Developmental changes in the organization of prereaching movements.

Abstract

How does the kind and amount of prereaching activity change with age during the period preceding successful grasping of objects, that is, during the first 4 months oflife? This question was studied longitudinally in 23 infants. They were seen every third week from the first week of life to 16 weeks of age. Twelve of the infants were also seen at 19 weeks of age. At each session the subject was presented with an object moving slowly in different ways in front of him or her. In the middle of each session there was also a 1-minute period with no object present. The results show that the amount of prereaching goes down at 7 weeks and that this decrease is contingent on the presence of the object. The infant does not seem to lose interest in the object at this age, but attending to the object inhibits prereaching activity in some way. The form of prereaching changes at this age, too. Instead of opening the hand during the forward extension of the arm the hand will be fisted. After this age, the amount of prereaching activity will go up again and the hand will start opening during the forward extension, but only when the infant looks at the object. A neonate, placed on its back in a horizontal or semiupright position will, if it is alert and has support for trunk and head, engage in rather extensive spontaneous movements of its arms and hands. These movements show a specific patterning, for example, the forward extension of the arm is often accompanied by an opening of the hand (von Hofsten, 1982; Trevarthen, 1974). This synergistic extension of arm and hand contrasts with the passive traction reflex where the arm is extended forward by pulling it at the wrist (Twitchell, 1965). In the latter case, all muscles will flex, including those of the fingers, that is. the hand will get fisted. Fisted hands are rare in spontaneous forward extensions of the arm. DiFranco,