Abstract
In a previous study on hand selection in a sequential reaching task, the authors showed a shift of the point-of-change (POC) to the left of the midline. This implies that participants conducted a number of contralateral reaches with their dominant, right hand. Contralateral movements have longer planning and execution times and a lower precision. In the current study, we asked whether lower mechanical costs of motor execution or lower cognitive costs of motor planning compensated for these disadvantages. Theories on hemispheric differences postulate lower mechanical costs in the dominant hemisphere and lower cognitive costs in the left hemisphere (independent of handedness). In right-handed participants, both factors act agonistically to reduce the total cost of right-handed reaches. To distinguish between the cost factors, we had left- and right-hand-dominant participants execute a sequential, unimanual reaching task. Results showed a left-shift of the POC in the right-handed and a right-shift in the left-handed group. Both shifts were similar in magnitude. These findings indicate that only the mechanical cost of motor execution compensates for the disadvantages of the contralateral reaches, while the cognitive cost of motor planning is irrelevant for the POC shift.
Highlights
When opening our sock drawer in the morning, we are blissfully unaware of the series of sensorimotor transformations our central nervous system has to perform to translate the retinal image of the drawer handle into a muscle activation pattern that guides our hand to the handle’s location
For the reduced left-handed group (19 participants), in contrast, we found a significant difference in probabilities, Z = 2.797, p = 0.005, r = 0.454
We asked whether a left-shift of the point-of-change (POC) observed for right-handed participants in a previous study (Rostoft et al 2002) reflected (a) a left hemisphere advantage in the cognitive cost of motor planning, (b) a dominant hemisphere advantage in the reaction time [ms]
Summary
When opening our sock drawer in the morning, we are blissfully unaware of the series of sensorimotor transformations our central nervous system has to perform to translate the retinal image of the drawer handle into a muscle activation pattern that guides our hand to the handle’s location. Due to these transformations, the creation of a reaching movement plan is associated with a cognitive cost. Motor hysteresis effects have been demonstrated in binary posture selection tasks (e.g., over- vs underhand grasp; Rosenbaum and Jorgensen 1992; Weigelt et al 2009). Reuse of the previous posture/motor plan is restricted to a range of indifference, in which participants are content with both alternatives (Rosenbaum and Jorgensen 1992)
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