According to a study using macaques, planned and immediate movements have distinct patterns in the brain before the movement is initialised. We asked Dr Benjamin Dann of the German Primate Center explains how it might help humans.
What’s the difference between planned movements and immediate reactions?
One example is when you’re waiting for a green light, preparing to press the gas pedal of your car, but if a child runs onto the street, you have to brake. Both are the same movement of putting your foot down, but in one case you have to react as quickly as possible, in the other you have time to prepare.
How did you compare the two movements?
We used the example of grasp movement. This study was done with macaque monkeys, which are the best model for humans. We trained two monkeys to grasp a handle in front of them with either a precision grip, like when you pick up a biscuit, or a whole-hand or power grip, like when holding a tennis racquet. On a monitor, we gave a visual cue which told the monkey how to grasp. The visual cues were circles: grey for precision grip, green for power grip. There was also a red circle for telling the monkey not to grip – they were only allowed to execute the grip once the red circle had disappeared. The researchers showed those circles at various 0.1-second intervals from 0 to 1.3 seconds. In the brain, the first decision was to choose the type of grip – power or precision – then wait for the visual cue [the red circle disappearing] to indicate the start of the movement.
And how did you measure brain activity?
We first implanted microelectrodes inside the skull. Unfortunately any non-invasive method is not suitable yet as the signal is too blurry. We were able to not just record individual neurons [brain cells], but hundreds of them – the whole network, actually – everything that’s participated in the planning, as well as in control of the movement itself.
How does the brain activity differ?
The movement itself is identically coded in the relevant motor areas of the cortex [the brain’s folded outer layers]. However, the population of neurons involved in this process has a completely different activity pattern before the movement is actually initialised. This distinct activity pattern within the brain, or ‘state’, is only present for working memory – short-term memory used while planning movements – and not when you react right away. All of a sudden this extra state appears when the minimum preparation time, the reaction time, is half a second.
Could identifying two brain states help in medicine?
This is basic research and we never know for sure if this will actually lead to a clinical application. It is a long-term goal. This could potentially be used for rehab of patients after stroke or tumour surgery: we can say, “We know that these two states have to exist in a healthy subject, now they’re both present again, the patient has re-learned the proper ability.” Or even a step further, knowing the configuration for a certain state and the errors involved, to try and reactivate that by artificial electric stimulation. Another potential application is prosthetics and how to control a robot limb by brain activity. Knowing that movement is a distinct state helps a lot because you want the arm to only move when the patient wants it to move.