Fetal behavior

Rapid Eye Movement (REM) sleep in adult and neonatal mammals is characterized by episodes of high variability and bursting in brainstem sites associated with spontaneous tonic and phasic behavioral events such as REMs, nuchal inactivity and twitches of the body. REM sleep is the principal behavioral state during fetal and neonatal life and as has been demonstrated by various REM deprivation procedures to be indispensable during this period and to lead to long lasting behavioral defects in adult life. The guiding hypothesis throughout this dissertation is that the variability of REM-associated nuchal atonia episodes and other spontaneous motor events reflects the fractal time patterns of a global fetal REM sleep state over multiple timescales serving as a transient behavioral ontogenetic adaptation to changing developmental environments. Further, spontaneous activity over many levels of organization, including phasic REM motor activity during ontogeny, could play a fundamental role in the development of appetitive behavioral processes (e.g., searching and orienting) and other forms of neuroplasticity (e.g., learning and dynamic regulation of receptor fields and maps). The nature of this variability was investigated by measuring the durations of nuchal atonia over extended periods in fetal sheep and neonatal rats, species which are in a REM sleep-like state >50% of the time. Hurst's rescaled range analysis, which affords comparisons between natural time series with short- and long-term correlated fluctuations, indicated that variability in both species over short time scales is statistically similar to longer time scales (i.e., is fractal in time) and remarkably stable over the developmental periods examined. Spontaneous nuchal events in both species were also found to be described by convolutionally stable self-similar Levy distributions, suggesting that activity associated with fetal REM sleep could provide a stable, scale invariant source of correlated stimulation, facilitating integration of new neural changes into developing motor and cortical networks over gestation. These fractal time descriptions of spontaneous prenatal behaviors have implications for conceptualizing the evolutionary mechanisms underlying heterochrony (shifting self-affine relationships between the timing of gene expression and behavioral activity) and the plasticity essential to the genesis of behavioral neophenotypes.