Abstract
Vision favors head stabilization in space during perturbations of standing balance. This is particularly obvious under conditions of continuous predictable perturbations as during sinusoidal antero-posterior (A-P) translations of the supporting platform. We tested here the hypothesis that under this condition the head can instead undergo large A-P oscillations, when a precision visual task is concurrently performed. We compared the head oscillations across four conditions while standing on a continuously translating platform. Eyes open (EO, no visual task), EO while reading a text fixed to the moving platform (EO-TP), EO while reading a text fixed to earth-ground (EO-TG), eyes-closed (EC). The platform translated at 0.2 and 0.6 Hz. Participants were young adult subjects, who received no particular instruction except reading the text aloud when required. Markers fixed on head, platform and text-sheet were captured by an optoelectronic device. We found that head oscillations were larger with EC than under all EO conditions. The oscillations were the least with EO and EO-TG, and intermediate with EO-TP. This was true under both low and high translation frequency, in spite of broadly smaller head oscillations at high frequency, common to all visual conditions. The distance between the head and the text was quite constant with EO-TP but fluctuated with EO-TG. The basic whole-body coordination features were moderately similar under all conditions, as assessed by the head-platform correlation coefficients and time lags. It appears that vision does not produce head stabilization in space when a concurrent visual task requiring focusing on a reading-text moving with the platform is performed. Contrary to traditional views centered on the stabilizing effect of vision under both static and dynamic conditions, the results show that head stabilization, normally ensuring a reference for inertial guidance for body balance, can be revoked by the CNS to allow performance of a non-postural task. This novel paradigm can shift long-standing views on the effect of vision on equilibrium control and be considered a potential exercise treatment for enhancing the multisensory integration process in people with balance problems.
Highlights
When a person balances on a platform that continuously moves fore and aft in a predictable way, vision definitely helps reduce head oscillations, to a different extent depending on the frequency of the translations [1, 2]
That study showed that head stabilization in space decreases gradually when visual acuity levels are progressively reduced by adding ad-hoc test lenses, to reach the oscillation level of the EC condition when acuity fell below 0.001/10
Vision per se does not produce automatic head stabilization in space when the body is subjected to a continuous predictable sinusoidal horizontal translation of the support base
Summary
When a person balances on a platform that continuously moves fore and aft in a predictable way, vision definitely helps reduce head oscillations, to a different extent depending on the frequency of the translations [1, 2]. Since decreased head and body sway with eyes-open (EO) compared to eyes-closed (EC) is observed both under static and dynamic conditions, reduced head oscillations are generally interpreted as the necessary result of the integration of the visual information into the neural processes responsible for the maintenance of equilibrium [10,11,12,13] This behavior has been observed in response to haptic information [14, 15] or information from the proprioceptors [16,17,18,19,20,21] and vestibular apparatus [22,23,24]. That study showed that head stabilization in space decreases gradually when visual acuity levels are progressively reduced by adding ad-hoc test lenses, to reach the oscillation level of the EC condition when acuity fell below 0.001/10
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