Cortical plasticity: how brains adapt
You can't teach an old dog new tricks, or so the dogma goes, and scientists say that this is because the brain becomes far less adaptable or plastic as we become older. But is this really true? Oxford University's Tamar Makin has been studying people lacking limbs to try to find out...
Tamar - Following limb loss, the major part of the brain that normally receives inputs and outputs to the hand is now deprived, and potentially, this could be very useful for people because perhaps we could tap into this deprived brain area and train it to support behaviour for other body parts, for example. And the question was whether that is possible and whether that opportunity for the brain to change would depend at the age at which individuals have lost their hands.
Chris - So, how did you do this? Who were you studying?
Tamar - We recruited several groups of individuals without a hand. One group is people that were just born without a hand, it was never fully developed, we call them congenitals. The other group is individuals that used to have a hand but it was chopped off due to traumatic circumstances, and these would be the acquired amputees. We had about 15 to 20 individuals in each group. On average, our participants were about 45 years old, but some of them were quite young, some of them were in their 20s, and others were in their late 50s.
Chris - So, good age range there.
Tamar - Yeah.
Chris - How did you actually study them?
Tamar - What we did is we put them in an MRI scanner and asked them to perform simple movements with different body parts. And using functional MRI, we were able to identify the brain areas that become active when they perform these range of movements.
Chris - And what do you see?
Tamar - We see that this deprived cortex specifically becomes activated either by the intact hand, that is the hand that hasn't been lost in the amputation and is still functioning, or the stump, so this is the residual arm or whatever was left from the arm after the amputation. And we see that the extent to which the deprived cortex becomes activated by movement depends not on the age in which amputees have lost their hand but rather to what extent they tend to use it in daily lives.
Chris - So, the more that they transfer the role of the lost hand to the intact hand, for example, then the greater the switch of allegiance of the patch of brain that was supplying their lost hand over to the now remaining hand then.
Tamar - Exactly.
Chris - Does that differ from what we thought might be going on before?
Tamar - Yes, in several ways. The important finding that we found in this study was that the deprived cortex is quite willingly being taken over. The intact hand is not a cortical neighbour of the missing hand. In fact, it's on the other side of the brain. And this is a very striking example of brain plasticity - of one brain area being reassigned or taken over to do something new. And the fact that this was more common in the acquired amputees, suggests that there's no upper limit in terms of age for plasticity, so that the brain is capable of changing even though people are quite adult and have had experience with that brain area and a particular body part for quite an extensive time.
Chris - How do you know - just playing devil's advocate for a minute - that the enhanced activation on the part of the brain corresponding to the lost hand isn't the reason they use the remaining body part more versus they use that body part more so that part of the brain develops to correspond to the movements in that body part more?
Tamar - So, this is actually a very good point because when we're using these tools, neuro-imaging, we can't pinpoint and say that it is the brain activation that is formed due to intense usage or rather individuals that have more plastic brains would therefore choose to use their intact hand more to make this inference, we actually have to follow people from the very first stage of losing their hand.
Chris - But, your inclination is that it is the use that begets the brain change, rather than the brain being capable of changing more which makes them use the surviving limb part more.
Tamar - Yes, and we also examined brain patterns of controlled participants. So, these are individuals with two hands and we were trying to see whether in just people that haven't lost their hand, activation in the hand area could be modulated by movement of the other hand. Let's say the dominant hand, when you look at the brain area of the non-dominant hand, and they all tended to show no activation when they were moving one hand in the cortex of the other hand. So, this leaves us to think that it is actually the behaviour that drives the plasticity and not the other way around.
Chris - Given what you found - what are the implications clinically for how we should rehabilitate people who are amputees or how we should help the people who are born with congenital limb loss?
Tamar - So, in terms of rehabilitation, we've always thought that there's a period in which the brain is more sensitive, is more capable of making changes, and that's the childhood. What our studies shows is that individuals have an opportunity to switch the way the brain is wired, and the way the brain is responding at every age. So, it doesn't matter if they lost their hand as children or were born without a hand, or lose it as adults, they should be able to use this deprived cortex to underlie adaptive behaviour.