In a standing subject, rapid turning of the shoulders to either left or right (eyes closed to avoid fixation) is associated with bilateral contraction of the cervical musculature so that the head is held in the original position. This nuchocephalic reflex is present in infants and children up to the age of about 4 years. Beyond this age the reflex is inhibited, such that the head is actively turned in the direction of shoulder movement after a time lag of about half a second. If the reflex is present in adults (i.e., disinhibited), it is claimed to be a "regressive" (primitive) sign, indicative of diffuse cerebral dysfunction.
Jenkyn LR, Walsh DB, Walsh BT, Culver CM, Reeves AG. The nuchocephalic reflex. Journal of Neurology, Neurosurgery and Psychiatry1975; 38: 561-566
Schott JM, Rossor MN. The grasp and other primitive reflexes. Journal of Neurology, Neurosurgery and Psychiatry 2003; 74: 558-560