Reporter: Michael Usher
Producer: Phil Goyen
Imagine being able to read another person's mind, to see what they see, to feel what they feel.
Well, Michael Usher has just met two amazing little girls who can do just that.
Tatiana and Krista Hogan are conjoined twins.
Their physical bond is obvious. They're joined at the head. But it's what you can't see that makes these sisters so special.
PHOTOS: Filming the Hogan twins
INTRO – MICHAEL USHER: Imagine being able to read another person's mind, to see what they see, to feel what they feel. Well, I've just met two amazing little girls who can do just that. Tatiana and Krista are conjoined twins. Their physical bond is obvious. They're joined at the head. But it's what you can't see that makes these sisters so special.
STORY – MICHAEL USHER: In a scenic corner of British Columbia, Canada, in an exceptional suburban park, I meet two extraordinary little girls. Tatiana and Krista are conjoined twins, a most unlikely result of life's genetic lottery. Like any five-year-olds, they've got energy and curiosity to burn – but they've also got the scientific world enthralled by their incredible brains.
FELICIA: It's like they're, it's almost like being a hybrid.
MICHAEL USHER: A hybrid.
FELICIA: They're hybrids, pretty much, like they can do things that nobody else can do.
MICHAEL USHER: Talk about teamwork! It's the definition of it, isn't it?
FELICIA: Oh yeah. Definitely.
MICHAEL USHER: When you first see Tatiana and Krista, it's hard not to be confronted by how they look. And given the risks and challenges that face all conjoined twins, you can understand any parents deciding to not proceed with the pregnancy. But Felicia Hogan never had any doubts.
FELICIA: No matter what you look like, you're still a human being, so it doesn't matter if you're conjoined, if you have an awful disease or don't look like everybody else, you are still human and that's all that really matters.
MICHAEL USHER: Did you ever contemplate that terrible thought that it wasn't fair to bring the girls into the world?
FELICIA: No, I think if anybody else would have had these girls, they probably wouldn't be here right now.
MICHAEL USHER: They chose the right mum, I think.
FELICIA: That's right. I think so too. I feel very lucky.
MICHAEL USHER: Each twin is very much her own person. Who's being the bossy one, now? Krista, in the blue dress, is 'bolshie'. Tatiana, in pink, is more gentle. Different personalities – but with an incredible mental connection. Two separate brains – that can behave as one.
FELICIA: They can sit there and not say anything to each other, and all of a sudden one will pop up and grab something to eat for the other one. There's no words being spoken between the two of them at all and they know exactly what the other one wants.
MICHAEL USHER: Tickle one -
FELICIA: Yeah, and the other one laughs. Yeah, you can pinch one, and the other one will cry with her, like she's feeling it.
MICHAEL USHER: So they know each other's thoughts?
FELICIA: Yeah, their brain stems are connected with a piece of tissue, and we believe that they can see and hear each other's thoughts through that connection, which is kinda awesome.
MICHAEL USHER: It's incredibly awesome, and such a world first that neuroscientists like Dr Todd Feinberg have gone from being initially sceptical to being simply astonished.
TODD: My first reaction was "Oh my god, I just can't believe that is real".
MICHAEL USHER: It's extraordinary isn't it?
TODD: It's quite amazing. Some of the things sound so unusual that you just almost can't believe they're really happening.
MICHAEL USHER: From the first prenatal scan, it was clear that the Hogan twins were unique. Here you can see each of their two heads – in medical terms known as craniopagus co-joined twins. What are the chances? Just 1 in 2.5 million.
MICHAEL USHER: It must have been terribly confusing news to hear.
FELICIA: It was very confusing. I mean I've seen co-joined twins on TV but for it to actually happen to you and know that you are having these twins it was like wow, like it was unreal.
MICHAEL USHER: That the girls are alive at all is a medical miracle. Their birth was a marathon operation. 16 Canadian medics packed the delivery room at British Columbia's Children's Hospital - two teams for each baby. The girls had just a 20% chance of survival. Everyone held their breath.
FELICIA: I was just like "Yes! Oh my god, OK." Their screaming, you know, that was like the greatest sound ever. I was like "Oh my god".
MICHAEL USHER: When you saw the twins for yourself for the first time how did you react?
LOUISE: I cried. They're beautiful.
MICHAEL USHER: It as a lovely thing to see wasn't it?
LOUISE: Yep, it sure was.
MICHAEL USHER: Raising Krista and Tatiana isn't easy – and the entire extended family share the load. Louise McKay is the girls' grandmother - she's more like a second mum.
