That is the finding of a new study published in the scientific journal Current Biology.
American and British scientists looked at 21 sleeping babies aged three to seven months old.
They used a special type of MRI scan to record how babies respond to laughing, crying, and background noises, like water.
When babies heard human voices, a part of the brain, the temporal cortex, became very sensitive, the same region as in adults.
The babies' brains responded strongly to negative or sad sounds, but did not differentiate between neutral and happy sounds.