The effect of light touch on standing sway when the stability of the external touch reference becomes unreliable

Lightly touching a stable reference is associated with sway reduction during standing. Unexpected displacement of the touch reference results in a false-positive balance reaction in some participants, but only with the first such disturbance. This study investigated whether light touch reduces standing sway (1) after the touch reference becomes unreliable, and (2) when participants are aware the touch reference is unreliable. 40 healthy adults, 20 that were naïve to the possibility of a touch reference displacement and 20 that were made aware prior to testing, were asked to stand while lightly touching (< 1 N) a reference with normal vision or vision occluded. Motion of the center of pressure was used to estimate standing sway before and after a single displacement, and then multiple displacements, of a touch reference. Sway area was always reduced while touching the reference, compared to standing with vision occluded without touch, even when the reference was known to be unreliable. In addition, sway area was further reduced following a single touch displacement in Naïve participants when vision was occluded. These results suggest that tactile cues from the finger interact with postural control in a complex manner, depending upon the expectation and experience of the characteristics of the touched object. Taken together, light touch can (1) be used as a spatial reference that assists in sway stabilization, (2) be a source of movement variability that impacts the performance of a skilled task, or (3) introduce noise in the sensory channels impacting fidelity.