Down regulation of vestibular balance stabilizing mechanisms to enable transition between motor states

The neural control of transition between posture and movement encompasses the regulation of reflex-stabilizing mechanisms to enable motion. Optimal feedback theory suggests that such transitions require the disengagement of one motor control policy before the implementation of another. To test this possibility, we investigated the continuity of the vestibular control of balance during transitions between quiet standing and locomotion and between two standing postures. Healthy subjects initiated and terminated locomotion or shifted the distribution of their weight between their feet, while exposed to electrical vestibular stimuli (EVS). The relationship between EVS and ground reaction forces was quantified using time-frequency analyses. Discontinuities corresponding to null coherence periods were observed preceding the onset of movement initiation and during the step preceding locomotion termination. These results show humans interrupt the vestibular balance stabilizing mechanisms to transition between motor states, suggesting a discrete change between motor control policies, as predicted by optimal feedback theory.

Crossing Abbey Road is something of a paradox in neuroscientific terms. As you stand waiting to cross, tiny movements of your body – such as those due to breathing – cause you to sway by small amounts. To prevent you from falling over, your brain makes active corrections to your posture. These posture-correcting mechanisms oppose movements such as sway and keep you standing upright. But what happens when you want to cross the road?

To get you moving, your brain has two options. It could temporarily suppress the posture-correcting mechanisms. Or it could reconfigure them so that they work in a different way. The posture-correcting mechanisms rely upon sensory input from various sources. These include the vestibular system of the inner ear. The vestibular system tells the brain about the position and movement of the head in space and relative to gravity. Monitoring vestibular system activity as a person starts to move should thus reveal what is happening to the posture-correcting mechanisms.

Tisserand et al. asked healthy volunteers to transition between standing still and walking, or to shift their weight from one foot to the other. At the same time, small non-painful electric currents were applied to the bones behind the volunteers' ears. These currents induced small changes in vestibular system activity. Sensors in the floor measured the forces the volunteers generated while standing or walking, thereby revealing how they adjusted their balance. The results showed that the brain suppresses its posture-correcting mechanisms before people start or stop moving.

These findings have implications for robotics. They could make it easier to program robots to show smooth transitions into and out of movement. The findings are also relevant to movement disorders such as Parkinson's disease. One common symptom of this disorder is freezing of gait, in which patients suddenly feel as though their feet are glued to the ground. Understanding how the brain controls movement transitions may reveal how such symptoms arise.

Does gravity influence the visual line bisection task?

The visual line bisection task (LBT) is sensitive to perceptual biases of visuospatial attention, showing slight leftward (for horizontal lines) and upward (for vertical lines) errors in healthy subjects. It may be solved in an egocentric or allocentric reference frame, and there is no obvious need for graviceptive input. However, for other visual line adjustments, such as the subjective visual vertical, otolith input is integrated. We hypothesized that graviceptive input is incorporated when performing the LBT and predicted reduced accuracy and precision when roll-tilted. Twenty healthy right-handed subjects repetitively bisected Earth-horizontal and body-horizontal lines in darkness. Recordings were obtained before, during, and after roll-tilt (±45°, ±90°) for 5 min each. Additionally, bisections of Earth-vertical and oblique lines were obtained in 17 subjects. When roll-tilted ±90° ear-down, bisections of Earth-horizontal (i.e., body-vertical) lines were shifted toward the direction of the head (P < 0.001). However, after correction for vertical line-bisection errors when upright, shifts disappeared. Bisecting body-horizontal lines while roll-tilted did not cause any shifts. The precision of Earth-horizontal line bisections decreased (P ≤ 0.006) when roll-tilted, while no such changes were observed for body-horizontal lines. Regardless of the trial condition and paradigm, the scanning direction of the bisecting cursor (leftward vs. rightward) significantly (P ≤ 0.021) affected line bisections. Our findings reject our hypothesis and suggest that gravity does not modulate the LBT. Roll-tilt-dependent shifts are instead explained by the headward bias when bisecting lines oriented along a body-vertical axis. Increased variability when roll-tilted likely reflects larger variability when bisecting body-vertical than body-horizontal lines.