Target viewing time and velocity effects on prehension

The goal of the present study was to understand which characteristics (move ment time or velocity) of target motion are important in the control and co ordination of the transport and grasp-preshape components of prehensile mov ements during an interception task. Subjects were required to reach toward, grasp and lift an object as it entered a target area. Targets approached a long a track at four velocities (500, 750, 1000 and 1250 mm/s) which were p resented in two conditions. In the distance-controlled condition, targets m oving at all velocities traveled the same distance. In the viewing-time-con trolled condition, combinations of velocity and starting distances were per formed such that the moving target was visible for 1000 ms for all trials. Analyses of kinematic data revealed that when, target distance was controll ed, velocity affected all transport-dependent measures; however, when viewi ng time was controlled, these dependent measures were no longer affected by target velocity. Thus, the use of velocity information was limited in the viewing-time-controlled condition, and subjects used other information, suc h as target movement time, when generating the transport component of the p rehensile movement. For the grasp-preshape component, both peak aperture an d peak-aperture velocity increased as target velocity increased, regardless of condition, indicating that target velocity was used to control the spat ial aspects of aperture formation. However, the timing of peak aperture was affected by target velocity in the distance-controlled condition, but not in the viewing-time-controlled condition. These results provide evidence fo r the autonomous generation of the spatial and temporal aspects of grasp pr eshape. Thus, an independence between the transport and grasp-preshape phas es was found, whereby the use of target velocity as a source of information for generating the transport component was limited; however, target veloci ty was an important source of information in the grasp-preshape phase.