Corrections in grasp posture in response to modifications of action goals

There is ample evidence that people plan their movements to ensure comfortable final
grasp postures at the end of a movement. The end-state comfort effect has been found
to be a robust constraint during unimanual movements, and leads to the inference that
goal-postures are represented and planned prior to movement initiation. The purpose
of this study was to examine whether individuals make appropriate corrections to
ensure comfortable final goal postures when faced with an unexpected change in
action goal. Participants reached for a horizontal cylinder and placed the left or right
end of the object into the target disk. As soon as the participant began to move, a
secondary stimuli was triggered, which indicated whether the intended action goal had
changed or not. Confirming previous research, participants selected initial grasp
postures that ensured end-state comfort during non-perturbed trials. In addition,
participants made appropriate on-line corrections to their reach-to-grasp movements to
ensure end-state comfort during perturbed trials. Corrections in grasp posture occurred
early or late in the reach-to-grasp phase. The results indicate that individuals plan their
movements to afford comfort at the end of the movement, and that grasp posture
planning is controlled via both feedforward and feedback mechanisms.