Timecourse of Preshaping for Functional Responses to Objects

The present study explores the unfolding of hand shaping in the context of functional responding to common objects. The principal issue addressed by this study is whether distinct, identifiable preshapes are evidenced in preparation for four distinct types of hand contact, including nonprehensile as well as prehensile hand shapes, and for small and large surfaces of contact. All four shapes considered here (poke, pinch, palm and clench) showed evidence of distinct preshaping, with similar timecourses in relation to the onset of reaching. The initial separation of the fingers, formation of first identifiable preshape feature, and completion of the preshape appeared at essentially the same time relative to the onset of reaching for all four hand shapes. The hand shapes did differ, however, in the time between the completion of a stable preshape and contact with the object. The time for preshape-to-contact was greater for hand shapes with small surfaces of contact than for those with large surfaces and for prehensile shapes than for non-prehensile. These differences are attributed to the precision required for the ultimate object-contact response.