First, they observed brain activity while the mice were resting. Stress had changed the activity in the amygdala of the less resilient mice much more than it had in the resilient ones.

When the researchers gave the mice a choice between plain and sugar-sweetened water, the resilient mice easily chose the sugar water. But the less resilient mice became obsessed and often opted for the plain water.

Xia looked at brain recordings of the mice who chose the sweet water. Their amygdala was communicating with a nearby brain region called the hippocampus that remembers and predicts.

She saw a different pattern in the mice that could not decide whether to drink the plain or sweetened water. In those mice, the conversation between the two brain areas sputtered.