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Two children, one mind?

In case you haven't seen it yet, a fascinating New York Times Magazine story about conjoined twins was published online this morning. The 4-year-old twins, Krista and Tatiana Hogan, are joined at the head and have an extremely rare bridge between their brains, one that raises the possibility that they could share some of their perceptions and thoughts:

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.

Tests by their neurosurgeon and the girls' everyday behavior both suggest that sensory signals are spilling over from one twin to the other:

As fantastic as it sounds, there is little doubt in Cochrane’s mind that the girls share some sensory impressions. When they were 2 years old, he performed a study in which Krista’s eyes were covered and electrodes were glued to her scalp. While a strobe light flashed in Tatiana’s eyes, Krista was emitting a strong electric response from the occipital lobe, which is where images are assembled. The test also worked when the girls switched roles. The results were not published, and some neuroscientists believe that this kind of test, which measures changes in brain activity beneath the skull, is imprecise in determining what region of the brain is at play; but most would agree that any response in the other twin’s brain suggests, at a minimum, connectivity.

The explanation Cochrane proposes is surprisingly straightforward for so unusual an outcome: that visual input comes in through the retinas of one girl, reaches her thalamus, then takes two different courses, like electricity traveling along a wire that splits in two ...

The results of the test did not surprise the family, who had long suspected that even when one girl’s vision was angled away from the television, she was laughing at the images flashing in front of her sister’s eyes. The sensory exchange, they 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.

The story is noteworthy both for its sensitivity - I've never read a piece about conjoined twins that did such a good job of making them seem like regular kids - and for the questions it raises about the nature of individuality. What does it tell us about the human mind if the mind can be shared by two people?

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