Canadian twins Tatiana and Krista Hogan are 4 years old.
They are the rarest of the rarest of the rare. Tatiana and Krista are not just conjoined, but they are craniopagus, sharing a skull and also a bridge between each girl’s thalamus, a part of the brain that processes and relays sensory information to other parts of the brain. Or perhaps in this case, to both brains. There is evidence that they can see through each other’s eyes and perhaps share each other’s unspoken thoughts. And if that proves true, it will be the rarest thing of all. They will be unique in the world.
They have been drawing international attention, both public and scientific, since before their birth. Dr. Douglas Cochrane, a neurosurgeon at Vancoouver Children’s Hospital, is part of the team that has been watching over them since they were in the womb. Last year he conducted tests in which one twin looked at an object while he measured the brain activity in the other. “Their brains are recording signals from the other twin’s visual field,” he cautiously concluded. “One might be seeing what the other one is seeing.”
The test and his comments were included in the documentary Twins Who Share a Brain, produced and directed by Kelowna filmmakers David McIlvride and Alison Love. It aired in the U.K. in May, and on CBC’s Doc Zone earlier this month.
The documentary created a minor sensation when it aired in the U.K., drawing 2.5 million viewers, almost 10 per cent of the viewing public. Among the more bizarre responses it engendered was a debate on the British student chat site The Student Room; the topic: “Conjoined twins sharing a brain—one person or two?”
The debate was of the angels-dancing-on-a-pin variety, as though science nerds had stumbled into a philosophy class. “I think it depends on whether they can develop their own personality,” opined one. “But then does that mean they are just one person with a very weird split personality disorder?” Added another: “If one is capable of thought that the other can’t interpret or won’t know about, then I’d say it’s two people. Otherwise I’d say it’s just one person with two functioning bodies.”
It’s safe to say no one in the twins’ family has any such doubts. The girls were distinct from the get-go, and they grow more so as they age. Krista is the larger and stronger. Tatiana, while smaller, is the work horse. Her heart does much of the pumping, her kidneys and liver do most of the filtering. “Krista is my bully. I think she always will be,” says Simms. “But [lately] Tati has taken a lot of the authority,” she adds. “ ‘If you’re going to be mean to me, I’m going to stop being nice.’ [Tati] is not as laid back as she was before. It’s a good thing.”
The twins receive weekly physiotherapy, most recently to improve their upper arm movement. Although some doubted they would ever walk, their mobility continues to improve. They walk and even run a bit, albeit awkwardly. They spin with remarkable ease, and they flop and wrestle on the floor with their siblings in a joyous tangle of limbs. Although they lean into each other like an inverted V, their necks so far have not suffered from the strain. Picking them up, one worries they’ll break. And yet they are remarkably flexible. “Their necks don’t seem to bother them,” says their mother, “When you see them stand up and stuff, you see all the muscle in their necks. It’s amazing.”

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