BODY-SCALED RATIO AS A CONTROL PARAMETER FOR PREHENSION IN 5-YEAR-OLDTO 9-YEAR-OLD CHILDREN

The purpose of the experiment was threefold: (a) To find evidence that grasping is body-scaled and thus remains invariant during development ; (b) to seek evidence that the body-scaled ratio of cube and hand siz e serves as a control parameter for the phase transition from one-hand ed to two-handed grasping by identifying the presence of sudden jump, enhanced variance, multistability, and hysteresis; and (c) to examine whether the stability properties of the observed grasping patterns inc rease with age. Thirty-three children aged 5, 7, and 9 years old were required to grasp and lift 14 cardboard cubes of different sizes (2.2, 3.2, 4.2, etc, to 16.2 cm diameter). Three conditions were used: (a) an increasing condition with sizes or-dec-ed from the smallest size to the largest; (b) a decreasing condition, with the sizes ordered from the largest to the smallest; and (c) twice in a different random order . Video recordings were analyzed and scored for the percentage of one- handed grasps. The results showed that the shift from one-handed to tw o-handed grasping occurred at the same body-scale ratio between cube s ize and finger span for all three age groups. Evidence was found for t he presence of a sudden jump, enhanced variance, multistability, and h ysteresis, indicating that the body-scaled ratio of cube and hand size selves as a control parameter. No change with age for the stability p roperties of the grasping patterns were observed. (C) 1998 John Wiley & Sons, Inc.