Lower visual field inputs reduce postural sway via ankle co-contraction, independent of vection

Optic flow stimuli in the lower visual field reduce center of pressure (COP) sway in the anteroposterior direction. The central nervous system (CNS) may adopt a stiffness control strategy in response to vection, defined as the visually induced illusion of self-motion.

Do visual stimuli presented in the lower visual field elicit stronger vection and promote a stiffness strategy?

Twenty-seven healthy young adults participated in this study. Visual stimuli were presented in two field-of-view conditions (upper and lower visual fields) and three velocity conditions (slow, medium, and fast) using a head-mounted virtual reality display. The optic flow consisted of small white spheres expanding radially toward the periphery. Participants stood quietly for 70 s. Postural responses were quantified using the root mean square (RMS), mean velocity, and power spectral density (PSD) of COP, as well as the co-contraction index (CCI) of ankle muscles. Subjective vection was assessed using a visual analogue scale.

Optic flow in the lower visual field significantly reduced the RMS of COP in the anteroposterior direction and produced a more anterior COP position compared with the upper visual field condition. Additionally, PSD in the low-frequency band (0-0.3 Hz) was significantly reduced, whereas PSD in higher-frequency bands (0.3-3 Hz) and CCI were significantly increased. In contrast, vection was weaker under lower visual field stimulation.

The CNS modulates ankle muscle co-contraction and postural orientation when optic flow is presented in the lower visual field, independently of vection strength.