Grasp Orientation in Ambiguous Settings is Sensitive to Prior Object Motion

Abstract

AbstractThe wrist posture chosen to grasp an object is normally consistent, determined by the optimal, most efficient strategy learnt from previous experience. Yet in certain settings, despite object properties remaining constant and intention the same, the action chosen by an individual can vary with a lack of clear preference for one posture over another. This is referred to as motor ambiguity. Here, we investigate the influence of preceding dynamically changing visual information on participants’ choice between two possible wrist postures when grasping an object at various orientations. We found that the decision is influenced by the participant observing rapid object rotation. Surprisingly, rather than being biased to choose the grasp that would have been appropriate for the initial position before object rotation, participants become more likely to use the alternate wrist posture. When object rotation is blocked from view, the bias effect is abolished. Furthermore, the bias strength was found to be independent of the motion magnitude, and persists even when greater decision time is allowed. We suggest that the influence of motion on ambiguous grasping could be attributed to participants overestimating the object’s final position when its motion is observed, preparing them to use the grasp appropriate for the upcoming region of certainty. The overestimation may be due to miscalculations in the cerebellar internal model or the gain computation induced by the pairing of rapid motion with the uncertainty of the ambiguous context.Significance StatementThe wrist posture chosen for a precision grasp is determined by the object’s visual properties/location, and a person’s comfort preferences. Situations whereby these factors are constant yet the motor response actioned is variable is known as motor ambiguity, though its neural mechanisms are unknown. Our findings suggest observation of object motion causes subjects to overestimate its final position, leading to a bias towards using a grasp opposite to that needed for the initial position. Our results provide a platform to study neural mechanisms and test motor control theories in situations of motor ambiguity when different choices do not lead to severe enough consequences to warrant adaptation in motor behaviour, and therefore can be revealed in our experimental design.