Taylor, Rachael and Linkenauger, Sally and Nuttall, Helen and Crawford, Trevor (2024) Can the Left Hand Benefit from being Right? The Influence of Body Side on Action Estimates. PhD thesis, Lancaster University.
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Abstract
Right-handed individuals (RHIs) possess various perceptual, visual, and somatosensory biases that facilitate more precise and efficient control of the right hand. For example, RHIs have asymmetries in the cortical representation of both hands, with the right hand representation in left motor cortex being significantly larger than the left hand representation in right motor cortex. These biases have consequences on RHIs’ perceptions of their bodies, with them estimating their right hand and arm to be visually larger than their left. Subsequently, they estimate that they can reach further distances or grasp larger objects using their right hand than their left, despite no significant differences in the real morphology of the hands. One key question is whether such visual biases RHIs experience sufficiently explain these asymmetries in action perception between the left and right hand. This thesis aims to explore both the visual and non-visual (somatosensory) factors that underlie both the strong right-hand preference and impression of greater capabilities using the right hand in RHIs. To do this, virtual reality (VR) and motion capture technology were used in a series of 9 experiments to isolate visual feedback associated with moving the right hand from the somatosensory feedback that would ordinarily be experienced. Three of these 9 experiments explored the impact of visual feedback specifying handedness on perceptions of reaching and grasping abilities, four explored the ability for people to embody virtual limbs that are visually presented incongruently to somatosensory feedback, and two explored whether the left-hemisphere processing advantage for visually guided actions could be exploited in the context of visual illusions. Overall, the key findings of this thesis were that: 1) visual feedback specifying hand use did not have a significant impact on action estimates; 2) action estimates were based on the physical hand that was being used, and the right hand was estimated as more capable even when viewed as the left; 3) differences in action estimates between the left and right hands is contingent on the complexity of the action being performed. Thus, the findings suggest that RHIs’ perceptions of greater action capabilities with their right hand are primarily rooted in cortical asymmetries that lead to an enlarged sensorimotor representation of the right hand. Moreover, more efficient sensory feedback aside from vision better accounts for differences in action perception than visual feedback specifying the hand being moved during visually guided actions. The findings of this thesis have broader implications for understanding the factors that underlie biases in action perception in RHIs.