The two cojoined twin sisters

When Krista and Tatiana Hogan were born in Vancouver, Canada, in 2006, doctors knew their case was extremely rare.

The sisters are craniopagus conjoined twins, meaning they are physically joined at the head. They have separate brains, but doctors discovered something unusual: a small bridge of neural tissue connects parts of their brains through the thalamus, an area involved in processing and relaying sensory information.

As the girls grew, their family and doctors began noticing remarkable things.

Tests suggested that visual information received by one twin could sometimes trigger activity in the other twin’s...