Research reportConstraints during bimanual coordination: the role of direction in relation to amplitude and force requirements
Introduction
A central question that has occupied researchers of motor control refers to the nature of the variables or movement parameters that are primary candidates for encoding in the central nervous system [42]. With respect to interlimb coordination in general and bimanual coordination specifically, this can be rephrased as the identification of the variables that constrain coordination. The underlying assumption is that those parameters that constrain bimanual coordination, indirectly reflect candidates for cortical encoding. Parameters undergoing substantial encoding are assumed to imply a substantial processing load within the central nervous system. The resulting neural activity is likely to spread to other brain areas and even to the contralateral hemisphere as a result of interhemispheric connections. Accordingly, it is not surprising that interference arises between both upper limbs, particularly when performing two different tasks simultaneously [20], [24], [28], [49].
Previous studies have mainly focused on the constraining role of timing by imposing tasks in which the upper limbs tap different rhythms simultaneously. When the temporal basis of one task is an integer multiple of the other, minor difficulties are observed [4] even though this depends on the precise nature of task execution as well as on the effectors used for the task [45], [51]. Complex rhythms with less compatible temporal ratios can become extremely difficult [4], [44].
The study of spatial constraints has received much less attention. Particularly, the role of movement direction has not yet been addressed systematically whereas movement amplitude has been studied occasionally. It has been observed that performing movements with different amplitude specifications in the upper limbs simultaneously, results in assimilation effects in which the larger of both amplitudes is often reduced and/or the small amplitude is increased [9], [28], [38], [39], [41]. Directional constraints have received very limited attention even though there are strong indications that direction is a primary coordination constraint in coordination of the homologous [10], [11], [47] as well as the nonhomologous limbs [1], [37], [46]. Furthermore, the relationship between direction, amplitude, and force specifications has not yet been addressed.
In the present study, the constraining role of movement direction, amplitude, and force in patterns of bimanual coordination was systematically studied in relation to each other. Psychophysical as well as basic neuroscientific studies on unilateral limb movements have addressed the role of amplitude and direction either separately or in combination. The results of psychophysics studies using reaction time, isometric force production, and reaching movement paradigms, have suggested that there is some degree of independence between both parameters or that they are subserved by (at least partly) different processes [7], [19], [29], [32], [40]. For example, Megaw [29] observed that specification of movement direction influenced reaction time more than specification of distance. Neurophysiological studies on single cell recording have revealed that both movement distance and direction are encoded in the discharge of premotor as well as primary motor cortex neurons [13] whereas evidence for the encoding of force has been less pervasive [15]. There is currently some debate, however, as to whether spatial encoding takes place with respect to an intrinsic versus extrinsic reference frame [21], [35].
The aforementioned studies on the production of various types of unilateral tasks hint that movement direction and amplitude are important movement parameters that are possibly subserved by (partially) distinct neural encoding processes. If so, they are likely to be primary candidates for constraining interlimb coordination. In the present study, the role of direction in relation to amplitude was investigated in a bimanual task paradigm (Experiment 1). Subjects were to maintain a single (vertical) movement direction in the left limb during cyclical line drawing (line task), whereas direction in the right limb was manipulated in eight sequential steps (star task). In the star task, subjects started with vertical movements and direction was shifted with 45° in a clockwise fashion, each time a series of five lines was completed. This resulted in eight movement directions. Subjects were instructed to produce the shifts in direction in one limb while maintaining the imposed direction in the other limb. The movements were performed at medium speeds in order to reveal the default central nervous system operations and to reduce the use of compensatory mechanisms. Amplitude was manipulated by having subjects perform left and right limb movements with either the same or different amplitude specifications. Whereas Experiment 1 focused on the role of direction and amplitude, Experiment 2 addressed direction in relation to force specification. Two types of force manipulations were applied: spring loading and mass loading. These loading conditions were intended to change the quality and/or quantity of muscle activation patterns underlying the different orientations of star drawing performance.
Section snippets
Subjects
Twelve undergraduate students of Katholieke Universiteit Leuven participated in the experiment. All subjects (four males, eight females) were right-handed (mean age=21 years, 7 months) and provided informed consent. They were not previously involved in a similar experiment. The procedures were reviewed by the ethical committee of Biomedical Research at K.U.Leuven.
Apparatus and task
The apparatus consisted of two XY-digitizing tables (LC20-TDS Terminal Display Systems) positioned in the horizontal plane in front
Discussion
The present discussion is focused on directional constraints in bimanual coordination in association with amplitude and force specifications.
Summary
In the present paper, a new psychophysical paradigm is proposed that focuses on a careful observation of the patterns of contralateral interference during the production of bimanual tasks. The underlying working hypothesis is that parameters which are preferably encoded in the higher cortical regions, will induce a minimal processing load in the central nervous system, giving rise to neural irradiation and structural interference within a highly linked neural medium when different parameters
Acknowledgements
Support for the present study was provided through a grant from the Research Council of K.U. Leuven, Belgium (Contract No. OT/99/39) and the Flanders Fund for Scientific Research (Project G.0285.98). Dr N. Dounskaia, Dr O. Levin, and Professor J. Duysens were supported by fellowships from the Research Council of K. U. Leuven (Contract No. F/95/51, F/00/096, and F/99/34, respectively).
References (53)
- et al.
Preferential coupling between voluntary movements of ipsilateral limbs
Neurosci. Let.
(1982) - et al.
Who tells one hand what the other is doing: the neurophysiology of bimanual movements
Neuron
(1999) Coordination
Interlimb amplitude differences, spatial assimilations and the temporal structure of rapid bimanual movements
Hum. Mov. Sci.
(1994)- et al.
Exploring interlimb constraints during bimanual graphic performance: effects of muscle grouping and direction
Behav. Brain Res.
(1998) - et al.
Are there unifying structures in the brain responsible for interlimb coordination?
- et al.
Making arm movements within different parts in space: the premotor and motor cortical representation of a coordinate system for reaching to visual targets
J. Neuroscience
(1991) Two-handed movement control
Res. Quart.
(1983)The generation of two isochronous sequences in parallel
Percept. Psychophys.
(1983)- et al.
Primary motor cortex is involved in bimanual coordination
Nature
(1998)