Interhemispheric interactions associated with unilateral ballistic motor tasks

Abstract

The aim of this research was to enhance our understanding of the spatial reference frames in which movement is represented in the primary motor cortices of both hemispheres during unilateral ballistic actions. The broader goal was to reveal how the coordinate systems of movement representation influence the interactions between the two cerebral hemispheres which presumably underlie cross limb transfer of motor skill. The issue of reference frames may be critical for cross-limb interactions because definitions of space and movement often conflict for opposite limbs in intrinsically-referenced coordinate systems (e.g. muscle or joint-based coordinates) due to their mirror symmetry. This thesis describes a series of experiments to investigate the reference frames in which movement representations are shared bilaterally, and whether alignment of reference frames influences the transfer of performance to the untrained limb after unilateral ballistic training. There is increasing evidence that the primary motor cortex ipsilateral to the active limb is active during unilateral movement. Yet, the extent to which such activity represents functional details of movement with the ipsilateral limb, and the coordinates of any such representation, is unclear. Hence, Chapter 2 reports work aimed at understanding the timing and coordinates of interactions between the two motor cortices when movement with one hand is being prepared. Studying the time course of changes in twitch directions evoked by transcranial magnetic stimulation (TMS) enabled us to examine whether activity in the “resting” motor cortex functionally represents the impending movement, and the reference frame of any such representation. Our results showed that twitch directions in the resting limb shifted toward the impending direction of the active hand in a muscle-based reference frame. Evident changes in TMS-evoked twitch parameters right before movement onset might be associated with decreases in interhemispheric inhibition and intracortical inhibition, brought about by the release of motor commands to the active limb. Use-dependent learning was previously reported to generalise according to extrinsic coordinates when posture changes were used to dissociate reference frames within the active limb, however it is unclear whether this form of learning generalises to the untrained limb. Chapter 3 assessed whether use-dependent bias in aiming performance occurred in the opposite untrained limb under postural manipulations that varied the extent to which spatial coordinates were aligned for the two limbs according to extrinsic, muscle-based and midline reference frames. The results revealed that systematic bias occurred only when both limbs were oriented such that training and aiming targets were aligned according to all reference frames. The data suggest that aiming biases in the untrained limb after contralateral ballistic training are represented according to a combination of coordinate systems. In order to understand the role of reference frame conflicts in transfer of use-dependent learning, we examined the representation of learned movements within the trained hemisphere with TMS. When resting TMS-evoked twitches were observed in different postures (Chapter 4), twitch directions followed with the wrist according to joint- and muscle-based reference frames. When posture was manipulated after ballistic training (Chapter 5), there were systematic shifts in evoked twitch directions toward the training direction, but adaptation was represented either in extrinsic or muscle-based reference frames for different subjects. In conclusion, the research described in this thesis increases understanding of the reference frames in which unilateral ballistic movements and learning are represented in the primary motor cortices. The findings showed that movement is represented in muscle-based reference frame during motor preparation, and that cross limb transfer improves when multiple reference frames of movement representation are congruent between limbs. We conclude that spatial reference frames play an important role in determining the functional effects of interhemispheric interaction during unilateral movement.