. Here, we investigated the role of cortical midfrontal theta dynamics of balance monitoring, driven by balance perturbations at different initial standing postures. We recorded electroencephalography, electromyography, and motion tracking of human participants while they stood on a platform that delivered a range of forward and backward whole-body balance perturbations. The participants' postural threat was manipulated prior to the balance perturbation by instructing them to lean forward or backward while keeping their feet-in-place in response to the perturbation. We hypothesized that midfrontal theta dynamics index the engagement of a behavioral monitoring system and, therefore, that perturbation-induced theta power would be modulated by the initial leaning posture and perturbation intensity. Targeted spatial filtering in combination with mixed-effects modeling confirmed our hypothesis and revealed distinct modulations of theta power according to postural threat. Our results provide novel evidence that midfrontal theta dynamics subserve action monitoring of human postural balance. Understanding of cortical mechanisms of balance control is crucial for studying balance impairments related to aging and neurological conditions (e.g. stroke).

A robust role for motor cortex

The role of motor cortex in non-primate mammals remains unclear. More than a century of stimulation, anatomical and electrophysiological studies has implicated neural activity in this region with all kinds of movement. However, following the removal of motor cortex, rats retain most of their adaptive behaviors, including previously learned skilled movements. Here we revisit these two conflicting views of motor cortex and present a new behavior assay, challenging animals to respond to unexpected situations while navigating a dynamic obstacle course. Surprisingly, rats with motor cortical lesions show clear impairments facing an unexpected collapse of the obstacles, while showing no impairment with repeated trials in many motor and cognitive metrics of performance. We propose a new role for motor cortex: extending the robustness of sub-cortical movement systems, specifically to unexpected situations demanding rapid motor responses adapted to environmental context. The implications of this idea for current and future research are discussed.

Quantitative Electroencephalography and Surface Electromyography Correlations upon Predictable and Unpredictable Perturbation in Older Adults

BACKGROUND

Quantitative Electroencephalography (qEEG) is a non-invasive method used to quantify electrical activity over the cortex. QEEG provides an accurate temporal resolution of the brain activity, making it a useful tool for assessing cortical function during challenging tasks.

OBJECTIVE

This study aimed to investigate postural adjustments in older adults in response to an external perturbation.

MATERIAL AND METHODS

In this observational study, nineteen healthy older adults were involved. A 32-channel qEEG was employed to track alterations in beta power on the electrodes over the two sensory-motor areas. Integrated electromyographic activity (IntEMG) of the leg muscles was evaluated in response to perturbations under predictable and unpredictable conditions.

RESULTS

The results indicated higher beta power during late-phase in the Cz electrode in both conditions. IntEMG was significantly greater in the tibialis anterior muscle during both conditions in the CPA epoch. In predictable condition, a positive correlation was found between the beta power over C₄ (r = 0.560, p = 0.013) and C₃ (r = 0.458, p = 0.048) electrodes and tibialis anterior muscle amplitude, and between beta power in C₄ and gastrocnemius amplitude (r = 0.525, p = 0.021). In unpredictable condition, there was a positive correlation between beta power over the C₄ and the tibialis anterior amplitude (r = 0.580, p = 0.009) and also it over the C₃ and the tibialis anterior amplitude (r = 0.452, p = 0.049).

CONCLUSION

Our findings demonstrate that sensorimotor processing occurs in the brain during response to perturbation. Furthermore, cortical activity appeared to be greatest during the recruitment of the muscles upon late-phase in older adults.

Cortical Activity During Postural Recovery in Response to Predictable and Unpredictable Perturbations in Healthy Young and Older Adults: A Quantitative EEG Assessment

Introduction:

To investigate the effects of predictable and unpredictable external perturbations on cortical activity in healthy young and older adults.

Methods:

Twenty healthy older and 19 healthy young adults were exposed to predictable and unpredictable external perturbations, and their cortical activity upon postural recovery was measured using a 32-channel quantitative encephalography. The absolute spectral power and coherence z-scores of cortical waves were analyzed through a 3-way mixed ANOVA.

Results:

During postural recovery from predictable perturbations, older adults exhibited higher frontoparietal beta power and higher alpha and beta coherence during the late-phase recovery than the young individuals. After unpredictable perturbations, the older group showed lower alpha power in the early phase and higher beta power in the late phase as compared to the young group. Results for the group × time and group × location interactions in the older group showed a higher alpha and beta coherence over the late phase, a higher alpha coherence in F3–P3 and F4–P4 regions, and a higher beta coherence in the F4–P4 region compared to the younger group.

