Response to auditory stimuli in 6- and 9-week-old human infants.

Response to 5-sec, 75-dB complex sounds was studied in groups of 24 infants each at 6 and 9 weeks of age. Under conditions appropriate for eliciting orienting, no significant cardiac response appeared at 6 weeks, but a significantly decelerative response did appear at 9weeks. Measurement of noncardiac responses suggested that sucking suppression is also a component of orienting to sound, that the status of eye-opening is equivocal, and that previously reported eye-movement responses are probably a component of startle. We concluded that age changes in orienting may occur in young infants, even when state is controlled, and that the probability of orienting varies with the kind of stimuli employed.