Catching small balls with a power grip and large balls with a precision grip: distinct roles for the reach and grasp

The dual visuomotor channel (DVC) theory proposes that reaching with a hand for an object is mediated by a reach channel that directs the hand in relation to spatial cues and a grasp channel that shapes the hand and fingers in relation to object cues. The theory derives from kinematic measures of reach and grasp movements that are used for obtaining static targets and targets rolling across a surface. Here we asked whether the theory extends to the movements used for catching. Measures from electromagnetic sensors, 3D video, and frame-by-frame video analyses were used to compare the hand movements of catching four balls (2.5 to 9 cm) to the hand movements of picking them up when static and intercepting them when they rolled across a surface. The results show that for catching, the reach features an open hand that is used to target and stop the flight of a ball. In turn, the grasp features apertures proportional to ball size, suggesting a relation to forthcoming grip organization, but not ball size per se. Grip analysis showed that a variety of finger synergies resulted in precision grips on larger balls and power grips on smaller balls. Once documented for catching, similar roles for the reach and the grasp are recognizable in the tasks of picking up static balls and intercepting balls rolling on a surface. The results are discussed with respect to their support for the DVC prediction of separate roles for visual reach and grasp channels and their integration with reach and grasp somatosensory channels.