Conclusion:

Our results revealed that the cortical activation after external perturbations increases with aging, particularly in frontoparietal areas. A shift from automatic (subcortical level) to attentional (cortical level) processing may reflect the contribution of attentional resources for postural recovery from an external threat in older individuals.

Quantitative evaluation of trunk function and the StartReact effect during reaching in patients with cervical and thoracic spinal cord injury

Objective.Impaired trunk stability is frequent in spinal cord injury (SCI), but there is a lack of quantitative measures for assessing trunk function. Our objectives were to: (a) evaluate trunk muscle activity and movement patterns during a reaching task in SCI patients, (b) compare the impact of cervical (cSCI) and thoracic (tSCI) injuries in trunk function, and (c) investigate the effects of a startling acoustic stimulus (SAS) in these patients.Approach.Electromyographic (EMG) and smartphone accelerometer data were recorded from 15 cSCI patients, nine tSCI patients, and 24 healthy controls, during a reaching task requiring trunk tilting. We calculated the response time (RespT) until pressing a target button, EMG onset latencies and amplitudes, and trunk tilt, lateral deviation, and other movement features from accelerometry. Statistical analysis was applied to analyze the effects of group (cSCI, tSCI, control) and condition (SAS, non-SAS) in each outcome measure.Main results.SCI patients, especially those with cSCI, presented significantly longer RespT and EMG onset latencies than controls. Moreover, in SCI patients, forward trunk tilt was accompanied by significant lateral deviation. RespT and EMG latencies were remarkably shortened by the SAS (the so-called StartReact effect) in tSCI patients and controls, but not in cSCI patients, who also showed higher variability.Significance. The combination of EMG and smartphone accelerometer data can provide quantitative measures for the assessment of trunk function in SCI. Our results show deficits in postural control and compensatory strategies employed by SCI patients, including delayed responses and higher lateral deviations, possibly to improve sitting balance. This is the first study investigating the StartReact responses in trunk muscles in SCI patients and shows that the SAS significantly accelerates RespT in tSCI, but not in cSCI, suggesting an increased cortical control exerted by these patients.

Reactive Postural Responses After Mild Traumatic Brain Injury and Their Association With Musculoskeletal Injury Risk in Collegiate Athletes: A Study Protocol

Background: Deficits in neuromuscular control are widely reported after mild traumatic brain injury (mTBI). These deficits are speculated to contribute to the increased rate of musculoskeletal injuries after mTBI. However, a concrete mechanistic connection between post-mTBI deficits and musculoskeletal injuries has yet to be established. While impairments in some domains of balance control have been linked to musculoskeletal injuries, reactive balance control has received little attention in the mTBI literature, despite the inherent demand of balance recovery in athletics. Our central hypothesis is that the high rate of musculoskeletal injuries after mTBI is in part due to impaired reactive balance control necessary for balance recovery. The purpose of this study is to (1) characterize reactive postural responses to recover balance in athletes with recent mTBI compared to healthy control subjects, (2) determine the extent to which reactive postural responses remain impaired in athletes with recent mTBI who have been cleared to return to play, and (3) determine the relationship between reactive postural responses and acute lower extremity musculoskeletal injuries in a general sample of healthy collegiate athletes.

Methods: This two-phase study will take place at the University of Utah in coordination with the University of Utah Athletics Department. Phase 1 will evaluate student-athletes who have sustained mTBI and teammate-matched controls who meet all the inclusion criteria. The participants will be assessed at multiple time points along the return-to-play progress of the athlete with mTBI. The primary outcome will be measures of reactive postural response derived from wearable sensors during the Push and Release (P&R) test. In phase 2, student-athletes will undergo a baseline assessment of postural responses. Acute lower extremity musculoskeletal injuries for each participant will be prospectively tracked for 1 year from the date of first team activity. The primary outcomes will be the measures of reactive postural responses and the time from first team activity to lower extremity injury.

Discussion: Results from this study will further our understanding of changes in balance control, across all domains, after mTBI and identify the extent to which postural responses can be used to assess injury risk in collegiate athletes.

Effect of Different Intensities of Transcranial Direct Current Stimulation on Postural Response to External Perturbation in Patients With Parkinson’s Disease

Background

Habituation of postural response to perturbations is impaired in people with Parkinson’s disease (PD) due to deficits in cortico-basal pathways. Although transcranial direct current stimulation (tDCS) modulate cortico-basal networks, it remains unclear if it can benefit postural control in PD.

