Conditioned activation of fetal behavior.

Previous demonstrations of prenatal learning have relied upon experimental protocols involving training at one gestational age and testing at a later age, potentially confounding effects of experience and retention with developmental changes in the fetus's behavioral repertoire. In this study, a single-session paradigm, which involved the pairing of a neutral chemosensory stimulus (sucrose CS) with a second stimulus that activates behavior (lemon US), was used to assess the learning capacity of the rat fetus. Four CS-US pairings were effective in promoting behavioral activity in 20-day-old rat fetuses when they were reexposed to the sucrose CS alone. The functional expression of conditioned behavioral activation on day 20 of gestation occurs at a time when CNS structures, many of which are involved in sensory processing and learning in older animals, are still undergoing rapid differentiation in the rat fetus.