Oscillation and complexity in early infant behavior.

Cyclic fluctuation is a ubiquitous property of spontaneous motor activity in the human fetus and neonate. Oscillation occurs on the scale of minutes, and irregularity is one of its most characteristic properties. To determine whether cyclic motor activity (CM) persists beyond the neonatal period (1 month after birth), and beyond what has been called the period of the fetus ex utero (2-3 months after birth), 30 infants were studied at monthly intervals from 1 to 4 months after birth during active sleep and awake. Spectral analysis was used to quantify the rate, strength, and irregularity of CM. To examine the characteristic irregularity of awake CM more closely, a nonlinear forecasting technique, developed to study chaotic dynamic in other fields, was used to estimate the predictability of spontaneous movement in a state space reconstructed from the time series. The rate and irregularity of CM during active sleep and awake, and its strength during active sleep, did not change across the first 4 postnatal months. However, there was a pronounced drop in the strength of awake CM from 2 months onward and a concurrent increase in the broad-band power of fluctuations faster than 2 cycles/min. The predictability of awake CM also dropped abruptly between 1 and 2 months, especially for prediction intervals less than 20 sec. The changes at 2 months reflect the introduction of a source of variation with a shorter time constant than fetal-neonatal CM, and which is specific to awake periods. The source of these effects may be moment-to-moment interactions between CM and attention.