32 REFLEX MODIFICATION BY ACOUSTIC AND TACTILE SIGNALS IN NEONATES

Abstract

In recent efforts directed to the analysis of sensory processing in the newborn, neonates and adults were exposed to various reflex modification procedures. Previous work with adults had indicated that when a tone precedes a reflex eliciting stimulus by an appropriate interval the amplitude of the reflex is inhibited. If the tone is presented simultaneously with the reflex-eliciting event, response to amplitude is augmented. The eye-blink eliciting device consisted of a miniature solenoid with a teflon striker which could deliver a controlled tap; a miniature photo reflective densiometer attached to a TDH 39 earphone assessed the eye blinks. For each experiment, 15 healthy term appropriate-for-gestational-age infants were studied between 13 to 90 hrs of age during quiet sleep; 15 adults were studied for comparison. Four experiments employing identical reflex modification procedures on neonates and adults suggest a developmental difference in processing sensory stimuli. Neonates failed to exhibit reflex inhibition by either prior tactile or acoustic stimuli (either tone pulses or sustained tones). Adults exhibited robust reflex inhibition to these same stimuli. Neonates, however, exhibited reflex augmentation when mild tones (70 dB) were presented simultaneously with a tap, while for adults tones of at least 90 dB were necessary to obtain reliable reflex augmentation. The present findings in conjuction with those of others suggest that neonates fail to exhibit inhibition because the neural system responsible for this effect are not fully developed at birth.