A Brain of Four Halves

Unique Craniopagus Twins

The story of Krista and Tatiana Hogan, 4-year old craniopagus twins, has featured on various TV channels, e.g. Channel 4’s ‘The Twins Who Share a Brain’ but the information below has been paraphrased from a New York Times article ‘Could Conjoined Twins Share a Mind?’, with my additions in square parentheses.

Craniopagus twins – those conjoined at the head – are rare but Krista and Tatiana Hogan are believed unique because of their neural anatomy. There is a ‘thalamic bridge’, which links the thalamus of one girl to the thalamus of her sister. (The thalamus filters sensory input and has been thought to be essential in the neural loops that create consciousness. The thalamus functions as a relay station.) In their case, the sensory input that one girl receives somehow crosses the bridge into the brain of the other: One girl drinks, the other girl feels it. This is an incomparable resource for neuroscientists. Further details:

  • Each girl has an unusually short corpus callosum and the two cerebral hemispheres also differ in size, with Tatiana’s left sphere and Krista’s right significantly smaller than is typical. (The conjoining is from Tatiana’s left to Krista’s right side of the head.)
  • When the girls were younger, each experienced several seizures, which medication has since controlled.
  • Tatiana’s heart was working harder than Krista’s because her heart supplies part of the blood to Krista’s brain.

Sensory Exchange

Examples of shared sensory impression, showing symmetry of behaviour:

  • Krista: ‘I am drinking really, really, really, really fast,’ and starts to power-slurp her juice. Tatiana: suddenly her eyes went wide and put hand below sternum – ‘Whoa! Now I do it,’ started to chug. Krista’s hand flew to her own stomach – ‘Whoa!’.
  • Experiment at 2 years old: Krista’s eyes covered and electrodes glued to her scalp. Flash a strobe light in Tatiana’s eyes. Strong electric response from Krista’s occipital lobe. The test also worked when the girls switched roles.
  • When the girls fight, they reach their fingers into each other’s mouths and eyes, scratching, slapping, hands simultaneously flying to their own cheeks to soothe the pain.

Other examples of ‘sensory exchange’:

  • Vision: 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.
  • Taste: Krista likes ketchup, and Tatiana does not. Tatiana tried to scrape ketchup off her own tongue, even when she was not eating it.
  • Touch: Tickle Tatiana’s foot, which Krista could not see. Krista: ‘Now do me,’.
  • Touch and Motor: Thermometer placed in Krista’s mouth. Tatiana: ‘Not in mouth,’. Her tongue, was curling in an unusual way.
  • Vision: Stuffed animal pulled out of a bag, a turkey, and handed to Tatiana so that Krista could not see. ‘Krista, do you know what Tatiana has in her hand?’ Krista: ‘Robin?’ (considered a close-enough answer).
  • Vision: Say the precise name of the toy that could only be seen through the eyes of her sister.
  • Touch: point precisely, without looking, to the spot on her sister’s body where she was being touched.
  • Touch: touch Krista’s birthmark. Krista: ‘Don’t touch my pen mark,’ and stroked it. Tatiana (without a birthmark) stroked the same spot on her own body in the same way and wore the same injured facial expression.
  • [No auditory example.]

One brain or two?

Do ‘they’ have one brain or two? Though they frequently move in near synchrony, mirroring each other’s gestures, the girls clearly have different personalities. Krista is physically stronger, ‘more of the bully’; Tatiana is more lighthearted and more submissive at the end of the day than at the start. Hence the impression that they are more like ‘one brain’ at the end of the day. The family says that the girls often get up silently and suddenly and walk over to, say, a sippy cup, which Tatiana then immediately hands to Krista, who drinks from it. Does one girl silently express her thirst to the other in the form of a higher thought?

Todd Feinberg’s book, ‘Altered Egos: How the Brain Creates the Self,’ describes patients with corpus callosum severed. A patient might find one of his hands is at odds, or in all-out war, with the other [Dr Strangelove]. ‘With split brain, you cut the brain in half, yet the person feels and acts as a whole. In these girls, they’re linked, yet each acts as a whole. It’s like [each girl] has one consciousness and can witness another’s.’

[Behaviour generally appears independent despite environment being extremely similar. This might explain their sometimes identical verbal responses – rather than any ‘commingling of consciousness’.]

In ‘Self Comes to Mind’, Antonio Damasio says ‘The fact that no one sees the minds of others, conscious or not, is especially mysterious.’ We may be capable of guessing what others think, ‘but we cannot observe their minds.’ And yet here are two girls who can possibly feel what the other feels. But: even that extraordinary dynamic would still put the girls on the continuum of connectivity that exists between ordinary humans.

[Given the coherent distinct personalities, it seems to me this is clearly a case of two ‘minds’ sharing sense input at the neural level (within one single ‘neural mass’ – I would hesitate to call it a single brain).]


  • Visual input comes in through the retinas of one girl, reaches thalamus, then takes two different courses. In the girl who is looking, the visual input continues on usual pathways, one of which ends up in the visual cortex. In the case of the other girl, the visual stimulus would reach her thalamus via the thalamic bridge, and then travel up her own visual neural circuitry, ending up in her own visual cortex.
  • In addition to sorting out the usual sensory experiences of the world, the girls’ brains, have been forced to adapt to sensations originating with the organs and body parts of someone else.
  • At other times, the connection seemed to fail them. Perhaps their brains are trying to filter out inputs that originates from the other girl’s body. (Family belief: effort to cross-tune tires them out.)
  • [Sense input from the ‘other’ is weaker than from ‘me’, but somehow conscious – awareness-raising rather than the guesswork of, say, blindsight.]
  • Unconvincing counter-conjecture: explain by something other than a neural bridge. ‘If they’re really close, through minute movements that one makes — maybe a typical movement her sister cannot see, but can feel’.


Postscript: Mirror Neurons and Synaesthetic Pain

When Krista feels something, Tatiana also actually feels something too, and vice versa. This is way beyond the usual the ability of people to empathize with others which has been attributed to the so-called ‘mirror neurons’. When we see others in pain, our mirror neurons fire.
But a recent New Scientist article discusses ‘synaesthetic pain’ – where seeing others in pain doesn’t just trigger empathy, it actually triggers pain. This is perhaps a bit like a ‘wireless’ version of Krista and Tatiana’s ‘thalamic bridge’!
In the article, Bernadette Fitzgibbon at Monash University thinks that:

  • Normally, mirror neurons do not cause real pain because there are inhibitory mechanisms that dampen the response.
  • In pain synaesthetes, this inhibitory mechanisms is itself inhibited.
  • Synaesthetic pain occurs mainly in people who have lost a limb.
  • The traumatic experience associated with losing a limb may heighten the sensitivity of pain synaesthetes to others’ pain. When threatened, our body naturally becomes hypervigilant to pain.
  • Pain synaesthesia may be a symptom of an abnormal, ongoing hypervigilance.

Intriguing, but this just raises questions on how mirror neurons work, including:

  • If I have lost my left arm and see that your left arm is hurt, maybe that could trigger real pain in my left arm (like a phantom limb pain). But the implication is that he mirror neurons only operate at a global (whole person) level.
  • Are mirror neurons implicated in the phenomenon that seeing my bleeding finger actually helps reduce the pain in my finger?
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