The asymmetrical tonic neck reflex in normal full-term infants.

The purpose of this study was to determine whether the asymmetrical tonic neck reflex was present in normal 1- to 12-week-old infants. Twelve infants were observed seven times within the first 12 weeks: at the first and second week and every other week thereafter. Spontaneous behavior was videotaped while the infant lay supine. Infants maintained a head-turned posture during every visit. The asymmetrical tonic neck reflex was present: 1) in the infants' arms during every visit, at a maximum at the sixth week; 2) in their legs, with the longest duration of the response at the first week's visit; and 3) in both arms and legs, with no change throughout seven visits. There were more movements of the face arm than of the skull arm during the seven visits, and the additional movements were in the extension range. The face leg moved more than the skull leg, but only at the 6th, 8th, and 10th weeks. The study confirms that the asymmetrical tonic neck reflex can be observed in the posture and movement of normal infants and that the pattern of appearance of the reflex differs in arms and legs.