Distinct Timing Mechanisms Produce Discrete and Continuous Movements

Raoul Huys,Nicole L Rheaume,Viktor K Jirsa,Breanna E Studenka,Howard N Zelaznik,Karl J Friston

doi:10.1371/journal.pcbi.1000061

Abstract

The differentiation of discrete and continuous movement is one of the pillars of motor behavior classification. Discrete movements have a definite beginning and end, whereas continuous movements do not have such discriminable end points. In the past decade there has been vigorous debate whether this classification implies different control processes. This debate up until the present has been empirically based. Here, we present an unambiguous non-empirical classification based on theorems in dynamical system theory that sets discrete and continuous movements apart. Through computational simulations of representative modes of each class and topological analysis of the flow in state space, we show that distinct control mechanisms underwrite discrete and fast rhythmic movements. In particular, we demonstrate that discrete movements require a time keeper while fast rhythmic movements do not. We validate our computational findings experimentally using a behavioral paradigm in which human participants performed finger flexion-extension movements at various movement paces and under different instructions. Our results demonstrate that the human motor system employs different timing control mechanisms (presumably via differential recruitment of neural subsystems) to accomplish varying behavioral functions such as speed constraints.

Highlights

Discrete movements constitute singularly occurring events preceded and followed by a period without motion for a reasonable amount of time, such as a single finger flexion or flexion-extension cycle [1,2]
We introduce the notion of phase flow topologies, identify the invariance separating two movement classes, and present an experimental study testifying to the existence of two different movement classes
In the mono-stable regime the actual timing deviated from the required timing due to a period-doubling when the movement pace exceeded approximately 2.0 Hz. (Figure 2A), which occurs due to the arrival of stimulus n before movement n21 has finished. These observations were robust under all parameter settings within each dynamical regime, the frequency at which the period doubling occurred showed a small variation as a function of one of the model parameters

Summary

Introduction

Discrete movements constitute singularly occurring events preceded and followed by a period without motion (i.e., with zero velocity) for a reasonable amount of time, such as a single finger flexion or flexion-extension cycle [1,2]. Continuous movements lack such recognizable endpoints, and normally are considered rhythmic if they constitute repetitions of particular events, in which case they often look quite sinusoidal. Its significance lies in the fact that the classification is model-independent; every behavior within a class can be mapped upon others, whereas maps between classes do not exist We use this principled approach to address the controversy whether discrete and rhythmic movements are fundamentally different. We introduce the notion of phase flow topologies, identify the invariance separating two movement classes, and present an experimental study testifying to the existence of (at least) two different movement classes

Methods

Results

Conclusion