Cardiac change of retarded and nonretarded infants to an auditory signal.

Five mentally retarded infants and 5 nonretarded infants were presented a tone for 15 trials. Heart-rate change from prestimulus level showed a deceleration on four 1-second intervals for the nonretarded babies and no change for the retarded babies. Comparison of the two groups of infants revealed a difference on five of the 1-second intervals following stimulus onset. When the total heart-rate change scores were examined for each baby, no overlap of the scores was evident for the infants in the nonretarded group with the individual scores of the retarded infants.