Characterization of electroencephalographic state in fetal baboons

Long-term recordings of the electroencephalograms (EEGs) from the non-human fetal primate have been visually scored for state. Data were obtained from three fetuses using recently developed techniques for chronic instrumentation of the pregnant baboon. Seven days or more after surgery, nine chart records of 4-5 h in duration were analyzed. These records were made during an interval in gestation from 143 to 153 days (term = 175 to 185 days). Criteria for differentiation of EEG state were based on the presence (state 1) or absence (state 2) of trace alternant, which is the predominant characteristic of EEG activity during quiet sleep in human preterm and term infants. Two patterns of EEG activity were readily distinguished at both standard (30 mm/s) and compressed (30 mm/min) EEG chart speeds. On a minute-by-minute basis, there was an overall concurrence of 82.9% for EEG state when scored from compressed and standard EEG records. From the compressed records, state 1 was present on average 38.3% of the time, with a mean epoch duration of 15.1 min, while epochs of state 2 averaged 25.9 min. We conclude that at least two EEG states are present at this gestation in the fetal baboon. These two states can be reliably defined visually by scoring methods directly comparable to those used for EEG recognition of sleep states in the human newborn infant. To validate the conclusion that these fetal state assignments based on patterns of EEG activity correspond to sleep states, it will be necessary to determine if the oscillations of other physiological parameters are coherent with these patterns.