How does a person get to grips with an extra thumb?

Test subjects got to grips with their new digit with relative ease...
31 May 2024

Interview with 

Dani Clode and Lucy Dowdall, University of Cambridge


Prosthetic thumb


Most of us have, at one time or another, wished we had an extra thumb on hand to help with fiddly tasks. And Dani Clode, who designs limb prostheses, thought the same, so she made one! It’s basically a rubber robotic thumb that straps onto the bottom of the hand in roughly the opposite position to the native thumb, and closes up onto the palm or against the little finger. She wanted to know how people would get to grips with using an additional digit, particularly as we move into an era when body augmentations like exosuits that can help us to perform certain tasks, are set to become more commonplace. The result, which was showcased and tested on hundreds of members of the public at the Royal Society’s famous “summer science exhibition” also offers an opportunity to explore neurologically what happens as someone gains a body part and learns to use it. I went to meet Dani and her colleague Lucy Dowdall at the Cambridge MRC Cognition and Brain Sciences Unit to test it out for myself…

Dani - Cool. So, we're going to put a third thumb on you today. I do need you to take your shoes off, if that's okay.

Chris - Good job I put decent socks on.

Dani - <laugh>

Chris - It's not every day I turn up to an interview and someone says to get your shoes off <laugh>.

Dani - Yeah. You know, we do things differently here. <laugh>,

Chris - I thought this was for my hand. Why have I got to get the shoes off?

Dani - It's foot controlled. So each of your big toes is going to control one, what we call, degree of freedom of the third thumb, which is the two kinds of movements that it does. So one movement is across the hand and back and the other movement is kind of towards your fingers and back. And so your left and right toes are going to do that. So this is the device here. So there's a part that's worn on your upper arm, like on your bicep, and then that's connected to a wire. That's to the motors that are worn on your wrist. And then there's a hand piece that's kind of got two straps. That one goes between your thumb and index and then the other kind of around the base of your thumb. And that's going to keep the third thumb on your hand.

Chris - Okay. So do you need my sleeves rolled up or is this jumper friendly? <laugh>I mean I know this is The Naked Scientists, right, but there are limits <laugh>.

Dani - Yeah, we have to be jumper friendly in the UK don't we? <laugh>?

Chris - So this is going over my right arm. Yep. So it's on the forearm at the moment. You're going to put that further up here?

Dani - I've got to kind of shimmy it up your arm.

Chris - So the neck goes around. The hand is going over my fingers.

Dani - So thumb in. It goes on your wrist.

Chris - Right. So I now have a strap around my wrist. A strap around my hand and a thing, which looks like an exceedingly large thumb, but on the bottom, sticking out from the bottom of my hand. That's sort of, if I had monkey hands, that's the length of the sort of digit I would expect to have <laugh>.

Dani - Yeah. I made a smaller one, but it wasn't quite as functional. So, yeah, it's become bigger over the years. <laugh>.

Chris - So I now have, I presume that's a battery pack as well. Now on the back of my arm.

Dani - The battery pack and also the kind of PCB, the electronics that connect wirelessly to the foot control. And then I'm going to put two sensors underneath your big toes. You've got your left control here. If you press down, there we go.

Chris - Oh. Oh, okay. So this is, I press my left big toe down and I'm moving towards the rest of my fingers. My right toe, I'm moving my new thumb into the palm of my hand.

Dani - Yep. Yes. So you can see that the speed is responding quite instantly and wirelessly to the pressure of your big toes. I

Chris - Actually it's three degrees of movement, isn't it? Because I can go into the palm, I can go outwards in the direction my fingers are pointing if I was sort of pointing my fingers away from my body. But I can also move the device so that it's in any of the positions in those two directions. It's not just into the palm or out towards my fingers.

Dani - Yeah, I was tricky about that and actually got a bit for free in how the thumb is fully flexible. So it's 3D printed out of a flexible material. And so you are actively pulling in and then the flexible material is kind of pulling back out. So it kind of seems like there's a lot more degrees of freedom, but there's only two controls and two motors.

