Krista and Tatiana Hogan are in many ways normal 5-year old twin girls. They like to play with their puppies and watch Dora the Explorer. However, they were born conjoined at the head; their two skulls merge together toward the top. They were introduced to the world in a National Geographic documentary and were the subject of a New York Times article last year that detailed their everyday lives, in addition to some of their unique features.
In addition to connected skulls, the girls’ brains also have a connection that is apparently unique in the history of medicine. Called a “thalamic bridge” by their neurosurgeon Douglas Cochrane, it shows up clearly in brain images taken of the two. This connection, along with the massive amount of connected skull and other tissue, has ruled out any prospect of attempting to separate them.
Cochrane believes that this special brain bridge allows events that are perceived by one of the twins to also travel to the brain of the other. When they appeared on a recent episode of the daytime talk show, Anderson Cooper, as Krista avidly consumed some ketchup, something she loves, Tatiana, who hates ketchup, made a horrible face. When the Times’ reporter tickled Tatiana’s foot, out of Krista’s view, Krista laughed. The family accepts that this happens, since they have frequently seen one girl laughing at a television program only the other could see. (Their heads are conjoined in such a way that their faces are angled away from each other, so that their fields of view have very little overlap.)
There are also interesting questions about their sense of self: are they two people, or one? The fact that they have different preferences and different personalities—their mother says that Tatiana is lighthearted, while Krista can be a bit of a bully—argues in favor of their being two distinct people.
Susan Dominus, the author of the Times article quotes prominent neuroscientist Antonio Damasio, who says that, “the fact that no one sees the minds of others, conscious or not, is especially mysterious,” and responds by saying, “and yet here are two girls who can possibly…feel what the other feels.”
For the twins to each experience their own copy of what one of them is sensing is different from having conjoined minds. This would involve a connection at a higher cognitive level, so that each could read the other’s thoughts, for instance. An initial test of whether the twins have conjoined minds would be simply to ask one of them to form a mental image, or think of a number, and see whether the other one can say what it is. If they can (and we can rule out non-mental explanations for a correct answer, such as muscle twitches) this would show that they have conjoined minds, not merely that sensory input travels to both of their minds. This would then raise an interesting question about whether there was one copy of the mental image or two, as happens with the twins’ sense perceptions. But if there are two copies, it is still open to Damasio to claim that one twin has not truly seen inside the mind of the other, but rather has a copy of what is going on in it.
If there is only one copy, this would be a case of what I call mindmelding, a case where two people are aware of the same conscious state residing in one of their brains. The question of whether this is possible has vast implications for an age-old philosophical debate about the nature of the mind. Is it physical, or is it something more than physical? The possibility of mindmelding would also falsify claims made by a large number of very prominent scientists and philosophers who, along with Damasio, believe that mindmelding is deeply, metaphysically impossible.
For instance: Consciousness is an “entirely private, first-person phenomenon” (Damasio). Conscious states “are directly experienced only by single individuals” (Gerald Edelman). “Conscious experience… is directly accessible only to the individual having that experience, not to an external observer” (Benjamin Libet). “If your friend is staring at something green, you cannot look at her and see the greeniness of her experience. Such intimacy is ruled out by the nature of consciousness" (Colin McGinn). “There is no way I can get into your mind and check the redness of your experience” (Chris Frith). “Though I can easily observe another person, I cannot observe his or her subjectivity” (John Searle). These writers draw several different conclusions from their claims. Some, such as Searle, are so convinced that mindmelding is impossible, that they believe that the mind itself deserves its own metaphysical category, since it is the one thing we know of in the universe that can only be known about by one person. Others, such as McGinn, believe that this alleged permanent privacy of experience means that the problem of understanding consciousness will never be solved. Yet others, such as Frith, believe that the privacy of the mental means that the mind is simply “not real.”
In my recent book Mindmelding: Consciousness, Neuroscience, and the Mind’s Privacy (Oxford, 2012) I argue that it is indeed possible for one person to directly experience the conscious states of another. A large body of evidence points to our conscious states themselves residing in the back of the brain’s cortex. If we connected one person’s conscious states to another person’s sense of self—which I argue resides in the front of the brain, in the prefrontal lobes—we would have achieved true mindmelding. Unlike Krista and Tatiana, there would not be two copies of the conscious state, but only one, so that the two people would be experiencing exactly the same conscious state. As with the twins, they might have different reactions to the state, but that is because they retain their individual selves. If I am correct that our sense of self resides primarily in our prefrontal lobes, this jibes with the observation that Krista and Tatiana each possess their own sense of self, since their brains each have their own prefrontal lobes, which are not connected.
If we allow the possibility of mindmelding, many of the problems surrounding the notion of consciousness and its place in nature become treatable in a straightforward scientific manner. And we do not have to invent an entirely new metaphysical category in order to understand consciousness, or give up on the problem, or claim that our minds are unreal.
So the answer to the question that forms the title of the New York Times piece, “Could conjoined twins share a mind?” is Yes. But so far, there is no evidence that they actually do.