Behavioural Brain Research 123 (2001) 201 – 218 Research report Constraints during bimanual coordination: the role of direction in relation to amplitude and force requirements Stephan P. Swinnen a, *, Natalia Dounskaia b , Oron Levin a , Jacques Duysens c a Laboratory of Motor Control, Department of Kinesiology, Group Biomedical Sciences, K.U.Leuen, Teruurse Vest 101, 3001 Leuen, Belgium b Motor Control Laboratory, Arizona State Uniersity, AZ, USA c Department of Medical Physics and Biophysics, K.U. Nijmegen, Belgium Received 29 September 2000; received in revised form 15 March 2001; accepted 15 March 2001 Abstract The present study addressed the status of spatial encoding during a bimanual task paradigm. This was based on the premise that patterns of contralateral interference during bimanual coordination provide a window into those movement parameters that are primarily encoded within the central nervous system. Results showed that both direction and amplitude were subject to (bilateral) interference when different specifications were to be generated simultaneously for each limb. Directional interference was found to be partially independent of the amount and pattern of underlying muscle activation, suggesting that direction is encoded at a rather abstract level in the central nervous system. The findings are consistent with single-cell recording studies that have pointed to the role of directional tuning in various brain areas. Moreover, the findings suggest that spatial parameters of movement constrain the coordination of limb movements in addition to temporal parameters. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Amplitude; Bimanual coordination; Direction; Human; Loading; Movement encoding; Spring www.elsevier.com/locate/bbr 1. 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 re- spect to interlimb coordination in general and bimanual coordination specifically, this can be rephrased as the identification of the variables that constrain coordina- tion. The underlying assumption is that those parame- ters that constrain bimanual coordination, indirectly reflect candidates for cortical encoding. Parameters un- dergoing 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 perform- ing two different tasks simultaneously [20,24,28,49]. Previous studies have mainly focused on the con- straining 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 tempo- ral ratios can become extremely difficult [4,44]. The study of spatial constraints has received much less attention. Particularly, the role of movement direc- tion 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 * Corresponding author. Tel.: +32-16-329071; fax: +32-16- 329197. E-mail address: stephan.swinnen@flok.kuleuven.ac.be (S.P. Swin- nen). 0166-4328/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0166-4328(01)00210-8