Beginning at about 4 months-of-age, the infant will grasp an appropriate-sized object with the ulnar side of the palm, but without involvement of the thumb. A month of so later, the object is grasped with the the whole hand and thus secured in the middle of the palm, but there is still little or no involvement of the thumb (see figure below). Between six and seven months, the thumb is used in the grasp, with the object secured in the radial side of the palm. With the radial palm grasp, there is now the first sign of thumb opposition together with flexion of the ulnar fingers (see figure below). During the rest of the first year, the pincer grasp is added to the prehensile repertoire, with the ability to grasp ever increasingly smaller objects becoming evident by means of being to achieve thumb-index finger opposition (see figure below for first ages of appearance). In a seminal descriptive study carried out more than 80 years ago using filmed recordings of changes in infant prehension, ten stages (or different patterns or prehension) were identified, and interpreted as following a proximal-distal developmental sequence (Halverson, H. M. An experimental study of prehension in infants by means of systematic cinema records. Genetic Psychology Monographs, 1931, 10, 107-283). The appearance of the palmar grasp and the subsequent changes in prehension reflect maturation of the motor cortex and corticospinal connections.
