Refractoriness in Sustained Visuo-Manual Control: Is the Refractory Duration Intrinsic or Does It Depend on External System Properties?

Abstract

Researchers have previously adopted the double stimulus paradigm to study refractoriness in human neuromotor control. Currently, refractoriness, such as the Psychological Refractory Period (PRP) has only been quantified in discrete movement conditions. Whether refractoriness and the associated serial ballistic hypothesis generalises to sustained control tasks has remained open for more than sixty years. Recently, a method of analysis has been presented that quantifies refractoriness in sustained control tasks and discriminates intermittent (serial ballistic) from continuous control. Following our recent demonstration that continuous control of an unstable second order system (i.e. balancing a ‘virtual’ inverted pendulum through a joystick interface) is unnecessary, we ask whether refractoriness of substantial duration (∼200 ms) is evident in sustained visual-manual control of external systems. We ask whether the refractory duration (i) is physiologically intrinsic, (ii) depends upon system properties like the order (0, 1st, and 2nd) or stability, (iii) depends upon target jump direction (reversal, same direction). Thirteen participants used discrete movements (zero order system) as well as more sustained control activity (1st and 2nd order systems) to track unpredictable step-sequence targets. Results show a substantial refractory duration that depends upon system order (250, 350 and 550 ms for 0, 1st and 2nd order respectively, n = 13, p<0.05), but not stability. In sustained control refractoriness was only found when the target reverses direction. In the presence of time varying actuators, systems and constraints, we propose that central refractoriness is an appropriate control mechanism for accommodating online optimization delays within the neural circuitry including the more variable processing times of higher order (complex) input-output relations.

Human Balance Control: Dead Zones, Intermittency, and Micro-chaos

“…However, it has long been speculated that there is a refractoriness in the execution of motor movements planned by the central nervous system such that corrective movements are made only intermittently [56,94,95]. Indeed the increased effectiveness of intermittent versus continuous feedback control has been demonstrated in a virtual stick balancing task [56].…”

“…In the case of stick balancing on the fingertip the intermittency manifests as a power law [12,19,75]. Second, the observed intermittency may reflect an intermittent motor control strategy [38,56,94,95,91]. Indeed control-theoretic considera-tions indicate that optimal control strategies in the presence of noise and delay are those in which corrective movements are made intermittently [75,92,93].…”

Intermittent control of unstable multivariate systems with uncertain system parameters

“…Recent theoretical and methodological advances have provided new experimental evidence that while human sustained tracking masquerades as continuous control, humans actually uses sensory feedback intermittently to sequentially update intervals of open loop predictive control [2,5,6]. An attribute of open loop predictive control of unstable systems, is that control is sensitive to inaccurate model values of physical system parameters.…”

Semidiscretization for Time-Delayed Neural Balance Control

“…Traditionally, mathematical models for the control of human balance [2,34,35,43,50,66,70,74,78] and stick balancing [22,33,42,53] have assumed that the feedback is a continuous and smooth function of time. Nonetheless, a number of experimental observations on human balance and movement suggest that the feedback exhibits a number of properties expected for digital control including the quantized nature of slow voluntary movements [52,71,76,79], the intermittent character of corrective movements [8,9,14,38,44,51,60], and the role of central refractory times [72,73,75,79]. Indeed, for certain balancing tasks, intermittent control works better than continuous control [46].…”