Hand movements and the hemispheres : asymmetries in hand and hemispace
This thesis presents a series of experimental investigations into left and right hand performance in right-handed participants. The experiments were designed to identify differences in movement kinematics when task characteristics varied in aiming and prehension movements, particularly along dimensions that were thought to differ between the hemispheres. The first part of the thesis reviews the existing literature on the relevant topics of motor control. There is extensive evidence to argue for distinct but complementary contributions from each hemisphere in the control of movements, with the emphasis on the left hemisphere, which plays a significant role in the organization of movements on both sides of the body. There is also support for the claim that the right hemisphere has a specialised role in visually-guided movements when visuo-spatial processing is involved in the form of localizing target positions. Furthermore, performance differences are investigated with respect to hemisphere. The second part of the dissertation presents a series of investigations of neurologically intact participants. In Experiment 1 and 2, 22 participants were required to make pointing movements with the index finger across the hemisphere in visual and non-visual conditions. The main finding of this study was ipsilateral movement advantages for both hands. Contrary to previous claims, these ipsilateral advantages, even when no vision was available, were the consequences of biomechanical factors rather than within-hemispheric processing. A right hand advantage was found for movement duration. In Experiment 3, 22 participants were required to point to suddenly-appearing targets aligned along the horizontal midline. The focus of this study was on right hemisphere participation in a pointing task in which the visuo-spatial demands were manipulated in the form of target location uncertainty. Contrary to previous claims, no evidence was found for the attenuation of right hand performance or even the facilitation of left hand performance. The dominant hand was noticeably more accurate and took less time decelerating. Main findings of this study were ipsilateral movement advantages for both hands. In Experiment 4, 20 participants were required to grasp and pick up target blocks of differing length and width to examine whether increased precision would exacerbate a previously reported hand difference on a grip aperture timing measure. The left hand reached maximum grip aperture earlier than the right hand but, contrary to expectations, this hand difference was not influenced by the manipulation of precision demands. Hemispatial effects were found for transport and grip related dependent measures. In the final Experiment 5, the area of hand performance differences was explored while both hands were grasping targets simultaneously by requiring 14 participants to grasp and pick up objects of the same or different lengths. The manipulation of hemispace resulted in a common movement duration of the limbs. An overall larger grip aperture was found for the left hand but was not affected by the task requirement of the right hand such that grip apertures of the hands were independently scaled.