A developmental study of the influence of task characteristics on motor overflow Patricia K. Addamo a, * , Maree Farrow b , Kate E. Hoy a,c , John L. Bradshaw a , Nellie Georgiou-Karistianis a a Experimental Neuropsychology Research Unit, School of Psychology, Psychiatry and Psychological Medicine, Monash University, Clayton, Vic. 3800, Australia b Alzheimer’s Australia Vic, Hawthorn, Vic., Australia c Alfred Psychiatry Research Centre, The Alfred, and Monash University School of Psychology, Psychiatry and Psychological Medicine, Prahran, Vic., Australia article info Article history: Accepted 17 September 2008 Available online 1 November 2008 Keywords: Children Corpus callosum Motor cortex Motor development Motor learning Motor skills Corticospinal tract abstract Motor overflow refers to involuntary movement or muscle activity that may coincide with voluntary movement. This study examined factors influencing motor overflow in 17 children (8–11 years), and 17 adults (18–35 years). Participants performed a finger pressing task by exerting either 33% or 66% of their maximal force output using their dominant or non-dominant hand. Attention was manipulated by tactile stimulation to one or both hands. Overflow relative to the target force was greater in children compared to adults, and at the lower target force for both groups, but was not influenced by attentional stimulation. Childhood overflow was greater when the left-hand performed the task. Although an imma- ture motor system may underlie an inability to suppress involuntary movement, childhood overflow may provide motor stabilization. Crown Copyright Ó 2008 Published by Elsevier Inc. All rights reserved. Symmetrical movements involving both upper limbs are argued to be ‘‘default” movements (Sadato, Campbell, Ibanez, Deiber, & Hallett, 1996; Swinnen, 2002). Proficient unimanual movement may therefore require the relevant motor sequence, in addition to the suppression of default movement (Lazarus & Todor, 1991; Lazarus & Whitall, 1999; Sadato et al., 1996). A failure to inhibit involuntary movement in the opposite limb is known as motor overflow (Lazarus & Todor, 1987; Mayston, Harrison, & Stephens, 1999; Todor & Lazarus, 1986a). Overflow is common in childhood and follows a non-linear and task dependant developmental de- cline in incidence and magnitude as a function of age (Connolly & Stratton, 1968; Largo et al., 2001; Lazarus & Todor, 1987; Todor & Lazarus, 1986a). While childhood overflow, and the overflow that can be induced in adults during forceful or effortful tasks, is likely to involve a network of to date largely unknown cortical and/or subcortical regions, the corpus callosum (CC) is suggested to be central to the mediation of this movement phenomenon (Addamo, Farrow, Hoy, Bradshaw, & Georgiou-Karistianis, 2007; Carson, 2005). Fittingly, functional maturation of the CC, enabling efficient interhemispheric transfer, continues at least throughout early adolescence (Giedd et al., 1999; Pujol, Vendrell, Junque, Mar- ti-Vilalta, & Capdevila, 1993; Rajapakse et al., 1996). Two theories, the Ipsilateral Activation Theory (IAT) and the Bilateral Activation Theory (BAT) have been proposed to account for motor overflow, each invoking involvement of the CC (Hoy, Fitzgerald, Bradshaw, Armatas, & Georgiou-Karistianis, 2004a). The corticospinal tract is comprised of the ipsilateral, and con- tralateral, pathways and is the main motor route controlling move- ment (Ropper & Brown, 2005). Recent findings suggest that in accordance with BAT, overflow in healthy populations may be mediated by contralateral corticospinal tract activation (Hoy, Geor- giou-Karistianis, Laycock, & Fitzgerald, 2007; Mayston et al., 1999). Mayston et al. (1999) used electromyography to record involuntary movement while adults and children performed finger tasks. As a measure of transcallosal inhibition (TCI), cutaneomuscular reflexes and dual pulse transcranial magnetic stimulation (TMS) were used to record cortical excitability. Indicative of increased cortical excit- ability contralateral to motor overflow, the cutaneomuscular reflex of the passive hand increased when the active hand performed fin- ger abduction, compared to when it was relaxed. It was concluded that overflow was a consequence of contralateral corticospinal tract activation resulting from sufficient bilateral cortical activa- tion, in children due to insufficient TCI transferred by a functionally immature CC. For adults, in the absence of effort or fatigue induc- ing circumstances, TCI was argued to suppress activity in the hemi- sphere ipsilateral to the voluntary active hand, otherwise leading to overflow (Mayston et al., 1999). However, according to the IAT of motor overflow, childhood overflow may result from activity in the ipsilateral corticospinal tract, a motor pathway usually inhibited in healthy adults by cor- tical signals transferred from the contralateral hemisphere via a functionally mature CC (Dennis, 1976; Muller, Kass-Iliyya, & Reitz, 1997; Nass, 1985; Tomasch, 1954). Findings such as Muller et al.’s (1997), whereby TMS produced only a contralateral response in adults, but generally a bilateral motor response in children, (albeit 0278-2626/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2008.09.005 * Corresponding author. Fax: +61 3 9905 3948. E-mail address: patricia.dredge@med.monash.edu.au (P.K. Addamo). Brain and Cognition 69 (2009) 413–419 Contents lists available at ScienceDirect Brain and Cognition journal homepage: www.elsevier.com/locate/b&c