Development of reaching in healthy infants: linearity?

Abstract

This study aimed at investigating phase transitions in the development of reaching in infants, as well as assessing whether changes in kinematics variables follow a linear or non-linear direction between the fourth and sixth months of age. Nine healthy infants were placed in a 50°-leaning baby seat and were shown attractive spherical objects of different properties. The infants’ movements were recorded, and 384 reaches were analysed by using 3D movement reconstruction. Results showed an increase with age in both straightness index and mean velocity, and a decrease in the number of movement units. Both straightness and movement units had a non-linear distribution, whereas mean velocity showed a linear one. The clearest phase transition occurred from the fourth to the fifth month. In the sixth month, the number of movement units was similar to the fifth month, and the straightness index was shown to reduce slightly. The fifth-to-sixth-month period, therefore, seems to be the most stable. Regarding the mean velocity, it was not possible to identify a remarkable point of transition. It is thus suggested that reaching movements improve during the analysed period of life, but such improvement does not necessarily follow a linear direction, since each kinematic variable shows a different developmental course.