The inimitable mouth: task-dependent kinematic differences are independent of terminal precision.

Abstract

Previous studies in our lab have described kinematic difference between grasp-to-eat and grasp-to-place movements, whereby participants produce smaller maximum grip apertures (MGAs) when grasping to bring the item to the mouth than when grasping to bring the item to a container near the mouth. This task difference is limited to right-handed movements, regardless of handedness; it has, therefore, been interpreted as evidence of left-hemisphere lateralization of the grasp-to-eat and other hand-to-mouth grasping movements. However, the difference in end-goal aperture may have accounted for both the kinematic signature (smaller MGAs) and their lateralized expression. Specifically, if the right hand is more sensitive to the precision requirements of secondary movements, it may have produced more precise MGAs for actions whose ultimate goal is the small-aperture mouth rather than a comparatively large aperture container. The current study addresses this question by replacing the previously-used bib with a small drinking glass whose aperture more closely resembles that of the mouth. 25 adult participants reached-to-grasp small cereal items to either (a) eat them, or (b) place them into a small-aperture glass hanging beneath their chin. Results once more showed a lateralised kinematic signature in the form of smaller MGAs for the eat action, demonstrating that the signature is not a result of lateralized sensitivity to a movement's secondary precision requirements. We discuss these results in terms of their impact on predominant theories regarding visual guidance of grasping movements.