[Autonomic reflexes, EEG and partial arousal reaction in the near-threshold region to acoustic stimuli in the newborn].

A sample of 19 neonates were exposed to sub- and suprathreshold acoustic stimuli. The experiment was performed with sleeping subjects. Stimuli, tones of 125, 250, 500 and 750 cps, third sounds of the same middle frequency and white noise, were applied only, if periods with no REM activity occurred. Stimulus intensity was varied from the subthreshold level (70-80 dB) to the suprathreshold level (80-100 dB). Polygraphic variables were recorded (EEG, EOG, instantaneous heart rate, respiration movements, actogram, motoric reactions and psychogalvanic reflex). The results showed, that with increasing stimulus intensity irregularities of respiratory parameters occurred. With further increase of stimulus intensity systematic changes in respiratory parameters and heart rate occurred. In addition to these changes, EEG activity and motoric reactions were obtained, when stimulus intensity reached a critical level. These data are consistent with the idea that at low stimulus intensities irregular vegetative reactions occur whereas systematic responses can be observed only, if stimulus intensity is above threshold. We conclude that with increasing stimulus intensity subcortical activity decreases whereas cortical activation increases.