Twins joined at the head — the medical term is craniopagus — are one in 2.5 million, of which only a fraction survive. The way the girls’ brains formed beneath the surface of their fused skulls, however, makes them beyond rare: their neural anatomy is unique, at least in the annals of recorded scientific literature. Their brain images reveal what looks like an attenuated line stretching between the two organs, a piece of anatomy their neurosurgeon, Douglas Cochrane of British Columbia Children’s Hospital, has called a thalamic bridge, because he believes it links the thalamus of one girl to the thalamus of her sister. The thalamus is a kind of switchboard, a two-lobed organ that filters most sensory input and has long been thought to be essential in the neural loops that create consciousness. Because the thalamus functions as a relay station, the girls’ doctors believe it is entirely possible that the sensory input that one girl receives could somehow cross that bridge into the brain of the other. One girl drinks, another girl feels it.
What actually happens in moments like the one I witnessed is, at this point, theoretical guesswork of the most fascinating order. No controlled studies have been done; because the girls are so young and because of the challenges involved in studying two conjoined heads, all the advanced imaging technology available has not yet been applied to their brains. Brain imaging is inscrutable enough that numerous neuroscientists, after seeing only one image of hundreds, were reluctant to confirm the specific neuroanatomy that Cochrane described; but many were inclined to believe, based on that one image, that the brains were most likely connected by a live wire that could allow for some connection of a nature previously unknown. A mere glimpse of that attenuated line between the two brains reduced accomplished neurologists to sputtering incredulities. “OMG!!” Todd Feinberg, a professor of clinical psychiatry and neurology at Albert Einstein College of Medicine, wrote in an e-mail. “Absolutely fantastic. Unbelievable. Unprecedented as far as I know.” A neuroscientist in Kelowna, a city in British Columbia near Vernon, described their case as “ridiculously compelling.” Juliette Hukin, their pediatric neurologist at BC Children’s Hospital, who sees them about once a year, described their brain structure as “mind-blowing.”