Video transcript: The quick and the dead: evidence that movement is swiftest in response to events in the environment


February 2010

Dr Andrew Welchman, University of Birmingham
I guess we are all familiar with Hollywood portrayals of gun fights in the Wild West? What we see there is that it is normally the good guy who is responding to the bad guy pulling their gun and shooting at them who wins the dual. And this idea that the response to what you see in the environment might be quicker to execute is really what was the seed that lay at the heart of this piece of research. So we wanted to go beyond that just quirky idea of a difference between reactive movements being faster than decisions to move made by themselves and see whether this might actually map on to what different parts of the brain might be doing to control those different types of actions.

So to try and test this idea about the difference between movements that we choose to make and movements that we make in response to something else that is happening, we designed an experiment where we got 2 people in the lab performing a virtual gun fight, a rather simple one that involved pressing 3 buttons. We have 2 people facing each other and they compete to make this movement as quickly as possible. On some trials 1 of the individuals would start off the movement sequence and their opponent would then react to them making a movement and then on other trials that same individual would not be making the decision to move but would rather be reacting to it. 

So what we found was that when people reacted to their opponent they made the movement about 20 milliseconds faster than if they had been making that movement as an intention movement - so reaction was quicker by about 20 milliseconds.  So that really suggests that there might be an advantage in responding to what is happening in the environment in that it makes you a little bit quicker. Although you are 20 milliseconds faster in making the movement there is obviously a delay between seeing what the other person is doing and then responding to that and that is in the region of 200 milliseconds. So the system has probably evolved as a useful means of getting out of the way of things. So if there is a bus coming towards us and want to get out of the way as fast as possible and this ability to make quick, fast movements is probably what we are tapping into with this experiment.

So having seen this difference in movement times the obvious question is “what is happening in the brain that is responsible for this?” Our experiment hasn’t tested that directly but 1 speculative idea is that there are different routes to the production of these different types of movement. One idea is that when we are reacting to things around us in the world then information comes in through the eyes, gets sent to the back of the brain and then from the back of the brain up towards the areas that control our movements.  In contrast to that when we make a decision to move the decision areas at the front of the brain get sent back towards the motor areas of the brain. And the key idea is that effectively the brakes get taken off faster when we make a reactive movements, so we can get moving faster than when we make an intention movements, and that would be responsible for us being 20 milliseconds quicker when we make a response to our opponent.