This science brief reports on a study that identified the effects of different experiences on critical periods of learning during early brain development. The study found that the presence of corticosterone in a part of the brain called the amygdala determined whether a normally painful experience was learned as an attractive or fearful event. When animals were treated with corticosterone during the first week after birth, the pairing of peppermint with a shock to the foot resulted in the peppermint being perceived as frightening. However, when animals were treated with a drug that blocks the actions of corticosterone during the second week after birth, the pairing of the peppermint and foot shock resulted in the odor being perceived as attractive. Thus, this study demonstrated the ability to change the time frame for a critical period of learning during which a specific experience is associated with later attraction or fear. [This scientific brief was produced by the National Scientific Council on the Developing Child at Harvard University. The brief is a summary of: Moriceau, S, Wilson, DA, Levine, S, and Sullivan, RM (2006). "Dual Circuitry for odor-shock conditioning during infancy: Corticosterone switches between fear and attraction via amygdala." J. Neurosci. 26:6337-6347.]