Object size as a determinant of grasping in infancy.

The early development and patterns of development of prehensile ability were examined. Infants 5, 7, and 9 months old were presented five objects, 0.5, 1.0, 3.5, 7.0, and 14.0 cm in diameter. The findings revealed that infants as young as 5 months old were able to differentiate grip configurations as a function of object size. The number of grasps involving the two or three most radial digits (thumb, index finger, and long finger) increased greatly over this age span. At 9 months of age these kinds of grasps were 10 times more frequent than at 5 months of age. However, at each age level, when only the two or three most radial digits were used, the reaches were typically directed at the two smallest objects. These findings suggest that it was not a perceptual problem that the younger infants were facing, nor was the problem knowing when to use different kinds of grasps; rather, the problem was one of cortico-motoneural connections, which are better established in older infants. The findings also suggest that traditionally described sequential development of infants' prehension is rigid and conservative. The discrepancy with earlier results may also be attributed to the difference in the objects' sizes and the way they were presented.