Nonlinear investigation of developmental changes of the ECoG activity in fetal sheep

The fetal sheep brain develops cyclic electrocortical activity from approximately 0.8 gestation alternating between NREM and REM sleep. The aim of this study was to investigate developmental changes of the electrocorticogram (ECoG) after the occurrence of cyclic electrocortical activity as a measure of functional brain maturation. The authors used nonlinear analysis because it has been recognized superior to linear analysis for describing the complex properties of the ECoG. The authors developed a nonlinear algorithm which calculates a mean prediction error (MPE) of the time series in the phase space. The ECoG of six chronically instrumented fetal sheep was recorded in utero at 124, 127, 131 and 135 day gestational age (dGA, term 146 dGA). The MPE of the ECoG was calculated for each animal at each investigated dGA from five 10 min epochs of REM and five 6 min epochs of NREM sleep. The MPE during REM sleep was significantly higher than during NREM sleep (p<0.05). With gestational age, the MPE of REM sleep increased and that of NREM sleep decreased significantly (p<0.05). These results suggest a further functional differentiation of both sleep states after transition to cyclic ECoG activity