Neuroscience Letters 416 (2007) 117–122 Differential after-effects of bimanual activity on mirror movements Alistair N. Vardy * , Andreas Daffertshofer, Arne Ridderikhoff, Peter J. Beek Institute for Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands Received 15 December 2006; received in revised form 15 January 2007; accepted 29 January 2007 Abstract Using a rhythmic isometric force production paradigm, we investigated the after-effects of in-phase and antiphase bimanual performance on the unintended recruitment of the homologous muscles of the opposite limb during subsequent performance of tasks that were unimanual by design. Electromyograms obtained from the muscles of the opposite limb were analyzed in terms of their amplitude and the distribution of their phase relative to that of the intended movements. Preceding bimanual activity had distinct effects on the relative phase (mean and uniformity) of the structured electromyograms. These were particularly pronounced following performance of the in-phase pattern. These findings are discussed in terms of interhemispheric excitation and inhibition. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Mirror movements; Bimanual coordination; EMG Mirror movements (MMs) are unintended activities of homolo- gous muscles on the opposite side of the body during intended unimanual performances that are also referred to as (contralat- eral) motor irradiations [7], motor overflows [1,4], synkinesis [13], or associated movements [15]. While MMs are frequently observed in pathologies like hemiparesis, X-linked Kallmann’s syndrome [17], and after stroke [13], they are also manifest in healthy subjects, in particular young infants—see e.g. [19], for a recent review. Expressions of MMs in healthy subjects typi- cally diminish with development and it is commonly stated that they disappear around the age of 10—see e.g. [10]. However, they may persist in adulthood [18] and can even become rather pronounced in the elderly [4]. Interestingly, MMs are modulated by the nature of the performed activity [1]. Generally speaking, the more difficult a task becomes, the more likely involuntary movements will occur; an increase in movement tempo (or rate in rhythmic tasks) may increase occurrence [4] and so does an increase in contraction level of the moving hand [2,23]. Also, motor overflows may be influenced by hand dominance [16] in that intended movements of the less dominant hand may yield more pronounced MMs in the dominant hand than vice versa. * Corresponding author. Tel.: +31 20 5988549; fax: +31 20 5988529. E-mail address: A.Vardy@fbw.vu.nl (A.N. Vardy). URL: http://www.fbw.vu.nl/∼avardy/ (A.N. Vardy). We examined distinct influences of performing different unimanual and bimanual tasks on MMs that occurred during subsequent unimanual tasks, with preceding unimanual tasks serving as control conditions. Subjects were invited to perform in-phase and antiphase coordination patterns (IP, i.e. simultane- ous activation of homologous muscles, and AP, i.e. alternating activation of homologous muscles, respectively). IP and AP can be quantified via the relative phase between moving or force producing limbs. IP refers to a relative phase of 0 ◦ , whereas AP implies a left/right phase difference of 180 ◦ . AP is known to be more difficult to perform than IP, in particular when per- formance tempo is high [14]. Likewise, MMs may exhibit a unimodal distribution of relative phases indicating that MMs occur predominantly in phase with the intended movement. The prominence of in-phase MMs suggests a strong relation between MMs and bimanual coordination and may be associ- ated with the higher stability of in-phase coordination [9,21]. To investigate possible relations between MMs and coordination patterns we looked for the presence of after-effects of IP and AP coordination on MM properties in subsequent unimanual tasks. In view of the aforementioned similarity between phase distributions of coordination patterns and MMs, we expected the subsequent MMs to reflect the phase relationship that character- ized the previously performed bimanual pattern (i.e. IP versus AP). We also expected the induced changes in MM patterns to shed light on interhemispheric interactions underwriting rhyth- mic bimanual coordination (see e.g. [22]), and to strengthen, 0304-3940/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2007.01.064