Neuroscience Letters 585 (2015) 82–87 Contents lists available at ScienceDirect Neuroscience Letters jo ur nal ho me p age: www.elsevier.com/locate/neulet Behavioral inflexibility and motor dedifferentiation in persons with Parkinson’s disease: Bilateral coordination deficits during a unimanual reaching task Shinichi Amano a , S. Lee Hong a , Jacob I. Sage b , Elizabeth B. Torres b,∗ a Department of Biomedical Sciences, Ohio University, Athens, OH, USA b Rutgers Robert Wood Johnson Medical School, Piscataway, NJ, USA h i g h l i g h t s • We examine movement slowing and bilateral coordination deficits in Parkinson’s disease (PD). • When reaching, PD patients exhibited slowness and velocity control deficits on the less affected arm. • Unlike healthy peers, PD patients exhibited excessive synchrony between shoulders. • Inflexibility in PD was accompanied by reduced difference across left and right limb (arm). a r t i c l e i n f o Article history: Received 23 May 2014 Received in revised form 1 October 2014 Accepted 4 October 2014 Available online 14 October 2014 Keywords: Parkinson’s disease Bradykinesia Motor freezing Phase synchrony Behavioral inflexibility Motor dedifferentiation a b s t r a c t We evaluated kinematics of people with Parkinson’s disease (PD) and age-matched controls during cued and uncued reaching movements. Maximum hand velocity, its variability and shoulder-to-shoulder cou- pling, quantified by phase locking value (PLV), were compared between PD (n = 14) and Control (n = 4). The PD group achieved significantly lower maximum hand velocities during the reaching movement in comparison to the Control group (p = 0.05), whereas the Control group exhibited significantly greater variability (i.e., larger SDs) of maximum hand velocities across the blocks than the PD group (p = 0.01). Persons with PD exhibited higher PLVs than the healthy elderly individuals when performing reaching movements with their dominant side (p = 0.05), while the PLVs did not differ between groups when the movements were performed with their non-dominant hand. The present study suggests that persons with PD have a reduced ability to: (1) modulate maximum reaching velocity; and (2) alter coordination across the shoulders to different reaching actions. In persons with PD, the velocity-oriented (dominant) limb becomes slowed and less flexible, to the point that its movement dynamics are effectively similar to that of the position-oriented (non-dominant) limb. Published by Elsevier Ireland Ltd. 1. Introduction When required to perform bimanual tasks, persons with PD appear to compensate for unilateral bradykinesia by facilitating the movement of the more affected side at the expense of the less affected side [8]. This compensatory strategy in persons with PD is a particularly important and intriguing phenomenon, given the fact that the dominant and non-dominant limbs perform spe- cialized functions [21]. The loss of specialization is known in the literature as a process of dedifferentiation, wherein systems that ∗ Corresponding author at: Department of Psychology, Rutgers University, Busch Campus, 152 Frelinghuysen Road, Piscataway, NJ 08854, USA. Tel.: +(848)445 8909. E-mail address: ebtorres@rci.rutgers.edu (E.B. Torres). normally serve specific functions lose this specificity and become simplified [2]. This process of dedifferentiation can occur at vari- ous levels: neural [16], muscular [13,17] and behavioral [18] level. As a result, a compensatory slowing of the dominant hand might reflect dedifferentiation in the dynamic functions of the dominant and non-dominant limbs. To date, studies of motor coordination during unimanual reach- ing in PD have focused exclusively on the reduced control and modulation of limb segments [1,11,19], whereas studies examin- ing bilateral coordination deficits in PD have generally employed a bimanual paradigm where both limbs are engaged in the perfor- mance of the task [4,7,10]. However, bilateral coordination during unimanual tasks has yet to be explored as a test of the poten- tial that dedifferentiation in limb dominance might be a problem in PD. Effectively, dedifferentiation in PD motor behavior could be http://dx.doi.org/10.1016/j.neulet.2014.10.007 0304-3940/Published by Elsevier Ireland Ltd.