A fascinating article about twins conjoined at the head which leads scientists asking whether they share a mind.  The girls are about three years old and if one senses an object, the other knows what the other is sensing!  How is this possible?

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.

Also, one of the girls started drinking water, and the other one could feel it in her stomach.  For a blood test, the doctor pricked one sister, and the other sister started crying.

The sensory exchange, they [the family] believe, extends to the girls’ taste buds: Krista likes ketchup, and Tatiana does not, something the family discovered when Tatiana tried to scrape the condiment off her own tongue, even when she was not eating it.

Yet, the girls do display differing personalities.  Nevertheless, they are so in sync it’s as if they are one being.

The girls surely have a complicated conception of what they mean by “me.” If one girl sees an object with her eyes and the other sees it via that thalamic link, are they having a shared experience? If the two girls are unique individuals, then each girl’s experience of that stimulus would inevitably be different; they would be having a parallel experience, but not one they experienced in some kind of commingling of consciousness. But do they think of themselves as one when they speak in unison, as they often do, if only in short phrases? When their voices joined together, I sometimes felt a shift — to me, they became one complicated being who happened to have two sets of vocal cords, no less plausible a concept than each of us having two eyes.

Although each girl often used “I” when she spoke, I never heard either say “we,” for all their collaboration. It was as if even they seemed confused by how to think of themselves, with the right language perhaps eluding them at this stage of development, under these unusual circumstances — or maybe not existing at all. “It’s like they are one and two people at the same time,” said Feinberg, the professor of psychiatry and neurology at Albert Einstein College of Medicine. What pronoun captures that?

Epistemic privacy seems to be lost just for these girls to each other.  Imagine all of your thoughts, desires, perceptions, memories, and hopes are shared with another person.  Sometimes, shared perceptions may be disastrous:

Someone ordered them chicken fingers, and Krista took a bite. Suddenly, Tatiana made a face. “It’s too yucky,” she said, starting to cry. The mayhem level went up a notch, and Tatiana crawled under the table, wailing, as Krista was trying to pull her back up by the force of her neck. Krista tried to put the chicken finger directly into Tatiana’s mouth. “Krista likes it!” she said. “It’s yummy!” Tatiana spit the food out, crying: “Let me hide! Let me hide!” She covered her mouth with her hand. “Don’t make her eat it, sweetie,” said their grandmother, as Doug sighed in frustration. “Sissy eat it!” Krista said again, trying to push it in Tatiana’s mouth. Krista started pulling her sister’s hair, and then both girls were crying. Tatiana’s futile declaration rose above the sounds of the restaurant. “I am getting out of here!” Tatiana sobbed. “Let me alone.”

So this brings up interesting questions: the neural networks makes both of their minds sync up.  Do they share one mind?  How does this affect personal identity?  Can one mind have different personal identities?