Gravitational and Somatosensory Influences on Control and Perception of Roll Balance.

INTRODUCTION: Blindfolded subjects used a joystick to orient themselves to the direction of balance in a device programmed to exhibit inverted pendulum behavior in the roll plane; they indicated with a trigger press when they were at that location. Our goal was to determine how otolith and somatosensory information about the gravitational vertical influenced the ability to locate the direction of balance. METHODS: The subjects (N = 12) were tested in each of three orientations of the body roll plane: vertical (Upright), 45° back (45_Degree), and 90° back (Supine), which provided progressively less salient otolith and somatosensory information about roll orientation with regard to the direction of gravity. For each pitch plane, subjects were tested with three directions of balance: 0° (aligned with the gravitational vertical in the Upright condition) and 30° right or left. RESULTS: The mean achieved and indicated orientations for the Upright and 45_Degree conditions were significantly displaced away from the direction of balance in the direction of gravity, with indicated angles less displaced. In the Supine condition, the mean achieved and indicated angles were closer to the direction of balance, but their within-trial standard deviations were significantly larger than in the Upright and 45_Degree conditions, which did not differ. This greater variability resulted from the frequent side to side "drifting" behavior that was a characteristic feature of the Supine condition only. DISCUSSION: These findings indicate that in the absence of vision accurate dynamic orientation requires gravity dependent shear forces on the otolith organs and body surface.Panic AS, Panic H, DiZio P, Lackner JR. Gravitational and somatosensory influences on control and perception of roll balance. Aerosp Med Hum Perform. 2017; 88(11):993-999.

Direction of balance and perception of the upright are perceptually dissociable.

We examined whether the direction of balance rather than an otolith reference determines the perceived upright. Participants seated in a device that rotated around the roll axis used a joystick to control its motion. The direction of balance of the device, the location where it would not be accelerated to either side, could be offset from the gravitational vertical, a technique introduced by Riccio, Martin, and Stoffregen (J Exp Psychol Hum Percept Perform 18: 624-644, 1992). Participants used the joystick to align themselves in different trials with the gravitational vertical, the direction of balance, the upright, or the direction that minimized oscillations. They pressed the joystick trigger whenever they thought they were at the instructed orientation. Achieved angles for the "align with gravity" and "align with the upright" conditions were not different from each other and were significantly displaced past the gravitational vertical opposite from the direction of balance. Mean indicated angles for align with gravity and align with the upright coincided with the gravitational vertical. Both mean achieved and indicated angles for the "minimize oscillations" and "align with the direction of balance" conditions were significantly deviated toward the gravitational vertical. Three control experiments requiring self-settings to instructed orientations only, perceptual judgments only, and perceptual judgments during passive exposure to dynamic roll profiles confirmed that perception of the upright is determined by gravity, not by the direction of balance.