Available online at www.sciencedirect.com Behavioural Brain Research 187 (2008) 361–370 Research report Directional constraints during bimanual coordination: The interplay between intrinsic and extrinsic directions as revealed by head motions R.L.J. Meesen a,b , N. Wenderoth a , J.J. Temprado c , S.P. Swinnen a,∗ a Motor Control Laboratory, Research Center for Motor Control and Neuroplasticity, Department of Biomedical Kinesiology, K.U.L., Tervuurse Vest101, 3001 Leuven, Belgium b REVAL Research Group, Department of Health Care Sciences, Provinciale Hogeschool Limburg, Guffenslaan 39, 3500 Hasselt, Belgium c UMR 6152 “Mouvement & Perception”, Universit´ e de la M´ editerran´ ee et CNRS, Facult´ e des Sciences du Sport, 163 Avenue de Luminy, BP 910, 13288 Marseille, France Received 22 February 2007; received in revised form 21 September 2007; accepted 25 September 2007 Available online 4 October 2007 Abstract The role of directional compatibility was investigated during the production of in-phase and anti-phase coordination patterns involving both arms as well as the head. Our first aim was to compare the quality of coordination between both arms when symmetrical arm posture manipulations were used to disentangle muscle homology from the mutual direction of limb motions in extrinsic space. Findings revealed that in-phase coordination, characterized by the simultaneous activation of homologous muscle groups, was resistant to posture manipulations. Conversely, during anti-phase coordination, the influence of extrinsic direction became more prevalent whereby isodirectionality in extrinsic space contributed to stabilization of anti-phase coordination patterns. The second aim was to study the effect of periodic head movements upon the assembling of a coordinative synergy among the body segments. The findings demonstrated that the in-phase patterns were hardly affected by directionality of head motion. Conversely, the anti-phase patterns were more vulnerable to the directional influence of head movements, showing less accurate and stable coordination during non-isodirectional than isodirectional head motions. These observations underscore the robust nature of coordination patterns based on muscle homology, even in the absence of symmetric arm positions. Moreover, isodirectional head movements became easily integrated with the overall coordination pattern, whereas head–limb coupling was poor when the head moved anti-directional with the limbs. © 2007 Elsevier B.V. All rights reserved. Keywords: Interlimb coordination; Egocentric; Allocentric; Homologous limbs; Head; Relative phase; Coordination constraints 1. Introduction In everyday life, we often manage to perform complicated coordination skills involving many joint and muscle combi- nations with little effort and high efficiency. Bernstein was already intrigued by this phenomenon which he referred to as the degrees-of-freedom problem [2]. His profound insights have inspired an intensive research program into the constraints that govern interlimb coordination. Constraints emerge within ∗ Corresponding author at: Motor Control Laboratory, Research Center for Motor Control and Neuroplasticity, Department of Biomedical Kinesiology, K.U.L., Tervuurse Vest 101, 3001 Leuven, Belgium. Tel.: +32 16 32 90 71; fax: +32 16 32 91 97. E-mail address: Stephan.Swinnen@FABER.KULEUVEN.BE (S.P. Swinnen). the context of the performer and his surroundings and refer to basic rules that enable the central nervous system to control the many degrees-of-freedom evident in biological systems, includ- ing humans. Two basic coordination constraints have received prominent attention in the context of cyclical bimanual coordi- nation tasks: the egocentric and the allocentric constraint. On the one hand, the egocentric constraint refers to a preference for simultaneous activation of the homologous muscle groups (in-phase, relative phase Φ =0 ◦ ), often resulting in mirror-symmetric movements with respect to the body midline and corresponding to movements in opposite direc- tions (non-isodirectional) in extrinsic space. However, these directional relationships depend on the particular task context (medio-lateral movements in the present case). When homolo- gous muscles are activated in alternation (anti-phase, Φ = 180 ◦ ), both limbs often move in the same direction (isodirectional) 0166-4328/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2007.09.030