Development of functional asymmetries in young infants A sensory-motor approach

Sammanfattning: Human functional laterality, typically involving a right-sided preference in most sensory-motor activities, is still a poorly understood issue. This is perhaps particularly true in terms of what underlying mechanisms that may govern lateral biases, as well as the developmental origins and course of events. The present thesis aims at investigating functional asymmetries in the upper and lower body movements of young human infants. In Study I, the presence of side biases in the stepping and placing responses and head turning in healthy fullterm newborns were explored. No evident lateral bias for the leg responses in terms of the first foot moved or direction of head turning was found. However, a lateral bias was revealed for onset latency in relation to the first foot moved in both stepping and placing. Asymmetries in head turning did not correspond to asymmetries in leg movements. In Study II, functional asymmetries in the stepping response of newborn infants were investigated in more detail by means of 3-D kinematic movement registration. Evident side differences were found in relation to smoother movement trajectories of the right leg by means of less movement segmentation compared to the left leg. Side differences were also found in relation to intralimb coordination in terms of stronger ankle-knee couplings and smaller phase shifts in the right leg than the left. In Study III, using the same movement registration technique, the kinematics of left and right arm movements during goal-directed reaching in infants were prospectively studied over the ages 6, 9, 12, and 36 months. Main findings included side differences and developmental trends related to the segmentation of the reaching movements and the reaching trajectory, as well as the distribution of arm-hand-use frequency. The results from Study I and II are discussed in relation to underlying neural mechanisms for lateral biases in leg movements and the important role of a thorough methodology in investigating newborn responses. Findings from Study III are discussed in terms of what they imply about the developmental origins for hand preference. An emphasis is also put on developmental differences between fullterm and preterm infants. Overall, the studies of the present thesis show that an increased understanding of subtle expressions of early functional asymmetries in the upper and lower body movements of young infants may be gained by means of refined measurements. Furthermore, such knowledge may provide an insight into the underlying neural mechanisms subserving asymmetries in the movements of young infants. The present studies also add new information to the current understanding of the development of human lateralized functions, in particular the findings derived from the longitudinal data. Apart from theoretical implications, the present thesis also involves a discussion with regard to the clinical relevance of investigating functional asymmetries in the movements of young infants.