Two Distinct Ipsilateral Cortical Representations for Individuated Finger Movements

Abstract

Movements of the upper limb are controlled mostly through the contralateral hemisphere. Although overall activity changes in the ipsilateral motor cortex have been reported, their functional significance remains unclear. Using human functional imaging, we analyzed neural finger representations by studying differences in fine-grained activation patterns for single isometric finger presses. We demonstrate that cortical motor areas encode ipsilateral movements in 2 fundamentally different ways. During unimanual ipsilateral finger presses, primary sensory and motor cortices show, underneath global suppression, finger-specific activity patterns that are nearly identical to those elicited by contralateral mirror-symmetric action. This component vanishes when both motor cortices are functionally engaged during bimanual actions. We suggest that the ipsilateral representation present during unimanual presses arises because otherwise functionally idle circuits are driven by input from the opposite hemisphere. A second type of representation becomes evident in caudal premotor and anterior parietal cortices during bimanual actions. In these regions, ipsilateral actions are represented as nonlinear modulation of activity patterns related to contralateral actions, an encoding scheme that may provide the neural substrate for coordinating bimanual movements. We conclude that ipsilateral cortical representations change their informational content and functional role, depending on the behavioral context.

Lateralized Sensitivity of Motor Memories to the Kinematics of the Opposite Arm Reveals Functional Specialization during Bimanual Actions

“…Neurons within the parietal reach region also show activity tuned for bilateral limb movement (Chang and Snyder, 2012). Such interhemispheric neuronal influences have also been reported in humans by using functional magnetic resonance imaging (Diedrichsen et al, 2013).…”

“…Although the neural substrates for the motor primitives are not fully elucidated, neurons in the frontal motor areas, the posterior parietal cortex, and the cerebellum are thought to be involved (Li et al, 2001;Della-Maggiore et al, 2004;Padoa-Schioppa et al, 2004;Xiao et al, 2006;Mandelblat-Cerf et al, 2011;Donchin et al, 2012). For example, neurons in the primary motor cortex or premotor cortex that are tuned with the movement direction of the reaching hand (Georgopoulos et al, 1982;Cisek et al, 2003) change their tuning properties after adaptation to a force field (Li et al, 2001;Xiao et al, 2006).…”

“…We suspect that the strength of this influence is greater from the dominant to the nondominant hemisphere rather than vice versa. In fact, it is well known that sensorimotor areas in the dominant hemisphere have greater influence over the nondominant hemisphere in both functional magnetic resonance imaging (Hayashi et al, 2008;Diedrichsen et al, 2013) and electrophysiological studies (Netz et al, 1995;Oda and Moritani, 1995;Ziemann and Hallett, 2001;Duque et al, 2007). Therefore, it is reasonable to suggest that such asymmetrical interhemispheric interactions are a possible neural substrate for the lateralized tuning pattern of the motor primitives.…”

Bihemispheric Transcranial Direct Current Stimulation Enhances Effector-Independent Representations of Motor Synergy and Sequence Learning

“…Twelve difficulty-matched five-finger sequences, which did not share any subsequential transitions longer than two presses, were selected based on a pilot study and a previous study (Wiestler and Diedrichsen, 2013). All sequences were performed during pretest and posttests, with eight trials per sequence per hand (96 trials total).…”

“…The strong intermanual transfer observed here could have been a consequence of the bihemispheric montage and may not extend to conventional unihemispheric tDCS: secondary motor areas that putatively represent movement skills in an effectorindependent manner, such as supplementary motor area (Perez et al, 2008), could have been targeted efficiently through the bihemispheric current, or the current flow may have changed the communication between motor cortices during training, possibly increasing the amount of mirrored activity in ipsilateral M1 (Diedrichsen et al, 2013). Additionally, high intermanual transfer could be attributable to cathodal enhancement of plasticity in the ipsilateral hemisphere: recent evidence suggests that cathodal unihemispheric tDCS at 2 mA, the same intensity used in our study, actually increases MEPs (Batsikadze et al, 2013).…”