The other night I happened across a documentary on TLC entitled “Inseparable: Joined.”
Here’s an article about the people the documentary explored: the conjoined Hogan twins of Canada, who are joined at the brain:
Craniopagus twins, joined at the head, are a rarity — one in 2.5 million. The vast majority do not survive 24 hours.
The Hogan twins are not just joined at the head. They also share a connection of the thalamus called a thalamus bridge. So each twin has a separate brain and her own thalamus, but each thalamus communicates with the other one and probably with more than the thalamus.
The documentary shows this extraordinary and fascinating situation. The twins are doing surprisingly well, but the fact that they are “doing” anything at all, including being alive, is surprising. They have different personalities, and although they have some learning disabilities – perhaps due to some of their other problems, which include epilepsy (and medication for it), diabetes, and celiac disease – if you watch them you can see they are quite intelligent, although slow for their age.
The twins cannot be surgically separated; it is considered too life-threatening. They were 10 years old at the time of the documentary and are 12 years old now. They also seem to have a good sense of humor.
But perhaps the most extraordinary element of their incredibly extraordinary situation is the way in which their brains communicate. It is being studied and is not that well understood, but for example one can be blindfolded and can tell what the other is seeing. In other words, they share some sensory input. But to make it even more complex, one of the twins can see from both eyes of the other, and the other twin can only see from one eye of her twin.
In a somewhat similar manner, one twin can voluntarily move three legs and one arm, and the other twin can voluntarily move three arms and one leg. But they are able to override this and just move the arms and legs on their side of the body, so that their movements can be coordinated. So it is partly voluntary, something they can access or turn off at will.
The thalamus acts like a switchboard relaying sensory and motor signals and regulating consciousness…
The twins say they know one another’s thoughts without having to speak. “Talking in our heads” is how they describe it.
In the TLC documentary I saw, after tobogganing down a hill one twin says, “I was so scared!” and the other proudly announces, “I wasn’t scared!” followed by, “I need to hold onto my sister because she’s scared!” and raucous laughter. They seem to mean this to be some sort of joke. And it’s actually pretty witty because of course they have no choice about “holding onto each other,” and they also share emotions in the sense of perceiving each other’s emotions. Sometimes there is even carryover in which one’s emotions affect the emotions of the other more directly and seem to influence the emotions of the other.
The Hogan twins’ situation conjures up questions involving identity, individuality, and the brain. Clearly, from the example of identical twins in general, we know that DNA does not determine identity because, although all identical twins share DNA, each twin is well aware of having a separate identity from the other twin. The Hogan twins take that one (or two, or three or four or more) steps further. They have separate identities and personalities, but their shared brain connection seems to give them not only a close bond (such as many identical twins have) but the ability to access sensory, cognitive, emotional, and motor elements of each others’ bodies.
[NOTE: I haven’t been able to find a link to the documentary I saw, but here’s a different one.]

