Table 1.
Sensory-derived signals represented as model parameters
| Sensory Origin | Definition | Model Parameter |
|---|---|---|
| Vestibular (EC) or vision + vestibular (EO) combined with intersegmental proprioception | Encodes θL | KLl |
| Encodes θ̇L | BLl | |
| UB-to-LB proprioception (muscle spindles, receptors in joints and ligaments) | Encodes θL − θU | KLlu, KUlu |
| Encodes θ̇L − θ̇U | BLlu, BUlu | |
| LB-to-surface proprioception (muscle spindles, receptors in joints and ligaments) | Encodes θS − θL | KLsl |
| Encodes θ̇S − θ̇L | BLsl | |
| Vestibular (EC) or vision + vestibular (EO) combined with intersegmental proprioception | Encodes θU | KUu |
| Encodes θ̇U | BUu | |
| UB-to-pelvis proprioception (muscle spindles, receptors in joints and ligaments) | Encodes θP − θU | KUpu |
| Encodes θ̇P − θ̇U | BUpu | |
| Kinetic proprioception (Golgi tendon organs, load receptors) | Encodes TL | GLt, τLt |
| Kinetic proprioception (Golgi tendon organs, load receptors) | Encodes TU | GUt, τUt |
Sensory signals are assumed to accurately encode physical variables. These sensory signals represent the output of central nervous system processing of information conveyed by primary sensory afferents (Angelaki et al. 1999; Casabona et al. 2004; Mergner et al. 1991, 1997). For kinematic feedback from sensory systems encoding joint motion, K is the proportionality constant between joint angle and corrective torque generation (a “stiffness” parameter) and B is the proportionality constant between joint angular velocity and corrective torque generation (a “damping” parameter). For kinetic feedback from sensory systems encoding torques, G is the proportionality constant between torques applied to joints of the body (TL or TU) and corrective torque generation, with the additional assumption that this corrective torque is low-pass filtered (filter time constants τLt and τUt). EC, eyes closed; EO, eyes open; θL, lower body (LB) sway angle; θU, upper body (UB) sway angle; θS, surface tilt angle; θP, pelvis orientation angle.