Visuo-manual tracking: does intermittent control with aperiodic sampling explain linear power and non-linear remnant without sensorimotor noise?

Abstract

The human operator is described adequately by linear translation of sensory input to motor output. Motor output also always includes a non-linear remnant resulting from random sensorimotor noise from multiple sources, and non-linear input transformations, for example thresholds or refractory periods. Recent evidence showed that manual tracking incurs substantial, serial, refractoriness (insensitivity to sensory information; 350, 550 ms; 1st 2nd order systems respectively). Our two questions are (i) the comparative merits of explaining the non-linear remnant using noise or non-linear transformations (ii) can non-linear transformations represent serial motor decision making within the sensorimotor feedback loop intrinsic to tracking?

Twelve participants (instructed to act in three prescribed ways) manually controlled two systems (1st, 2nd order) subject to a periodic multi-sine disturbance. Joystick power was analysed using three models, continuous-linear-control (CC), continuous-linear-control with calculated noise spectrum (CCN), and intermittent control with aperiodic sampling triggered by prediction error thresholds (IC). Unlike the linear mechanism, the intermittent control mechanism explained the majority of total power (linear and remnant) (77–87% v. 8–48%, IC v CC). Between conditions, IC used thresholds and distributions of open loop intervals consistent respectively with instructions and previous measured, model independent values; whereas CCN required changes in noise spectrum deviating from broadband, signal dependent noise. We conclude manual tracking uses open loop predictive control with aperiodic sampling. Because aperiodic sampling is inherent to serial decision making within previously identified, specific frontal, striatal and parietal networks we suggest that these structures are intimately involved in visuomanual tracking.

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