The condition involves a kind of tangle in the brain called an arteriovenous malformation, or AVM.

NPR: Govt Stops Study Seeking To Prevent Type Of Stroke

Early studies suggested a risk of limb malformation with tricyclic antidepressants, but the risk hasn't been confirmed by more-recent studies.

CNN: Antidepressants: Safe during pregnancy?

For example, sometimes a fetus might be diagnosed with having a congenital cystic adenomatoid malformation -- an abnormal formation in the lungs -- but actually turns out to have a diaphragmatic hernia.

CNN: Five diagnoses that call for a second opinion

Researchers have tried to link problems with the brain either through trauma or malformation to extraordinary mental abilities - one of the theories being that damage to one area prompts compensation in another.

BBC: How does a human calculator do it?