The Directed Head Turning Response and Allied Movements of the Human Baby

This paper contains the results of a qualitative and quantitative analysis of the nervous mechanisms of the to side and of the head turning in human babies. In the side to side movement, the head is turned from one side to the other repeatedly. This movement may occur either in response to tactile stimulation near the mouth, or, in hungry newly born babies, also in the absence of such stimuli. It helps the baby find the nipple of its mother's breast. At most two or three weeks after birth, the rhythmical movement develops into a spatially oriented movement. The latter at first retains a rhythmical component (a damped oscillation), but this soon disappears completely, and a fully oriented turning of the head towards the stimulus (directed head turning) takes the place of the original rhythmical movement. In premature babies rhythmical searching persists considerably longer. The receptive field of the side to side movement is a large, undifferentiated area of skin around the mouth. On the other hand, the reflexogenous zone of directed head turning is highly differentiated (fig. 5). The afferent system of both behaviour patterns consists of the tactile sense organs in this area of skin, and their sensory fibres in the maxillary and mandibular branches of the fifth cranical nerve. The quantitative analysis deals primarily with the properties of the directed response. How soon a response wil occur depends not only on intensity and frequency of the stimulus, but also on the level of excitation of the motor neurons and the gamma fibres to the muscle spindles, and on proprioceptive afferent messages. The same factors also influence the duration of the persistence of the response after cessation of the stimulus. If directed turning is released repeatedly by the application of an adequate stimulus always to the same spot, no further reactions will occur after a certain number of responses have been performed (central adaptation). The number of reactions obtained depends on frequency, intensity and place of stimulation. Central adaptation is probably due to processes in the reticular system. Further stimulation after the onset of adaptation may lead to responses of a different type: turning away from instead of towards the stimulus. This phenomenon is termed central reset (zentrale Umstimmung). The occurrence of rebound has been demonstrated in directed head turning. Head turning, being a preliminary to food intake, is one link of the system subserving the homeostasis of the energy content of the body. It can easily be shown that the state of this system influences threshold, duration, intensity, etc., of the turning movement. Real insight into the rules governing head turning can be gained only by taking into account, as far as possible, all physiological processes involved in energy metabolism and particularly in food intake.