Chris - Do you reckon it'll hold a microphone? Shall I have a go? This is quite brave. It's quite heavy, this microphone. Okay, I'm going to have a go. It's quite strong.

Dani - <laugh> You want to go the other way around so you've got to change the way that you actually use your hand. So I would go that way. So push your right toe.

Chris - So you're putting the microphone along my hand? So it's gonna be grabbed. So let me just try that.

Dani - So right toe. And press and hold

Chris - You've got to hold quite tight. Yeah. <laugh>, but it has got it. I mean that's quite good. I'm not sure what I would do with it, because it's completely occupying the rest of my hand.

Dani - Yeah, it's a big microphone <laugh>, but what you're doing is you're freeing up the use of your fingers, especially your pinch grip for example, which is a super functional part of your hand. The third thumb can grasp something whilst then freeing up that part of your hand.

Chris - So what sorts of use cases would you see for this? I grant you, it's very easy to use this. It's my first go, I grabbed hold of something and I feel pretty confident that I can put this thing into any position relative to my hand. That's really easy to do. But what's the use case for this? What sort of applications do you foresee?

Dani - Yeah, well this kind of device is augmentation. So extending the function of the hand. It can kind of grasp an object whilst freeing up your kind of index or rest of your fingers. So for example, you know, holding a bottle whilst unscrewing the cap, that can be done one handed. So all these kinds of tasks that usually require two hands you can explore just using one hand.

Chris - So do you think surgeons might be interested in something like this? Because there are some aspects of surgery where surgeons say, I wish I was an octopus.

Dani - I would love to collaborate with surgeons on this. Especially people with professions where they are kind of hyper trained with their hands. You know, to then expand the function of a hyper trained hand surgeon is an interesting example because they use a lot of assistance. There's people that are perhaps having their eyes or other hands during the surgery and so to augment them and create more fluid function during surgery would be an incredible opportunity.

Chris - Lucy, you are the neuroscientist on the team. What's your role? What are you actually doing and trying to find out?

Lucy - So we're interested in how our experiences and our environment shape our brain. After five days of training with the thumb, we see changes in how our hand is represented in the brain and in our next steps in this research we're looking at how the third thumb itself is understood within this body representation. And that's really important. When thinking about if people are using these technologies in their everyday life, how is that then affecting our biological body?

Chris - When you say you see changes in the brain, what are those changes? How does the brain seem to incorporate it into its model of the body?

Lucy - In our brain we have a body representation. So for example, if I move any one of my individual fingers, I'll see an air of my brain devoted to that finger lighting up. And after five days of training, we find that after the training the fingers all become more similar to each other. So we're having a reduced representation of our hand in the brain. But we want to know is that because we're using our hand differently every day because we know that how we use our hand does affect how it's understood by the brain. Or is it something very specific to the third thumb itself and that being incorporated.

Chris - And if I put this on the other hand, which is obviously being controlled by the opposite side of my brain, if I put this on the other hand, does the learning transfer or do I have to start from scratch with the other side?

Lucy - So we're actually looking at this right now. We don't have the final data, but we are quite surprised to see it does seem to generalise very well. We've also looked at changing the controller, so moving from the toes to controlling it by the heels. And again, this learning seems to generalise. So there's something about this hand and foot coordination, people are able to learn very quickly and generalise very quickly.

Dani - With our current research that we were talking about as well, we actually found that we had a really inclusive range of people that tried the third thumb on for the first time at the Royal Society Summer Science Exhibition. And although it was only a right-handed thumb that I had, we included left-handers and right-handers and everyone had a very similar response to it.

Chris - That was why I was asking the question. Because I was thinking 90% of the population are right-handed. You've put this on my right hand. Yes. Did I find it easy because I am right-handed? If you'd given me a left-handed one, would I have struggled the same way a left-hander might if I gave them a right-handed one? Or does it just generalise?

Dani - It seems to just generalise, which <laugh> as the designer of it. I was quite surprised. I'm left-handed. So <laugh>, I use mine on my right hand. I dunno why I chose that, but it was probably so I could work on it at the same time. <laugh>.


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