Maternal presence serves as a switch between learning fear and attraction in infancy.

Odor-shock conditioning produces either olfactory preference or aversion in preweanling (12-15 days old) rats, depending on the context. In the mother's absence, odor-shock conditioning produces amygdala activation and learned odor avoidance. With maternal presence, this same conditioning yields an odor preference without amygdala activation. Maternal presence acts through modulation of pup corticosterone and corticosterone's regulation of amygdala activity. Over-riding maternal suppression of corticosterone through intra-amygdala corticosterone infusions permits fear conditioning and amygdala activation.