Haptic feedback enhances rhythmic motor control by reducing variability, not improving convergence rate

Abstract

Stability and performance during rhythmic motor behaviors such as locomotion are critical for survival across taxa: falling down would bode well for neither cheetah nor gazelle. Little is known about how haptic feedback, particularly during discrete events such as the heel-strike event during walking, enhances rhythmic behavior. To determine the effect of haptic cues on rhythmic motor performance, we investigated a virtual paddle juggling behavior, analogous to bouncing a table tennis ball on a paddle. Here, we show that a force impulse to the hand at the moment of ball-paddle collision categorically improves performance over visual feedback alone, not by regulating the rate of convergence to steady state (e.g., via higher gain feedback or modifying the steady-state hand motion), but rather by reducing cycle-to-cycle variability. This suggests that the timing and state cues afforded by haptic feedback decrease the nervous system's uncertainty of the state of the ball to enable more accurate control but that the feedback gain itself is unaltered. This decrease in variability leads to a substantial increase in the mean first passage time, a measure of the long-term metastability of a stochastic dynamical system. Rhythmic tasks such as locomotion and juggling involve intermittent contact with the environment (i.e., hybrid transitions), and the timing of such transitions is generally easy to sense via haptic feedback. This timing information may improve metastability, equating to less frequent falls or other failures depending on the task.

Implicit guidance to stable performance in a rhythmic perceptual-motor skill

“…Prior studies on ball bouncing have also demonstrated the importance of ball-racket impact in behavior. For instance, in contrast to haptic guidance throughout the entire racket trajectory, haptic feedback at the ball-racket impact does improve performance and learning (Sternad et al 2001; Ankarali et al 2014). An intervention that focuses on physical contacts has also been shown a promising direction for robotic assistance in locomotor recovery.…”

System identification of rhythmic hybrid dynamical systems via discrete time harmonic transfer functions

“…, t n−1 , such that t 0 < t 1 < · · · < t n . Each event time corresponds to a hybrid transition, giving rise to a sequence of discrete states α 0 , · · · , α n−1 and smooth trajectories q αi (t) ∈ V αi , t ∈ [t i , t i+1 ], such that each q αi is a trajectory of the continuoustime dynamical system on V αi given in (2). Further, the…”

“…Rhythmic dynamic behaviors can be observed in a wide variety of biological and robotics systems, such as terrestrial locomotion [3,17] and juggling [2,4]. Such behaviors often include hybrid characteristics in that they exhibit both smooth flows punctuated by discrete jumps and are often modeled as hybrid dynamical systems [10].…”

“…There are also recent studies that attempt to estimate and quantify the dynamics around the limit cycles of such systems from data [2,15]. The most popular mathematical framework used in such analysis is based on Poincaré return maps which reduce the rhythmic dynamical system to a lower dimensional discrete-time system that describes the behavior in terms of its cycle-to-cycle transitions.…”

Haptic proprioception in a virtual locomotor task

“…Also, visual and haptic feedback can improve performance of lower extremity training than visual-only and haptic only mode [8]. Cowan et al have shown that providing sensory feedback-like force impulse to the hand while performing rhythmic motor tasks, such as virtual paddle juggling, enhances performance by reducing variability in the rhythmic movement [9].…”