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Exposure to postural threat has been documented to influence the sensory contributions of proprioceptive and vestibular information in standing balance control. Contributions from the visual system to balance are also crucial, yet the degree to which postural threat may modulate visual control of balance is not well characterized. Therefore, the aims of this study were to assess the feasibility of eliciting visual evoked postural responses (VEPRs) using head-mounted virtual reality (VR) and use this method to examine the potential influence of virtual postural threat on the visual control of balance. Thirty-six healthy young adults were exposed to a pseudorandom, translational visual stimulus of a real-world environment in VR. The visual stimulus was presented in virtual conditions of LOW and HIGH postural threat in which participants stood at ground level, and on a 7 m elevated platform, respectively. VEPRs were successfully produced in both postural threat conditions. When exposed to the visual stimulus while at an elevated surface height, participants demonstrated significant changes to their physiological arousal and emotional state. Despite significant coherence across the stimulus' frequency range, stimulus correlated VEPRs were not significantly modulated during exposure to the visual stimulus under virtual postural threat. This study supports the future utility of VR head-mounted displays in examining emotional influences on the visual control of balance., (Copyright © 2022. Published by Elsevier B.V.)

Mechanical muscle tendon vibration activates multiple sensory receptors in the muscle and tendon. In particular, tendon vibration tends to activate the Ia afferents the strongest, but also will activate group II and Ib afferents. This activation can cause three main effects in the central nervous system: proprioceptive illusions, tonic vibration reflexes, and suppression of the stretch response. Noisy tendon vibration has been used to assess the frequency characteristics of proprioceptive reflexes and, interestingly there appeared to be no evidence for proprioceptive illusions or tonic vibration reflexes during standing [9]. However, it remains unknown if noisy vibration induces a suppression of the muscle stretch response. Therefore, the purpose of this study was to investigate the effects of noisy and periodic tendon vibration on the stretch response in the flexor carpi radialis muscle (FCR). We examined FCR stretch responses with and without periodic (20 and 100 Hz) and noisy (∼10-100 Hz) tendon vibration. We additionally had participants perform the task under the instruction set to either not respond to the perturbation or to respond as fast as possible. The key finding from this study was that both periodic and noisy vibration resulted in a reduced stretch response amplitude. Additionally, it was found that a participant's intent to respond did not modulate the amount of suppression observed. The findings from this study provide a more detailed understanding of the effects of tendon vibration on the muscle stretch response., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Prior studies have shown that enhanced levels of arousal can increase specific aspects of visual perception. The current study investigated the effect of a height-induced postural threat on the detection of central and peripheral visual targets. Ten healthy young adults performed a modified useful field of view task in a virtual environment under low and high postural threat. Each individual completed two blocks of standing trials at ground level (low postural threat), and on a virtual platform raised 7 m above the ground (high postural threat). Under high compared to low postural threat, individuals demonstrated decreases in self-reported balance confidence and increases in state anxiety and fear. With increased threat, detection rates for visual stimuli increased, independent of the location of the stimulus in the field of view. These findings suggest that detection of visual stimuli is facilitated in threatening environments, likely driven by a combination of emotion, attention and other higher cognitive influences., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Primary (Ia) sensory afferents that innervate muscle spindles provide strong synaptic input to homonymous motoneurons and are thought to play a role in balance control. In addition, Ia afferents have broad heteronymous connections; i.e., projections to motoneurons that innervate other muscles that act at the same joint as well as at different joints. The purpose of the current study was to investigate heteronymous Ia afferent connections from the triceps surae muscles to lower limb and back muscles during quiet standing in humans. We applied supra-threshold noisy vibration (10-115 Hz) to the right Achilles tendon of 12 participants maintaining quiet stance and recorded EMG activity bilaterally from homonymous (Soleus) and heteronymous muscles (Semitendinosus, Vastus Lateralis, Erector Spinae). We estimated coherence, phase, and gain between the tendon probe acceleration and rectified EMG from each muscle. We found significant coherence between the probe acceleration and EMG in ipsilateral Soleus (5-100 Hz), Semitendinosus (10-75 Hz), Vastus Lateralis (5-70 Hz), and bilateral Erector Spinae muscles (10-70 Hz). These results provide evidence that triceps surae muscle spindle afferents can influence the activity of muscles proximal to the ankle joint across a broad frequency band during quiet standing., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Research indicates that threat-induced changes in standing balance are associated with shifts in attention focus. This study investigated whether distracting attention modifies threat-induced changes in standing balance. Twenty-five healthy young adults stood without (No Threat) and with (Threat) the possibility of receiving a temporally unpredictable anteroposterior support surface translation. In both conditions, participants completed a distractor task that consisted of counting how often a pre-selected letter occurred in an auditory sequence, or no distractor task. Emotional responses to threat were quantified using electrodermal activity and self-report measures, while attention focus was quantified using self-report. Centre of pressure (COP) was measured to assess changes in standing balance. Results indicate that postural threat induced an emotional response, as well as broad shifts in attention focus and changes in standing balance. Distracting attention with a cognitive task mitigated threat-induced increases in medium-frequency COP displacements (0.5-1.8 Hz). These results provide support for a relationship between threat-related changes in balance control and attention focus., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Previous research has shown that cutaneous reflexes are modulated when walking with a threat to stability. It is unclear if this reflex modulation is purely related to the context of the imposed threat or if emotional changes associated with the threat exert an independent influence on reflex excitability. This study investigated the influence of emotional stimuli on lower limb cutaneous reflexes during treadmill walking. Twenty-eight healthy young adults walked at a self-selected pace while viewing pictures that manipulated emotional arousal and valence (confirmed with electrodermal and self-report measures). Throughout each trial, cutaneous reflexes were evoked by electrically stimulating the sural nerve at heel contact, mid-stance, or toe off. Surface electromyography of the ipsilateral soleus (SOL), medial gastrocnemius (MG), tibialis anterior (TA), biceps femoris (BF), and vastus lateralis (VL) was recorded to assess reflexes. Highly arousing pictures, independent of valence, significantly facilitated TA, and trended toward facilitating SOL and BF reflexes during mid-stance. Unpleasant pictures, independent of arousal, significantly reduced reflex amplitudes in BF during mid-stance and TA during toe off. While changes in background muscle activity and step cadence were observed, they did not correlate with reflex changes. This study provides the first evidence that emotional stimuli exert an independent influence on cutaneous reflex excitability during gait. As cutaneous reflexes contribute to stability during gait, these findings support the notion that emotional state influences important sensorimotor processes underlying balance control., (Copyright © 2017 Elsevier B.V. All rights reserved.)