MICHAEL USHER: They certainly haven't been wrapped in wrapped in cotton wool, have they?
LOUISE: No. Most definitely not, no. We've allowed them to be as normal any two little girls as they can be.
MICHAEL USHER: You're very proud of them, aren't you?
LOUISE: Yep, I sure am!
MICHAEL USHER: It's not medically possible to separate Tatiana and Krista. Each girl has her own organs but their vascular system works as one. Their blood pumps through one girl and then through to the other. Tatiana's doing most of the pumping, her metabolism works double time, which is why she is smaller - less robust. To look at Tatiana physically seems the weak one.
MICHAEL USHER: But she's the engine room.
LOUISE: She's the engine room, yeah.
MICHAEL USHER: She's pumping all that blood over to Krista.
LOUISE: She's doing all the hard work to keep them both going.
MICHAEL USHER: How their bodies work together is remarkable – but it's how their brains work together that has really caught the imagination of medical researchers. Early tests when they were still babies revealed that their two brains are so connected that they can even see for each other.
FELICIA: They covered one baby's eyes completely, and they flashed a light in her eye, and those signals were actually found on her sister's side as well as hers, which means she was seeing what her sister was seeing.
MICHAEL USHER: Wow, four sets of eyes. Two brains
MICHAEL USHER: And the older the girls get, the more incredible their connection.
FELICIA: So if I was to go 'Hi. How are you?'
MICHAEL USHER: To demonstrate, mum Felicia covers Krista's eyes, and places a stuffed toy in front of Tatiana. Despite her eyes being covered, Krista can still see the pig.
FELICIA: Krista, what's Mummy holding?
KRISTA: Um a piggy?
FELICIA: That's right, Mummy's holding the piggy.
MICHAEL USHER: Goodness me. Here's how it works: the girls have completely separate brains and personalities, but their brain stems are connected by what's called a thalamic bridge. The thalamic bridge is like a high-speed optic fibre, and in their case, it appears to be a two-way highway. Thoughts, feelings, ideas can cross this bridge and into the other brain.
TODD: They still have functioning full brains on each side connected by this robust thalamic bridge that allows all manner of information to cross.
MICHAEL USHER: So they're plugged in, essentially?
TODD: It's like you've got two independent television sets, OK? But there's a cable that's running between the two of them and even though you can watch Channel 2 over here, you can watch Channel 4 over there, every once and a while, this one can turn on the station that this one is watching and tune into it.
MICHAEL USHER: That looks really tasty. That's a good looking cheese sandwich. When they tune in, how they do, why they tune into each other - are some of the myriad of questions that Dr Todd Feinberg would love answered.
TODD: In order for it to work, the one side which functions quite independently most of the time from the other side, this one needs to tune into the channel that the other one is watching most of the time in order to get what the other one is seeing or experiencing. There are other times, perhaps when one's a little tired or distracted, that it spontaneously leaks over, so one will taste something that the other one is eating when they really don't want to.
FELICIA: Can you see your sister?
FELICIA: Is she pretty?
MICHAEL USHER: Now, with the girls starting school this year, a new wide world will open up for them. And for researchers - as they explore the potential of these two beautiful minds – to not just share feelings, but detailed thoughts, even knowledge.
TODD: We know they can share sensations. The question will be as they mature and they get to explore a little more clearly, would it be conceivable that one side could think of a tiger and the other girl would be able to say 'tiger' for example? That would really be scientifically a breakthrough and an amazing observation. It would change a lot of the way we think about consciousness for example if that were the case.
MICHAEL USHER: For all the scientific wonder, it's difficult not to worry about Krista and Tatiana's future, and at the local park, we saw how tough the stares and taunts can be.
KID: They're oddly stuck together.
FELICIA: They are stuck together.
MICHAEL USHER: Do you know what that's called? They're called co-joined twins.
KID: That's really freaky.
LOUISE: The world is cruel to us sometimes, and sometimes it's okay. And that's just the way it is.
MICHAEL USHER: Knowing their personalities now, how are they going to handle all of that, do you think?
LOUISE: Well I think Krista's going to be the mouthy one and she's going to be telling people off, whereas I think Tatiana will try and smother it with love.
MICHAEL USHER: For all the challenges, all the triumphs that lie ahead, these happy, mischievous five-year-olds are an astonishing reminder that it's what's on the inside that really counts.
MICHAEL USHER: Before you know it they'll be grown up, teenagers – two teenage girls.
FELICIA: I know, I'm dreading!
MICHAEL USHER: What are you hopes and dreams for them?
Just to be themselves. And to do what they want to do in life. They're very special little girls.