I believe it was playing Mozart, since it was called “The Mozart Effect”. And it helps their brains develop more logical connections, as any structured music does. But the “improvement” ceases to matter by about age 4 or 5, because they’re then building connections based on life experiences. If the classical music is reinforced, however, along with deliberate practice, you create young children with perfect pitch. (The ability to recognize a note in isolation, instead of how most of us do it, which is to recognize how far away a note is from another note.)