Habituation of infants' cardiac response to speech stimuli

In an attempt to explore the relation between the amount of variability in a stimulus and rate of habituation to that stimulus, 4 groups of infants 4 1/2--5 1/2 months of age were presented with repeated speech stimuli which were synthesized exemplars of [baba], natural exemplars of [baba] or [kaba], or novel syllables of each trial. Infants' cardiac responses to the auditory stimuli were recorded, and the number of trials to a proportional criterion of habituation of the heart-rate decelerative response was determined. Infants were found to habituate most rapidly to the synthesized [baba] stimulus which remained constant across trials and least rapidly to the speech syllables which changed from trial to trial.