(A) Behavioral task of holding (blue throughout this paper) and carrying (red with yellow background throughout this paper) by the mother-infant dyad.

(B) An example of the task consisting of repetition of holding and carrying. Each condition lasted approximately 20 s. Amount of voluntary movement, presence of crying, and the interbeat interval (IBI) of the infant are presented.

(C–E) Time course of voluntary movements (C), crying (D), and the normalized IBI (nIBI) (E) of the holding-carrying transition of 12 human infants under 6 months of age. Event-triggered averaging of the effect of holding-carrying transition on the behavior and IBI was performed with the last heartbeat in the holding as a trigger (x = 0), over the time segment of −50 to +50 heartbeats from the trigger. The movement, crying, and IBI were averaged at each beat for each participant and were then averaged for all participants. Time-scale bars (s) in this figure were calculated from the average IBI. Maternal cadence (footsteps) per min during carrying was mean = 79.4, SD = 14.6, with a range of approximately 60–120 footsteps per min. Prior to data analysis, we tested the effect of possible covariates (child sex and age, maternal age, cadence) on the dependent variables (IBI, cry, voluntary movement) and found no significant correlations for any of the dependent variables; therefore, these were not considered as covariates in the subsequent analysis.

(F–H) Time course of the nIBI of the holding-carrying transition with more than 50% crying in holding but no cry in carrying (n = 13; F), without crying (n = 25; G), and with more than 50% crying in both holding and carrying (n = 6; H).

(I) The fundamental frequency (F0) of crying in the infants in (H).

(J) Time course of the nIBI when infants were sleeping (n = 7) throughout the holding-carrying period.

(K) rMSSD (heart rate variability index of parasympathetic activity) during the holding and carrying.