While Parkinson's disease (PD) is traditionally viewed as a motor disorder, there is mounting evidence that somatosensory function becomes affected as well. However, conflicting reports exist regarding whether plantar sensitivity is reduced in early-onset PD patients. Plantar sensitivity was assessed using monofilaments and a gold-standard, two-interval two-alternative forced choice vibrotactile detection task at both 30 and 250Hz. Lower-limb cutaneous reflexes were assessed by delivering continuous, sinusoidal vibration at 30 and 250Hz while recording muscle activity in Tibialis Anterior. We found no evidence of elevated plantar thresholds or dysfunctional lower-limb cutaneous reflexes in PD patients ON medication. We also found no acute effect of ceasing L-dopa intake on either plantar sensitivity or cutaneous reflexes. Our finding of intact cutaneous function in PD supports the further exploration of therapeutics that enhance plantar sensitivity to minimize postural instability, a source of considerable morbidity in this clinical population., (Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.)

This study examined how changes in threat influenced conscious perceptions of postural sway. Young healthy adults stood on a forceplate mounted to a hydraulic lift placed at two heights (0.8m and 3.2m). At each height, subjects stood quietly with eyes open and eyes closed for 60s. Subjects were instructed to either stand normal, or stand normal and track their perceived sway in the antero-posterior plane by rotating a hand-held potentiometer. Participants reported an increased level of fear, anxiety, arousal and a decreased level of balance confidence when standing at height. In addition, postural sway amplitude decreased and frequency increased at height. However, there were no effects of height on perceived sway. When standing under conditions of increased postural threat, sway amplitude is reduced, while sway perception appears to remain unchanged. Therefore, when threat is increased, sensory gain may be increased to compensate for postural strategies that reduce sway (i.e. stiffening strategy), thereby ensuring sufficient afferent information is available to maintain, or even increase the conscious perception of postural sway., (Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.)

Introduction: Balance problems during virtual reality (VR) have been mentioned in the literature but seldom investigated despite the increased use of VR systems as a training or rehabilitation tool. We examined the influence of VR on body sway under different stance conditions., Methods: Seventeen young subjects performed four tasks (standing with feet close together or tandem stance on firm and foam surfaces for 60s) under three visual conditions: eyes open without VR, eyes closed, or while viewing a virtual reality scene which moved with body movements. Angular velocity transducers mounted on the shoulder provided measures of body sway in the roll and pitch plane., Results: VR caused increased pitch and roll angles and angular velocities compared to EO. The effects of VR were, for the most part, indistinguishable from eyes closed conditions. Use of a foam surface increased sway compared to a firm surface under eyes closed and VR conditions., Conclusion: During the movements of quiet stance, VR causes an increase in postural sway in amplitude similar to that caused by closing the eyes. This increased sway was present irrespective of stance surface, but was greatest on foam.

This study investigated the effects of postural threat on the cortical response associated with postural reactions to predictable and unpredictable perturbations to upright stance. Postural threat was manipulated by having individuals stand on an elevated surface to alter the context in which the postural task was performed. Ten healthy young adults experienced a series of predictable and unpredictable trunk perturbations when standing at ground level and at the edge of a platform located 3.2 m above the ground. Participants felt less confident, more fearful of falling, more anxious and less stable when standing at the high surface height. Unpredictable perturbations generated a large negative potential (N1) which was increased by 84% when standing at the high compared to low surface height. The magnitude of change in this potential was related to the magnitude of change in balance perceptions, such as confidence and fear. Predictable perturbations did not generate a N1 potential but instead produced an anticipation-related potential prior to the perturbation. This cortical activity observed in response to predictable perturbations was not influenced by postural threat. A large N1 potential was observed for a 'surprise' perturbation that followed a series of predictable perturbations. There was a trend for the amplitude of this potential to be increased when standing at the high compared to low surface height. The results of this study provide evidence for the modulating influence of psychological factors related to postural threat on the cortical activity associated with postural reactions to unpredictable perturbations.