Susan Dominus has penned a remarkable piece for the New York Times about Krista and Tatiana Hogan, the 4-year old conjoined twin girls from British Columbia who are attached at the head. Scans show that the two girls have brains that are interconnected by a never-seen-before "thalamic bridge," an indication that they might share conscious thoughts. And if their early behavior is any indication, this may very likely be the case.


Twins joined at the head, what's referred to as craniopagus in the medical literature, are extremely rare occurrences, affecting only one in 2.5 million births. Most don't survive. But the Hogan twins are an even rarer case yet. Their neural anatomy is something that has never been seen before by scientists: their scans reveal an attenuated line stretching between their two brains connecting each of their thalamus together. What makes this observation particularly interesting is that the thalamus happens to be a kind of neural switchboard, a two-lobed organ that filters most sensory input.


It's also thought to play an important role in the rise of conscious awareness.

Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming neural circuits that are thought to be involved in consciousness. The thalamus also plays a major role in regulating arousal, levels of awareness, and activity.

For the Hogan twins, this raises the interesting possibility that they may share both sensory experiences and conscious thoughts. Susan Dominus, who spent a considerable amount of time with twins while researching the story, witnessed a number of occasions that strengthened this suggestion. There are also the results from scientific observation:

As fantastic as it sounds, there is little doubt in [neurosurgeon Douglas] 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.


Now that the girls are getting older, their family is starting to notice more and more of this kind of behavior. The girls use the word "I' interchangeably, at times using it to refer to themselves as individuals, and at other times as a collective unit. In fact, they often behave as a single unit, often getting up simultaneously without uttering a word to one another and fulfilling a task like getting a cup of water. At the same time, they're also prone to disagreements and fights. Sometimes they scratch away at the other girl's face, and then have to console themselves after inflicting the shared pain. And there's more:

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.


It's fairly obvious that the girls share sensory experience, but as time passes it's also becoming evident that the girls also share conscious experience. How else to explain their wordless cooperation? Only time will tell as the girls get older, and as they become better at articulating the exact nature of their special connection.

Susan Dominus's piece is detailed and lengthy, but definitely worth reading from top to bottom. It's insightful, touching, and very personal. The girls have an extremely supportive extended family — one that's not willing to make their unique twins into circus freaks or medical test subjects. All they want are their girls to grow up as normally as possible given their rather special circumstances.


Top image via Gizmodo. Body image via NYT.