Behavioral states and their ontogeny: human studies.

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Abstract Despite numerous attempts in describing the many underlying events correlated with changes in behavioral states, the biological significance of state alternation remains unknown. There does not yet exist a satisfactory explanation for the biological need for such state changes, nor does there exist sufficient physiological evidence to explain the underlying processes leading to these state transitions. Nonetheless, the study of the ontogeny of behavioral states is important because it shows the progressive ability of the organism to integrate multiple neuronal systems. Available literature suggests that some rudimentary form of behavioral state is established in fetal life throughout the animal kingdom, although there is substantial difference in the degree and rate of development of organized behavioral states among the different animal species. Highly organized behavioral states that involve concurrence of many physiologic systems are present in the third trimester in the fetal lamb, while they are not seen until 2 to 3 weeks postnatally in the rat, rabbit, and cat. Subsequent maturation consists primarily of increased cycle length in all species. In addition, there is some evidence that a circadian rhythm may be superimposed on the ultradian rhythm of behavioral states in immature animals, but there is currently no information on the ontogenesis of this circadian rhythm. It is currently hypothesized that behavioral states cycling in the adult is generated by the interaction of multiple, anatomically distributed sets of neurons. 40 Electrophysiological and pharmacological studies have suggested that both the cholinergic and aminergic neuronal systems play important roles in the maintenance of state cycling. Studies in the fetal lamb also support these proposed mechanisms in early development. 41, 42 However, there is little evidence to indicate that changes in transmitter release or receptor sensitivity occur in the course of sleep deprivation in the adult. A number of other neuropeptides may also be involved in this complicated control system. The progressive organization of behavioral states in early development and the agerelated changes in cycle duration may provide valuable insight in understanding the neurobiological events that underlie state alternation and homeostasis.