A critical examination of the Moro response in newborn infants--symmetry, state relation, underlying mechanisms.

The primary purpose of the present study was to analyze quantitatively the vestibular evoked Moro response, and the symmetry of the movement pattern involved, as the traditional descriptions bring about. Another aim was to determine the segmental movements involved and determine whether the components of the Moro response are dependent on changes in the infant's behavioral state. Another question concerns whether the form of these movements changed over repeated trials on the same day, or from the first to the fifth day after birth. Vestibular evoked Moro responses of 52 neonates, 1-5 days of age, elicited in different behavior states (State 1-5), were examined and quantitatively analyzed. The response was evoked by a predefined, rapid, downward, vertical body motion, without any dorsiflexion of the infant's head. Optoelectronic device (SELSPOT II) were used to monitor the arm/hand movement patterns involved in the response. The three-dimensional movement pattern in space, duration, velocity, latency, and the acceleration of both arms/hands were analyzed in relation to the infant's behavioral state. The response movements were structured into phases of abduction/extension, adduction/flexion and the extension/flexion of the fingers. The vestibular stimulation used was found to be sufficient for eliciting an adequate Moro response. The segmental movement pattern of the Moro response was found to be sensitive to the infant's behavioral state at the time when the response was elicited. This was found in the movement pattern, duration, latency, and the velocity of the response. The response was found to be asymmetrical, in 82% of the infants it was found to be a predominant shorter onset latency of the right arm, in 12% the opposite was found. These findings suggest that there is a fundamental, spinal asymmetry involved in the Moro response which is subject to supraspinal influences emanating from the vestibulospinal system. No differences were found between 1 and 5 days of age for any of the scoring categories, and no differences were found within groups over six successive trials.