Ontogeny of behavioral arousal: the role of environmental stimuli

During the course of ontogenesis the developing rat has been reported to pass through a transient period of intense behavioral arousal which peaks at 15 days of age, a phenomenon that has been interpreted to reflect a sequential caudal to rostral development of excitatory and inhibitory systems in the mammalian brain. In a series of four experiments it was shown (a) that this period of intense hyperactivity occurs only when the animal is tested alone in an unfamiliar environment, that the degree of arousal is proportional to the dissimilarity between the home cage and the test environment, and that isolation per se is insufficient to elicit the arousal response; (b) that environmental temperature has a minimal influence on the degree of behavioral arousal seen in either familiar of unfamiliar environments; (c) that unlearned responses to pheromonal or other naturally occurring nest odors do not suppress the high levels of locomotor activity evoked by unfamiliar environments in the 15-day old rat pup; and (d) that it is fear or distress evoked by the unfamiliar environment rather than curiosity that underlies this developmental phenomenon. It is concluded that the sequential increase and decrease in locomotor activity that occurs during ontogenesis cannot be used to support the principle of caudal to rostral development of excitatory and inhibitory centers in the central nervous system.