Stepping, placing and headturning biases in newborn infants A neurodevelopmental perspective

Abstract: In the present thesis the stepping, placing and head turning responses in healthy humanfullterm newborns are investigated. The main focus is put on a study of these newbornresponses in relation to functional asymmetries, while at the same time exploring anddiscussing different factors that possibly can affect the outcome of such studies. Study I aims to examine one such factor in relation to underlying mechanisms controlling leg movements in focusing on the effects of glucose on newborn stepping and placing responses. The results revealed that glucose, as well as an inactive state, resulted in less pronounced stepping responses and difficulties in eliciting them. There was also a tendency towards a similar finding for placing in that both glucose and an inactive state were associated with a less vigorous placing response, although this could not be proved significant. However, there was no effect of glucose on expressions of laterality in either ofthe responses studied. A theoretical debate in progress concerns whether different newborn functional asymmetries can be said representing a single neural system for lateralization or multiple sub-systems rooted in different neural mechanisms (Grattan, De Vos, Levy, & McClintock, 1992). In order to look closer at this issue in relation to newborn head- and leg preferences, Study II was designed to investigate the presence of side biases in newbornstepping and placing responses (lower-body), together with head turning preference (upperbody), and whether observed lateral biases of the upper- and lower body are congruent with each other. No evident lateral bias could be found for either response in terms of the first foot moved or direction of head turning. Furthermore, asymmetries in head turning did not correspond to asymmetries in leg movements, in support for multiple sub-systemsrather than a single lateralized system. However, a lateral bias was found for onset latency in relation to the first foot moved in both stepping and placing. The findings 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.