Vision Does Not Necessarily Stabilize the Head in Space During Continuous Postural Perturbations

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.

Adaptation of balancing behaviour during continuous perturbations of stance. Supra-postural visual tasks and platform translation frequency modulate adaptation rate

“…These were performed in accordance with the Declaration of Helsinki and approved by the institutional Ethics Committee (Istituti Clinici Scientifici Maugeri, approval number # 2257 CE). These individuals participated in a previously published study on the effects of vision on the balancing behaviour [34]. Here, the data recorded in the same cohort have been analysed further in order to address the effects of visual task and translation frequency on the adaptation of the balancing behaviour across the period of repeated perturbations.…”

“…The methodology has been described in detail in [34]. Here, we briefly report the main procedures and the new analytical methods.…”

“…The head antero-posterior (AP) displacements have been recently studied under conditions requiring the execution of a visual supra-postural task [34]. That investigation has shown that the 'head-fixed-in-space' behaviour with EO can be revoked during the performance of a visual task consisting in reading a text that moves with the platform.…”

“…In the present study, we wanted to quantify the habituation rate of the body AP displacement under conditions that feature distinct visual states, including those implying the performance of supra-postural visual tasks. Indeed, the 'fixed-in-space' head behaviour EO becomes a 'head-riding-the-platform' behaviour when individuals focus on an object moving jointed with the platform, in spite of full vision availability [34]. In this case, vision helps accomplish the visual task optimally by keeping a constant head-text distance, rather than by producing head and body stabilization in space as a necessary outcome.…”

Human Balance in Response to Continuous, Predictable Translations of the Support Base: Integration of Sensory Information, Adaptation to Perturbations, and the Effect of Age, Neuropathy and Parkinson’s Disease

“…As a matter of fact, under certain conditions, sighted subjects can tolerate ample head and body oscillations, when balancing on the translating platform, despite having normal vision, for instance while reading a text. In this case, as the text moves with the moving platform to which it is fixed as an integral whole, the head moves as much as the platform, and the distance between the eyes and the text is kept constant [44]. Therefore, these findings suggest that head stabilization in space can be revoked by the brain to enhance the performance of a non-postural task.…”