Psychophysiologic Studies of the Neonate: An Approach Toward the Methodological and Theoretical Problems Involved

The heart rate and behavioral responses to a repetitive series of air-stream stimulations were measured in 40 3–5 day old babies and their heart rate responses were correlated with their pre-stimulus heart rate levels. These correlations were plotted as a series of individual statistical regression lines that reflect the apparent lawful inverse relationship between the size of the heart rate response and the heart rate of the babies at the time of stimulation. As the pre-stimulus heart rate level increases, the reactivity decreases and at a certain value becomes negative--the same stimulus now produces a decrease in heart rate. The babies differ as to the level at which a stimulus starts to produce this negative response (cross-over point). In the 15 babies who were tested on 2 successive days the individual values of these cross-over point levels show a test-retest agreement constancy, p < .001. Since these cross-over points are independent of the state of the baby, they can be useful in comparing different modalities of stimulation and different organ systems in individual babies. In addition, each baby's individual regression line differed from every other in 2 qualities. They have different slopes as measured by their regression coefficients and different amounts of scatter or deviation about their regression line as measured by their estimates of standard error. These 2 qualities that have a test-retest agreement, p < .05, may be used as indices of differences in aspects of homeostatic functioning that have been hypothesized as possible precursors of some ego functions. When the babies were either very quiet or very excited, they did not show any behavioral responses to a certain proportion of stimuli, and this phenomenon may be related to the functioning of a hypothetical neonatal stimulus barrier. In the comparative evaluation of differences in the autonomic responses of individuals the concept of reactivity or lability is meaningless without first defining its parameters which are: (1) the initial state as related to the individual's own regression equation and also, (2) what might be called the individual's receptivity to stimulation at the time of stimulation. Comparing an individual to the total population, may lead, at least in neonates, to spurious conclusions. It was determined that the change-initial level regression was statistically equivalent to the stress-initial level regression.