Objective

To analyze the effect of different intensities of anodal tDCS on postural responses and prefrontal cortex (PFC) activity during the habituation to the external perturbation in patients with PD (n = 24).

Methods

Anodal tDCS was applied over the primary motor cortex (M1) with 1 mA, 2 mA, and sham stimulation in 3 different sessions (~2 weeks apart) during 20 minutes immediately before the postural assessment. External perturbation (7 trials) was applied by a support base posterior translation (20 cm/s and 5 cm). Primary outcome measures included lower limb electromyography and center of pressure parameters. Measures of PFC activity are reported as exploratory outcomes. Analyses of variance (Stimulation Condition × Trial) were performed.

Results

Habituation of perturbation was evidenced independent of the stimulation conditions. Both active stimulation intensities had shorter recovery time and a trend for lower cortical activity in the stimulated hemisphere when compared to sham condition. Shorter onset latency of the medial gastrocnemius as well as lower cortical activity in the nonstimulated hemisphere were only observed after 2 mA concerning the sham condition.

Conclusions

tDCS over M1 improved the postural response to external perturbation in PD, with better response observed for 2 mA compared with 1 mA. However, tDCS seems to be inefficient in modifying the habituation of perturbation.

Cortical responses to whole-body balance perturbations index perturbation magnitude and predict reactive stepping behavior

The goal of this study was to determine whether the cortical responses elicited by whole‐body balance perturbations were similar to established cortical markers of action monitoring. Postural changes imposed by balance perturbations elicit a robust negative potential (N1) and a brisk increase of theta activity in the electroencephalogram recorded over midfrontal scalp areas. Because action monitoring is a cognitive function proposed to detect errors and initiate corrective adjustments, we hypothesized that the possible cortical markers of action monitoring during balance control (N1 potential and theta rhythm) scale with perturbation intensity and the eventual execution of reactive stepping responses (as opposed to feet‐in‐place responses). We recorded high‐density electroencephalogram from eleven young individuals, who participated in an experimental balance assessment. The participants were asked to recover balance following anteroposterior translations of the support surface at various intensities, while attempting to maintain both feet in place. We estimated source‐resolved cortical activity using independent component analysis. Combining time‐frequency decomposition and group‐level general linear modeling of single‐trial responses, we found a significant relation of the interaction between perturbation intensity and stepping responses with multiple cortical features from the midfrontal cortex, including the N1 potential, and theta, alpha, and beta rhythms. Our findings suggest that the cortical responses to balance perturbations index the magnitude of a deviation from a stable postural state to predict the need for reactive stepping responses. We propose that the cortical control of balance may involve cognitive control mechanisms (i.e., action monitoring) that facilitate postural adjustments to maintain postural stability.

Does severity of motor impairment affect reactive adaptation and fall-risk in chronic stroke survivors?

Background

A single-session of slip-perturbation training has shown to induce long-term fall risk reduction in older adults. Considering the spectrum of motor impairments and deficits in reactive balance after a cortical stroke, we aimed to determine if chronic stroke survivors could acquire and retain reactive adaptations to large slip-like perturbations and if these adaptations were dependent on severity of motor impairment.

Methods

Twenty-six chronic stroke participants were categorized into high and low-functioning groups based on their Chedoke-McMaster-Assessment scores. All participants received a pre-training, slip-like stance perturbation at level-III (highest intensity/acceleration) followed by 11 perturbations at a lower intensity (level-II). If in early phase, participants experienced > 3/5 falls, they were trained at a still lower intensity (level-I). Post-training, immediate scaling and short-term retention at 3 weeks post-training was examined. Perturbation outcome and post-slip center-of-mass (COM) stability was analyzed.

Results

On the pre-training trial, 60% of high and 100% of low-functioning participants fell. High-functioning group tolerated and adapted at training-intensity level-II but low-functioning group were trained at level-I (all had > 3 falls on level-II). At respective training intensities, both groups significantly lowered fall incidence from 1st through 11th trials, with improved post-slip stability and anterior shift in COM position, resulting from increased compensatory step length. Both groups demonstrated immediate scaling and short-term retention of the acquired stability control.

Conclusion

Chronic stroke survivors are able to acquire and retain adaptive reactive balance skills to reduce fall risk. Although similar adaptation was demonstrated by both groups, the low-functioning group might require greater dosage with gradual increment in training intensity.

