By Dr Chesney Craig.

In order to maintain an upright stance, the central nervous system (CNS) utilises the most reliable sensory sources in the current environment. For example, when navigating under poor lighting conditions, the CNS will rely less on vision and more on somatosensory and vestibular information sources. This process is known as sensory reweighting. Sensory reweighting is slower in older adults (aged 65+), resulting in prolonged instability during sensory transitions, in which information from one or more sensory sources is deficient. For example, older adults show longer postural sway after-effects following reinstatement of stable conditions after destabilisation from exposure to a moving support or visual surround. However, it is unclear whether postural delays after “low risk” transitions (e.g. from unstable to stable conditions) are a sensory reweighting deficit or a compensatory response to preserve fast postural responses for conditions of greater perceived fall risk (e.g. from stable to unstable conditions). We hypothesised that if the after-effect was accompanied by prolonged perception of instability then this would support an age-related deficit, rather than a compensatory response.

In two experiments, we assessed postural sway in young and older adults during blindfolded transitions from a stable platform to an unstable platform and then back to a final stable platform. The unstable platform moved in proportion to the participant’s body sway (e.g. sway-referenced). Our main outcome measures were the magnitude and duration of the postural sway after-effect during the transition to the final stable platform, and the time taken to perceive platform stabilisation as assessed using a button press. In both studies, older adults demonstrated significantly larger and longer postural after-effects, compared to young adults (Figure). Additionally, the after-effects were more pronounced in fall-prone older adults (Figure panel A) and more importantly, they were accompanied by a delayed perception of platform stabilisation in older adults (Figure panel C and D). In both experiments, older adults took five times longer than young adults to perceive platform stabilization. Interestingly, despite showing a larger after-effect, fall-prone older adults did not show prolonged perceptual delays compared to healthy older adults.

This is the first report, to our knowledge, in which age-related postural sway after-effects are accompanied by a perceptual illusion of continued movement. Our findings support that instability during sensory transitions is due to inefficient sensory reweighting, which delays reliable postural perception until accurate sensory weights are re-established. This suggests that everyday sensory transitions may pose a significant risk of instability to older adults. Furthermore, the fact that fall-prone older adults showed a similar perceptual delay to healthy older adults but showed larger postural after-effects, could suggest that sensory reweighting delays are similar in both groups but the body’s ability to compensate for this delay may determine fall risk.

Figure 1. TOP: Mean anteroposterior sway path length for the duration of baseline and for each 30s window (R1-R6) following reinstatement of a stable platform after sway-referencing for each group in (A) Experiment 1 and (B) Experiment 2. ○ indicates between group difference at p< .05. Dashed brackets represent duration that this remained significant for. * indicates within-group difference from baseline, p< .008. Colour represents group categorisation. BOTTOM: Mean time that each group’s postural sway returned to baseline levels and mean time taken to perceive platform stabilisation (button press), relative to when the platform stopped moving (time = 0) for (C) Experiment 1 and (D) Experiment 2. † indicates within-group difference between button press and time taken to reduce sway, p < .001.

Publication

Craig C.E. and Doumas M. (2019). Slowed sensory reweighting and postural illusions in older adults: the moving platform illusion. Journal of Neurophysiology, 121: 690-700. doi: 10.1152/jn.00389.2018

About the Author

By Dr Brian Dalton.

The central nervous system produces motor responses to maintain standing balance through complex processing and integration of multiple signals regarding the position and motion of the body in space. Activation of trunk, leg and foot muscles contribute to whole-body balance control to varying degrees. Even though the feet provide an excellent source of sensory information, it is uncertain whether foot muscles play an active functional role in maintaining quiet standing balance; or whether the activity of these muscles is simply a by-product of preserving rigidity of the feet for the plantar flexors to produce postural responses to keep the body upright. Electrical vestibular stimulation (EVS) can be used to evoke whole-body balance responses that arise from the summation of all muscles involved in the compensatory response. The presence of vestibular-evoked balance responses can elucidate the specific muscle’s role during standing. Thus, the purpose of our study was to determine whether foot muscles – specifically, the abductor hallucis (AH) and abductor digiti minimi (ADM) – displayed postural responses driven by vestibular stimulation during quiet standing.

Seven healthy, young participants were exposed to a continuous, random EVS signal during quiet standing (Figure below). We assessed postural responses as variations in anterior-posterior forces under the feet, and AH, ADM, and medial gastrocnemius muscle activity using surface electromyography. We characterised the relationships between the EVS input and subsequent motor output in both time and frequency domains via multivariate Fourier analyses. We found that vestibular-evoked balance responses were present in anterior-posterior forces and in all muscles. These responses were modified similarly via head orientation (affecting the direction of EVS-evoked balance responses) and removal of vision (affecting the weighting of sensory information), which is characteristic of a postural adjustment driven by vestibular stimulation. The current findings emphasize that foot muscles provide an active role in balance control during standing.

Our results indicate that a complete model of the sensorimotor control of quiet standing should include foot muscles. Future research should focus on examining whether decrements within foot muscles lead to impairments in standing, and whether rehabilitative strategies involving these muscles can improve postural control in those with standing balance problems.

Figure: Experimental setup

Publication

Wallace JW, Rasman BG, Dalton BH. Vestibular-evoked responses indicate a functional role for intrinsic foot muscles during standing balance. Neurosci 377: 150-160, 2018.

https://doi.org/10.1016/j.neuroscience.2018.02.036

About the Author