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Light touch contact: not just for surfers
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Surfing most closely illustrates the challenge that people with balance problems overcome every day. The surface on which they stand is not the firm surface that healthy individuals perceive. Instead, each step is a continual challenge, requiring a wide base of support and conscious avoidance of soft, spongy surfaces, such as grass or a sandy beach. These balance problems often stem from deficits in one of the three sensory systems that are considered crucial for flexible balance control: vision, somato-sensation, and vestibular. The cost of poor balance to society is enormous. Imbalance is a major cause of falls, and in older adults is associated with functional decline and frailty. The total cost of fall injuries for people 65 and older was $20.4 billion in 1994 and is expected to exceed $32 billion by 2020, moving a U.S. Congressman to introduce legislation that would expand fall-related research and risk reduction programs.1
One of the ways that people with balance problems stabilize themselves is to subconsciously seek out other forms of sensory information that substitute for their deficit. Like the surfer touching the side wall of the tube, people with balance problems naturally seek out surfaces to touch when their balance is threatened, such as when entering a darkened room or walking along an uneven or narrow surface: a log in the woods, for example. Our research group has been studying this behavior over the past 10 years to determine what information humans derive from lightly touching surfaces. Recent investigations have shown that very light contact cues from just a single fingertip provide information that leads to enhanced control of body sway, even when the applied contact forces are physically inadequate to stabilize the body.2,3 Subsequent work has shown that sighted and congenitally blind individuals can use a cane to stabilize their upright stance in the same fashion as the fingertip, even at very low force levels.4
Light-touch contact studies
The light-touch studies paradigm is illustrated in Figure 1. Subjects stand on a force platform in a heel-to-toe stance to challenge their balance while touching a small force plate designed to measure the forces applied by the right index fingertip. The touch device consists of a horizontal metal plate attached to a metal stand situated to the side of the subject. The subjects place their right index finger on the middle of the bar while strain gauges mounted on the metal bar transduce the lateral and vertical forces applied by the fingertip. Subjects were initially tested with eyes opened and closed in three contact conditions: no contact, during which the subjects’ arms hung passively by their side; touch contact, in which the subjects could apply only up to 1N of force; and force contact, during which subjects could apply as much force as desired. In the light-touch condition, an auditory alarm went off if 1N of force was exceeded, indicating that the subject should apply less force without losing contact with the surface. The light-touch task is very easy to perform: after just a few seconds of practice to get a feel for the threshold force, subjects rarely set off of the alarm.
Figure 2 shows the typical results. Average displacement of the center of mass was highest with no contact/eyes closed and reduced in all other conditions. Despite mean fingertip force levels that were more than 10 times greater with force than touch contact, light touch reduced body sway equivalently.
In subsequent studies, a servomotor was attached to the plate to move it sinusoidally at different frequencies (0.1-0.8Hz) to derive a frequency response function between touch plate motion and body sway. The results were unequivocal. Body sway adopted the frequency of the touch plate with maximum gain at 0.2-0.4Hz. Modeling showed that subjects derived velocity information about their own body sway by touching the plate and using that as feedback to correct for sway deviations.5 Subjects were not told beforehand that the plate might move, but rarely reported perception of the moving plate. They often attributed the increased frequency of body sway driven by touch plate movement to a squishy floor, indicating that cognitive processes influence how the sensor information at the fingertip is interpreted.
How do these touch cues serve as a source of sensory information about body orientation? While cutaneous receptors are distributed across the entire body surface, they are particularly dense in the fingertip and hand. Analogous to the fovea of the retina, the fingertips are referred to as the somesthetic macula.6 Two-point discrimination studies have shown that the fingertip can resolve differences as small as 2mm,7 which is approximately the mean level of sway that we observe with light touch contact. Interestingly, two-point discrimination at the bottom of the foot is approximately 8-10mm, which is approximately the mean level of sway observed when subjects stand without fingertip contact and eyes closed.
In summary, a series of studies on postural control with light touch contact of the fingertip have demonstrated that somatosensory cues are a powerful orientation reference for improved control of upright stance. The movement of contact forces across the skin surface of remote extremities is providing orientation cues about movement of the body and signaling muscular activation for corrections of body sway. Small applied forces are not capable of physically moving the body, but still provide information about body orientation relative to the surfaces upon which we stand, lean, and touch.
The improvement in balance control observed with a mobility aid such as a cane is often attributed to the cane acting as a third leg, with the concomitant widening of the base of support. The light touch studies argue that in cases of a sensory deficit, improved balance control arises from the precise cues about body sway provided by somatosensory information from the fingertips and hand. The third leg is uniquely different from the real legs. It has the high resolution of the fingertip to detect movement related to body sway, resulting in postural corrections well before the boundaries of upright stability.
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