Cortical dynamics during preparation and execution of reactive balance responses with distinct postural demands

The contributions of the cerebral cortex to human balance control are clearly demonstrated by the profound impact of cortical lesions on the ability to maintain standing balance. The cerebral cortex is thought to regulate subcortical postural centers to maintain upright balance and posture under varying environmental conditions and task demands. However, the cortical mechanisms that support standing balance remain elusive. Here, we present an EEG-based analysis of cortical oscillatory dynamics during the preparation and execution of balance responses with distinct postural demands. In our experiment, participants responded to backward movements of the support surface either with one forward step or by keeping their feet in place. To challenge the postural control system, we applied participant-specific high accelerations of the support surface such that the postural demand was low for stepping responses and high for feet-in-place responses. We expected that postural demand modulated the power of intrinsic cortical oscillations. Independent component analysis and time-frequency domain statistics revealed stronger suppression of alpha (9-13 Hz) and low-gamma (31-34 Hz) rhythms in the supplementary motor area (SMA) when preparing for feet-in-place responses (i.e., high postural demand). Irrespective of the response condition, support-surface movements elicited broadband (3-17 Hz) power increase in the SMA and enhancement of the theta (3-7 Hz) rhythm in the anterior prefrontal cortex (PFC), anterior cingulate cortex (ACC), and bilateral sensorimotor cortices (M1/S1). Although the execution of reactive responses resulted in largely similar cortical dynamics, comparison between the bilateral M1/S1 showed that stepping responses corresponded with stronger suppression of the beta (13-17 Hz) rhythm in the M1/S1 contralateral to the support leg. Comparison between response conditions showed that feet-in-place responses corresponded with stronger enhancement of the theta (3-7 Hz) rhythm in the PFC. Our results provide novel insights into the cortical dynamics of SMA, PFC, and M1/S1 during the control of human balance.

Cortical contributions to anticipatory postural adjustments in the trunk

KEY POINTS

Increases in activity of trunk muscles that occur prior to, or concurrent with, a voluntary limb movement are termed anticipatory postural adjustments (APAs). APAs are important for maintaining postural stability in response to perturbations but the neural mechanisms underlying APAs remain unclear. Our results showed that corticospinal excitability of erector spinae (ES) muscle increased at 40 ms prior to rapid shoulder flexion, with a reduction in intracortical inhibition and no change in spinal excitability. Changes in corticospinal excitability were observed in ES, with similar excitability profiles between standing and lying positions, but were not observed in rectus abdominis. We suggest that the neural control of postural adjustments involves changes at a cortical level, which in part are due to reduced inhibition.

ABSTRACT

Voluntary limb movements are associated with increases in trunk muscle activity, some of which occur within a time window considered too fast to be induced by sensory feedback; these increases are termed anticipatory postural adjustments (APAs). Although it is known that the function of APAs is to maintain postural stability in response to perturbations, excitability of the corticospinal projections to the trunk muscles during the APAs remains unclear. Thirty-four healthy subjects performed rapid shoulder flexion in response to a visual cue in standing and lying positions. Transcranial magnetic stimulation (TMS) was delivered over the trunk motor cortex to examine motor evoked potentials (MEPs) in erector spinae (ES) and in rectus abdominis (RA) muscles at several time points prior to the rise in electromyographic activity (EMG) of anterior deltoid (AD) muscle. TMS was also used to assess short-interval intracortical inhibition (SICI) and cervicomedullary MEPs (CMEPs) in ES in the standing position. MEPs in ES were larger at time points closer to the rise in AD EMG in both standing and lying positions, whereas MEPs in RA did not differ over the time course examined. Notably, SICI was reduced at time points closer to the rise in AD EMG, with no change in CMEPs. Our results demonstrate that increasing excitability of corticospinal projections to the trunk muscles prior to a voluntary limb movement is likely to be cortical in origin and is muscle specific.

Perturbation-evoked potentials: Significance and application in balance control research

Historically, balance control was thought to be mediated solely by subcortical structures based on animal research. However, recent findings provide compelling evidence of cortical involvement during balance reactions evoked by whole-body postural perturbations. In humans, an external perturbation elicits an evoked potential, termed the perturbation-evoked potential (PEP). PEPs are widely distributed over fronto-centro-parietal areas with maximal amplitude at the FCz/Cz electrode. From our literature review it is evident that the PEPs are comprised of a small positive potential (P1) that peaks around 30-90ms after perturbation onset, a large negative potential (N1) that peaks around 90-160ms, followed by positive (P2) and negative (N2) potentials between 200 and 400ms. Converging results across multiple studies suggest that these different PEP components are influenced by perturbation characteristics, postural set, environmental, and psychological factors. This review summarizes and integrates seminal research on the PEP, with a special emphasis on the PEP N1. Implications for future studies in PEP research are discussed to encourage further empirical investigation of PEP characteristics in healthy and patient populations.

