Perceptual classification task. Before a trial began, each participant placed his or her index and middle
fingers of the right hand on the two buttons of the response box. The shutter devices were opaque at
this time so that the participant was unable to see the table. After the experimenter positioned one of
the four stimuli on the experimental table, she pressed a key and the shutters became transparent.
When the shutter devices opened, the participant classified the rectangle’s width as quickly and accurately
as possible into narrow or wide categories by pressing either the index or middle finger, respectively.
After classification, the participant grasped the stimuli by their width using his or her thumb
and index finger and lifted and placed the stimuli on the table to ensure that they received similar
haptic feedback as during grasping. The shutter devices were open while the participant classified
the rectangles (closed-loop condition) but became opaque when the participant reached out to grasp
the object (open-loop condition) (see next paragraph for further explanations). The participant returned
his or her hand to the response box to begin the next trial.