Meet Tatiana and Krista Hogan, 4-year-old conjoined twins whose brains are linked by a neural bridge. They were born in 2006 to parents Felicia Simms and Brendan Hogan of British Columbia, Canada. “These twins, I think to our current knowledge, are the only ones that do have a common neurological connection,” said Dr. Doug Cochrane, the twins’ pediatric neurologist. Twins joined at the head — the medical term is craniopagus — are one in 2.5 million, of which only a fraction survive. An incomparable resource for neuroscientists interested in tracing neural pathways, in the malleability of the brain and in the construction of the self, Tatiana and Krista are also a study in the more expansive neural system of sociology: the feedback loop of how their family responds to difference, how the world outside the walls of their home responds to the family’s response and how the girls respond in turn. For now, for the most part, the girls are not treated as if they were, as one neuroscientist described them, “a new life form.” Although they rarely venture outside their home, they spend most days the way many preschoolers do, chasing after an uncle’s puppy or watching television or testing their grandmother’s considerable patience as they play their private games at bedtime.