The Importance of Being in Touch

Abstract

This paper describes a series of studies resulting from the finding that when free floating in weightless conditions with eyes closed, all sense of one's spatial orientation with respect to the aircraft can be lost. But, a touch of the hand to the enclosure restores the sense of spatial anchoring within the environment. This observation led to the exploration of how light touch of the hand can stabilize postural control on Earth even in individuals lacking vestibular function, and can override the effect of otherwise destabilizing tonic vibration reflexes in leg muscles. Such haptic stabilization appears to represent a long loop cortical reflex with contact cues at the hand phase leading EMG activity in leg muscles, which change the center of pressure at the feet to counteract body sway. Experiments on dynamic control of balance in a device programmed to exhibit inverted pendulum behavior about different axes and planes of rotation revealed that the direction of gravity not the direction of balance influences the perceived upright. Active control does not improve the accuracy of indicating the upright vs. passive exposure. In the absence of position dependent gravity shear forces on the otolith organs and body surface, drifting and loss of control soon result and subjects are unaware of their ongoing spatial position. There is a failure of dynamic path integration of the semicircular canal signals, such as occurs in weightless conditions.

Keywords: dynamic balance; non-orientation; path integration; position cues; spatial orientation; vehicle control; velocity storage; vestibular loss.

Figure 1

(A) Illustration of a deep knee bend made in 1 g. The surface of support and the visual surroundings are felt and seen to remain stationary as the body is lowered. (B) During a deep knee bend made during initial exposure to 1.8 g, it feels as if the knees have flexed too rapidly and the aircraft is seen and felt to displace upwards under the feet causing too rapid flexion of the knees. (C) Following about 50 deep knee bends made over subsequent parabolas, the deep knee bends again feel normal and the aircraft is seen and felt to be stable again as the body is lowered. (D) Following adaptation to 1.8 g, the initial deep knee bends made during 1 g straight-and-level flight again seem abnormal with the body seeming to move downward too slowly because the aircraft seems to move downward slowing the flexing of the legs.

Figure 2

Typical test situation for our light-touch stabilization of posture studies. The alarm sounds when the force on the finger touch plate exceeds 1 N (≈103 g). Subjects in practice never reach this force level in their experimental trials. The alarm is off during force touch trials.

Figure 3

The relationship between the lateral CP and fingertip force changes and EMG activity that leads to CP changes. For light-touch, force changes at the fingertip lead EMG activity by 125 ms and CP changes by ≈275 ms. The EMG reflects muscle activation that 150 ms later alters the CP to counteract body sway. With force touch, changes in applied finger force lead CP changes by ≈80 ms, indicating some mechanical stabilization by the finger force level.

Figure 4

The results are illustrated for the no-contact, von Frey Filament with 10 g bending resistance, rigid metal filament, and light-touch finger conditions. The von Frey filaments with larger bending constants are not shown but all attenuate sway more than the 10 g filament. The rigid filament and finger touch attenuate CP mean sway amplitude more than other conditions.

Figure 5

Time course of haptic stimulation after finger contact is made ≈ 12 s into 25 s long trial. (A) shows medial-lateral finger position (cm) and (B) medial-lateral center of pressure (cm). Note rapid decrease in sway even prior to full finger stabilization.

Figure 6

The effects of eliciting tonic vibration reflexes in the peroneus longus muscle of the standing subject's right leg or in the biceps of the right arm are shown for the different touch conditions. Vibrating the peroneus longus destabilizes subjects and can evoke falling. Light touch fully eliminates the effect. By contrast, during light-touch, biceps vibration is destabilizing and can induce falling or protective stepping.

Figure 7

Entrainment of body sway to cyclical lateral oscillations of the touch bar at frequencies of 0, 0.1, 0.2, 0.3, 0.4, and 0.5 Hz. TB_x = lateral motion (≈4 mm amplitude) of touch bar, CP_x = lateral motion of the center of pressure, and H_x = lateral head motion. Subjects (n = 5) show entrainment of head and center of pressure to the touch bar frequency of oscillation.

Figure 8

Mean sway amplitude of center of pressure (CP), top panel, and of head, bottom panel of labyrinthine loss (LL), N = 5 and normal subjects, N = 5. There are no entries in the dark no-touch conditions for the LL subjects because they lose balance within several seconds. With light touch in the dark, the LL subjects have a significantly lower CP mean sway amplitude than the normal subjects in their no-touch vision condition.

Figure 9

Illustration of the MARS configured for vertical plane roll balancing. The axis of roll motion is indicated by the ⊗ symbol, the deviation from the gravitational vertical is angle φ. Kp = 600°/s² is the inverted pendulum constant.

Figure 10

Results for the conditions “align with the upright” and “align with the direction of balance.” Seven directions of balance (DOB) were presented twice for each instruction type for 16 subjects. The DOBs were −30°, −15°, −5°, 0°, +5°, +15°, and +30°. Red entries represent achieved angles, blue represents indicated angles. The shading indicates standard deviations. The b entries represent slopes of MARS angles in degrees vs. DOB angles. The sign convention is minus entries represent rightward tilts of the subject and DOB, positive is leftward tilts. The intersection of the dotted horizontal and diagonal lines corresponds to 0°, the direction of gravity.

Figure 11

Velocity-position phase plots for a typical subject in vertical roll plane balancing. Solid lines in quadrants 1 and 3 represent crash boundaries where joystick commands cannot prevent the MARS from exceeding + or −60 degrees. Dots outside these lines represent crashes. The MARS is reset to the start position after a crash and the trial then continues. Performance is near perfect after 20 trials. The red dots represent anticipatory joystick commands that slow the MARS down as it approaches the balance point. Blue dots represent destabilizing joystick commands that accelerate the MARS in the direction of a crash boundary.

Figure 12

Velocity-position phase plots for a typical subject in horizontal roll plane balancing show cyclical patterns of looping and drifting. The drifting and looping are reduced but not eliminated over repeated trials. Crashes are decreased but not eliminated as shown by dots outside crash boundaries.

Figure 13

Velocity-position phase plots for vertical yaw axis balancing. The drifting and looping and crashes characteristic of horizontal roll plane balancing (Figure 12) are apparent for the yaw axis, where there are also no gravity dependent positional cues available about ongoing yaw rotational position. Subjects, as in horizontal roll, report being unable to sense their ongoing position relative to the direction of balance.

Figure 14

Position-velocity phase plots for a typical subject performing horizontal yaw axis dynamic balancing. In this situation, gravity dependent positional shear forces are present and learning is obvious from Trial 1 to 20, just as in vertical roll plane balancing shown in Figure 11.

Figure 15

Recumbent subjects (N = 6) indicated the amplitude of imposed passive yaw-axis rotations during the 0 and 1.8 g phases of parabolic flight maneuvers and in straight-and-level flight. Rotations were 30° or 60° in amplitude and lasted <1.5 s. Subjects used a gravity neutral pointer and tried to keep it aligned with their start position. In 0 g, rotations were greatly underestimated. Similar results were obtained for vertical yaw axis rotation in 0 g.