Balance in Blind Subjects: Cane and Fingertip Touch Induce Similar Extent and Promptness of Stance Stabilization

Assessment of functional mobility and gait during a timed up and go test in adults with total blindness

BACKGROUND

The loss of vision leads to behavioral and motor adaptations that do not necessarily translate to good functioning with regards to daily tasks.

AIM

To investigate differences in functional mobility in adults with total blindness, and analyze differences in spatiotemporal gait variables with and without the use of a cane, and wearing shoes or barefoot.

METHODS

We used an inertial measurement unit to assess the spatiotemporal parameters of the gait and functional mobility in seven subjects with total blindness and four sighted participants during the timed up and go test (TUG) test performed under conditions: barefoot/shod; and with/without a cane (blind subjects).

RESULTS

Significant differences between groups were found in total TUG test time and in the sub-phases when the blind subjects executed the TUG barefoot and without a cane (p < .01). Other differences were found in trunk movement during sit-to-stand, and stand-to-sit where blind subjects when without cane and barefoot, they had a greater range of motion than sighted subjects (p < .01). Also, BMI has a moderate to strong influence in the execution of the TUG in blind subjects (p < .05) CONCLUSION: This study showed that, when using a gait-assistance device and wearing shoes, blind subjects have similar functional mobility and gait as sighted subjects, suggesting that an external haptic reference can compensate for the lack of vision. Knowledge of these differences can provide a better understanding of the adaptive behavior in this population, thereby assisting in minimizing the occurrence of trauma and falls.

Specific Posture-Stabilising Effects of Vision and Touch Are Revealed by Distinct Changes of Body Oscillation Frequencies

We addressed postural instability during stance with eyes closed (EC) on a compliant surface in healthy young people. Spectral analysis of the centre of foot pressure oscillations was used to identify the effects of haptic information (light-touch, EC-LT), or vision (eyes open, EO), or both (EO-LT). Spectral median frequency was strongly reduced by EO and EO-LT, while spectral amplitude was reduced by all "stabilising" sensory conditions. Reduction in spectrum level by EO mainly appeared in the high-frequency range. Reduction by LT was much larger than that induced by the vision in the low-frequency range, less so in the high-frequency range. Touch and vision together produced a fall in spectral amplitude across all windows, more so in anteroposterior (AP) direction. Lowermost frequencies contributed poorly to geometric measures (sway path and area) for all sensory conditions. The same subjects participated in control experiments on a solid base of support. Median frequency and amplitude of the spectrum and geometric measures were largely smaller when standing on solid than on foam base but poorly affected by the sensory conditions. Frequency analysis but not geometric measures allowed to disclose unique tuning of the postural control mode by haptic and visual information. During standing on foam, the vision did not reduce low-frequency oscillations, while touch diminished the entire spectrum, except for the medium-high frequencies, as if sway reduction by touch would rely on rapid balance corrections. The combination of frequency analysis with sensory conditions is a promising approach to explore altered postural mechanisms and prospective interventions in subjects with central or peripheral nervous system disorders.

Induced stabilization of center of mass decreases variability of center of pressure regardless of visual or tactile information

Traditional theories claim that center of pressure (COP) is oscillating to minimize the center of mass (COM) movements, contrary to exploratory theories which propose that COP oscillates to increase sensory information flow from the environment. The aim of this work was to better understand the underlying postural control mechanisms, specifically the interplay of COP oscillations and sensory information flow on keeping the COM stable. Eighteen volunteers took part of the experiment divided into three parts based on sensory conditions: eyes opened, eyes closed and eyes closed with lightly touching a fixed object with one finger. Throughout each part the participants had to quietly stand for 335 s. In the middle of each part, we stabilized their COM for 105 s using a robotized waist-pull system. We recorded whole-body kinematics, COP oscillations, electromyographic activity of soleus and tibialis anterior muscles and the force applied by the finger during light touch conditions. The variability of COP significantly decreased when the COM was stabilized in all sensory conditions. The interaction between sensory condition and stabilization was also significant with different decline of COP variability between quiet standing and stabilization part in all three different sensory conditions. Ankle and knee angle variability decreased significantly while the hip angle variability did not. Our findings suggest that COP is not moving to explore the environment, but to attenuate oscillations of the COM. However, possible functional aspect of movement variability to keep the COM stable still remains.

Cutaneous and muscular afferents from the foot and sensory fusion processing: Physiology and pathology in neuropathies

The foot-sole cutaneous receptors (section 2), their function in stance control (sway minimisation, exploratory role) (2.1), and the modulation of their effects by gait pattern and intended behaviour (2.2) are reviewed. Experimental manipulations (anaesthesia, temperature) (2.3 and 2.4) have shown that information from foot sole has widespread influence on balance. Foot-sole stimulation (2.5) appears to be a promising approach for rehabilitation. Proprioceptive information (3) has a pre-eminent role in balance and gait. Reflex responses to balance perturbations are produced by both leg and foot muscle stretch (3.1) and show complex interactions with skin input at both spinal and supra-spinal levels (3.2), where sensory feedback is modulated by posture, locomotion and vision. Other muscles, notably of neck and trunk, contribute to kinaesthesia and sense of orientation in space (3.3). The effects of age-related decline of afferent input are variable under different foot-contact and visual conditions (3.4). Muscle force diminishes with age and sarcopenia, affecting intrinsic foot muscles relaying relevant feedback (3.5). In neuropathy (4), reduction in cutaneous sensation accompanies the diminished density of viable receptors (4.1). Loss of foot-sole input goes along with large-fibre dysfunction in intrinsic foot muscles. Diabetic patients have an elevated risk of falling, and vision and vestibular compensation strategies may be inadequate (4.2). From Charcot-Marie-Tooth 1A disease (4.3) we have become aware of the role of spindle group II fibres and of the anatomical feet conditions in balance control. Lastly (5) we touch on the effects of nerve stimulation onto cortical and spinal excitability, which may participate in plasticity processes, and on exercise interventions to reduce the impact of neuropathy.

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

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.