Lateral differences and head-turning responses to somesthetic stimulation in premature human infants

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Abstract
Head-turning responses to somesthetic stimulation of the perioral region were studied at weekly intervals in 25 prematurely born infants. No regular changes associated with age were evident. The premature infants were more likely to turn towards than away from a stimulus; however, they were less so than are full-term infants. The infants were as likely to turn towards the stimulus when it was on the left as when it was on the right; however, they were more likely to turn away from a stimulus on the left than on the right. Comparisons with full-term infants indicated reduced lateral differences in responding. Such a reduction is in sharp contrast with the previous finding of marked postural asymmetry in a comparable sample of prematurely born infants. These data indicate a possible sequence in the development of head-turning responses to somesthetic stimulation and the assumption and maintenance of an asymmetrical head posture which consists of (1) a right-turning postural bias, (2) contralateral responding to somesthetic stimulation, (3) lateral differentiation of contralateral responding, (4) ispilateral responding, and (5) lateral differentiation of ipsilateral responding.