LATERAL BIASES IN HEAD TURNING AND THE MORO RESPONSE IN THE HUMAN NEWBORN - ARE THEY BOTH VESTIBULAR IN ORIGIN

Head turning after release from the midline and the Mole response to a full-body drop in 15 full-term newborns lying supine on a custom-buil t platform was studied. While the lateral bias for head turning was no t as pronounced as for the Moro response, it was still assumed in the ratio of 2 (right): 1 (left) as predicted by Previc (1991). Onset late ncy and time-to-peak acceleration were both significantly shorter In t he right arm during the initial phase of the Moro response. For both m easures, this right arm bias persisted over four consecutive elicitati ons in most infants. Vaginally delivered infants and those born by Cae sarean section did not differ in terms of head preference and the two measures of arm advantage. Our main finding was that infants with a ri ght-sided head preference had a consistently shorter onset latency for the right arm. We interpret this association as stemming from a commo n labyrinthine asymmetry that involves different vestibulospinal pathw ays for the neck and arm muscles. In general, our findings are discuss ed in the context of Previc's (1991) left-otolithic dominance hypothes is and Grattan, De Vos, Levy, and McClintock's (1992) model of newborn functional asymmetries. (C) 1998 John Wiley & Sons, Inc.