Abstract
Vision plays a central role in maintaining balance. When humans perceive their body as moving, they trigger counter movements. This results in body sway, which has typically been investigated by measuring the body’s center of pressure (COP). Here, we aimed to induce visually evoked postural responses (VEPR) by simulating self-motion in virtual reality (VR) using a sinusoidally oscillating “moving room” paradigm. Ten healthy subjects participated in the experiment. Stimulation consisted of a 3D-cloud of random dots, presented through a VR headset, which oscillated sinusoidally in the anterior–posterior direction at different frequencies. We used a force platform to measure subjects’ COP over time and quantified the resulting trajectory by wavelet analyses including inter-trial phase coherence (ITPC). Subjects exhibited significant coupling of their COP to the respective stimulus. Even when spectral analysis of postural sway showed only small responses in the expected frequency bands (power), ITPC revealed an almost constant strength of coupling to the stimulus within but also across subjects and presented frequencies. Remarkably, ITPC even revealed a strong phase coupling to stimulation at 1.5 Hz, which exceeds the frequency range that has generally been attributed to the coupling of human postural sway to an oscillatory visual scenery. These findings suggest phase-locking to be an essential feature of visuomotor control.
Highlights
Despite its apparent naturalness, bipedal upright standing is inherently unstable and involves a myriad of complex underlying neural and biomechanical control processes (Peterka 2002; Horak 2006)
Wavelet decomposition of the center of pressure (COP) signals yielded time–frequency resolved power spectra, which revealed the extent to which each frequency was present in the postural response at each point in time
We calculated the time–frequency resolved inter-trial phase coherence (ITPC), which gave insight into how stable the phase of the postural response in a given frequency band remained across trials
Summary
Bipedal upright standing is inherently unstable and involves a myriad of complex underlying neural and biomechanical control processes (Peterka 2002; Horak 2006). Understanding how humans control their balance and posture can benefit many applications such as diagnosing and preventing disease, as well as accelerating rehabilitation (Horak 2006). Stable balance is mainly achieved by controlling the body’s center of mass (COM) and keeping it within a distinct area of stability which physically supports the upright human body (Horak and Macpherson 1996; Scholz et al 2007, 2012; Sousa et al 2012). Locating the COM within the complex mass distribution of a human body is a difficult endeavor, which is why foot center of pressure (COP) is widely used to investigate postural sway and whole-body movements (Winter 1995; Winter et al 1996). COP describes the accumulation of all forces the human body enacts on the ground on one spot, which can be measured via the reactive ground forces using a force
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