Cognition and balance control: does processing of explicit contextual cues of impending perturbations modulate automatic postural responses?

Processing of predictive contextual cues of an impending perturbation is thought to induce adaptive postural responses. Cueing in previous research has been provided through repeated perturbations with a constant foreperiod. This experimental strategy confounds explicit predictive cueing with adaptation and non-specific properties of temporal cueing. Two experiments were performed to assess those factors separately. To perturb upright balance, the base of support was suddenly displaced backwards in three amplitudes: 5, 10 and 15 cm. In Experiment 1, we tested the effect of cueing the amplitude of the impending postural perturbation by means of visual signals, and the effect of adaptation to repeated exposures by comparing block versus random sequences of perturbation. In Experiment 2, we evaluated separately the effects of cueing the characteristics of an impending balance perturbation and cueing the timing of perturbation onset. Results from Experiment 1 showed that the block sequence of perturbations led to increased stability of automatic postural responses, and modulation of magnitude and onset latency of muscular responses. Results from Experiment 2 showed that only the condition cueing timing of platform translation onset led to increased balance stability and modulation of onset latency of muscular responses. Conversely, cueing platform displacement amplitude failed to induce any effects on automatic postural responses in both experiments. Our findings support the interpretation of improved postural responses via optimized sensorimotor processes, at the same time that cast doubt on the notion that cognitive processing of explicit contextual cues advancing the magnitude of an impending perturbation can preset adaptive postural responses.

Cortical control: learning from the lamprey

The function of the motor cortex has been a persistent mystery. A recent study has found striking correspondence between the descending projections of lamprey pallium and mammalian motor cortex, encouraging comparative studies of the origin (and role) of forebrain motor control.

The role of prefrontal cortex during postural control in Parkinsonian syndromes a functional near-infrared spectroscopy study

Postural instability represents a main source of disability in Parkinsonian syndromes and its pathophysiology is poorly understood. Indirect probes (i.e., mental imagery) of brain involvement support the role of prefrontal cortex as a key cortical region for postural control in older adults with and without Parkinsonian syndromes. Using functional near infrared spectroscopy (fNIRs) as a direct online cortical probe, this study aimed to compare neural activation patterns in prefrontal cortex, postural stability, and their respective interactions, in (1) patients with Parkinsonian syndromes; (2) those with mild parkinsonian signs; (3) and healthy older adults. Among 269 non-demented older adults (76.41 ± 6.70 years, 56% women), 26 individuals presented with Parkinsonian syndromes (Unified Parkinson's disease rating scale (UPDRS): 11.08 ± 3.60), 117 had mild parkinsonian signs (UPDRS: 3.21 ± 2.49), and 126 individuals were included as a healthy control group. Participants were asked to stand upright and count silently for ten seconds while changes in oxygenated hemoglobin levels over prefrontal cortex were measured using fNIRs. We simultaneously evaluated postural stability with center of pressure velocity data recorded on an instrumented walkway. Compared to healthy controls and patients with mild parkinsonian signs, patients with Parkinsonian syndromes demonstrated significantly higher prefrontal oxygenation levels to maintain postural stability. The pattern of brain activation and postural control of participants with mild parkinsonian signs were similar to that of normal controls. These findings highlight the online role of the prefrontal cortex in postural control in patients with Parkinsonian syndromes and afford the opportunity to improve therapeutic options for postural instability.

The role of the cerebral cortex in postural responses to externally induced perturbations

The ease with which we avoid falling down belies a highly sophisticated and distributed neural network for controlling reactions to maintain upright balance. Although historically these reactions were considered within the sub cortical domain, mounting evidence reveals a distributed network for postural control including a potentially important role for the cerebral cortex. Support for this cortical role comes from direct measurement associated with moments of induced instability as well as indirect links between cognitive task performance and balance recovery. The cerebral cortex appears to be directly involved in the control of rapid balance reactions but also setting the central nervous system in advance to optimize balance recovery reactions even when a future threat to stability is unexpected. In this review the growing body of evidence that now firmly supports a cortical role in the postural responses to externally induced perturbations is presented. Moreover, an updated framework is advanced to help understand how cortical contributions may influence our resistance to falls and on what timescale. The implications for future studies into the neural control of balance are discussed.