MATCHING OBJECT SIZE BY CONTROLLING FINGER SPAN AND HAND SHAPE

The ability of human subjects to accurately control finger span (dista nce between thumb and one finger) was studied. The experiments were pe rformed without visual feedback of the hand and were designed to study the dependence of accuracy on object size, shape, distance, orientati on and finger configuration. The effects of finger combination and sen sory modality used to perceive object size (vision and haptics) were a lso studied. Subjects were quite proficient at this task; the small er rors tended to be predominantly negative, i.e., finger span < object s ize. The thumb-little finger combination was less accurate than the ot her finger combinations, irrespective of the sensory modality used. Su bjects made larger under-estimating errors when matching the size of c ylinders than when matching cubes and parallelepipeds. No effect of vi ewing distance, object orientation and finger configuration was found. Accuracy in matching object size was not dependent on the sensory mod ality used. The question of how the individual degrees of freedom of t he fingers and thumb contributed to the control of finger span was als o addressed. Principal components analysis showed that two components could characterize the hand postures used, irrespective of object size . The amplitude of the first principal component was constant, and the amplitude of the second scaled linearly with object size. This findin g suggests that all of the degrees of freedom of the hand are controll ed as a unit. This result is discussed in relation to the 'virtual fin ger' hypothesis for grasping.