Effects of Tendon Vibration, Light Touch, and Mechanical Noise on Postural Control: Implications for Somatosensory Reweighting

In order to maintain balance equilibrium, the body relies on sensory feedback from the visual, vestibular, and somatosensory systems. It is hypothesized that postural control is maintained by dynamically weighting the contributions of afferents from these systems based upon the relevance and accuracy of their inputs, a concept known as sensory reweighting. The reweighting of sensory afferents for balance is more commonly explained holistically as entire sensory systems being up-weighted or down-weighted based on their appropriateness; however, all three sensory modalities utilized in balance (visual, vestibular, and somatosensory) have various types of sensory receptors whose inputs could be reweighted accordingly within a modality, as opposed to reweighting the entire modality as a whole for postural control. This study investigated contributions from various receptor types specifically within somatosensation to postural control. Tactile and muscle spindle receptors from both the upper- and lower-body were manipulated by utilizing combinations of tendon vibration, fingertip light touch (FLT), and small amounts of mechanical noise intended to induce stochastic resonance (SR), a phenomenon where weak sensory inputs may be enhanced by the addition of noise. Three separate experiments were conducted to assess interaction effects on balance among: 1) mechanical noise delivered to the bottom of the feet and Achilles tendon vibration (Aim 1), 2) FLT conditions and Achilles tendon vibration (Aim 2), and 3) FLT and arm tendon vibration conditions (Aim 3). Results revealed that combinations of somatosensory stimuli produced differing postural effects than the individual stimuli themselves. It was inferred that these effects during interactions were evidence of reweighting occurring within the somatosensory system itself. This study provides further insight into how the sensory reweighting hypothesis accounts for human postural control and how such forms of somatosensory manipulation might be utilized in the development of countermeasures to combat balance deficits in a multitude of populations at greater fall risk.