A New Method Applied to Kinematic Data Reveals Hidden Influences on Reach and Grasp Trajectories


Researchers at the University of Victoria (Canada) have developed new statistical procedures
that allow them to measure the influence of competing action intentions on the execution of a
reach and grasp response. This method is capable of detecting perturbations as small as a
fraction of a degree in the rotation of the hand and as little as a few millimeters in its position.

Previous approaches to statistically analyzing kinematic data have been limited to analyzing
position and rotation using separate methods of calculation. In contrast, this new method
simultaneously captures for the first time, statistical differences not only in the position of the
hand but also its rotation. This capability can be applied to a large variety of experimental and
applied questions, including assessment of subtle changes in action over time associated with
learning, such as the development of skilled motor performance, and the analysis of action in
special populations.

This new methodology has been used by the Victoria researchers to clearly reveal how a competing action intention alters the dynamics of a reach and grasp action over time. Surprisingly, the online control of movement is continuously modulated by the influence of an action plan evoked by a competing object, even when that object is no longer visible. The effect of this competition is captured by a "difference trajectory" representing moment by moment alterations in the trajectory of the hand. Positional effects are shown in a box plot depicting perturbations in the translation of the hand, and rotational effects are shown in a ball plot reflecting perturbations in its rotation.

Till, B., Masson, M., Bub, D., Driessen, P. (2014). Embodied Effects of Conceptual Knowledge Continuously Perturb the Hand in Flight. Psychological Science (in press, April 2014).