Making decisions is something we do continuously throughout the day. But what's going on neurologically while this is happening? There are two theories: one says that we weigh up all of the possible options, pick the best one and then make a motor plan to achieve the desired outcome. In contrast, the other theory posits that the decision is part of the motor process itself and that different neuronal assemblages representing the possible outcomes compete with each other until one wins out and becomes the decision we make. Unfortunately, there's data supporting both theories! And they can't both be right. But what no one had done before was to test both theories at the same time, to see what the real pecking order is. And this is what Veit Stuphorn set out to do...
Veit - So what we tried to do is set up an experiment in which information that would allow you to make a selection based on the outcomes and the selection of a motor process is available simultaneously. The idea that we had is so that we then can see whether there's one kind of selection process that leads the other. We used these macaque monkeys. We trained them to do the following - so they looked into a computer screen and on the computer screen, there were images basically. These images indicated to the animals both possible amounts that they could get at two different ones, and the probability for each of the two amounts. So basically there were gambles. For each trial in the experiment, there were two different gambles and from trial to trial, the monkey couldn’t really predict which pair of gambles hewould have to make a decision between.
Chris - So, he’s choosing whether he wants to take a big risk and have a big reward or a smaller risk and a degree of certainty. So you're stressing the system but in a different way each time so it’s not just a pre-planned motor action each time.
Veit - That’s right. So this is sort of the option part and the action part is basically, we could record to which part of the screen the monkey was looking. By looking at either one or the other of these visual targets, the monkey indicated which one he wanted. He presented this, let’s say, the same pair of gambles, we presented in different positions on the screen so that the monkey had to make different eye movements to indicate the same choice. In that way, across many different trials, we could differentiate between activity in the brain that was related to the motor response that the monkey chose or the option that he chose.
Chris - Which one seems to hold the key to what the monkey’s ultimate choice is?
Veit - So, the main finding in our study was that these neurons indicated both the options that he selected and the action that he chose, but there was a sequence between them. So early in the trial, the neuron showed us which of the two options the monkey preferred, which one he would choose – independent of the eye movement he made on that particular trial. And then sometime later, that activity changed and started to show the eye movement – the action that the monkey had to do in order to indicate this choice. So this first showed the selection of an option and then a little bit later – about 100 milliseconds later – selection of particular eye movement.
Chris - Which population of nerve cells was it specifically that you were recording from to get this data?
Veit - So we recorded in an area that is called the supplementary eye field. It’s an area in the middle frontal part of the primate brain and humans have also such an area. It has been known for a long time that it’s somehow responsible or involved in the control of eye movements. That’s why we chose it because we had selected eye movements as the type of movement of the monkey to do in order to indicate his choice.
Chris - That’s a visual stimulus though. So, what would’ve happened say, they had to feel something? It wouldn’t obviously involve those same pre-motor areas but on supplementary motor areas corresponding to visual movements because they… or would it? I mean, would they do it with their eyes closed and still perform the same way?
Veit - Yes, whether or not the supplementary eye field specifically would be involved in some other form of decision making that’s strictly speaking, unclear because that experiment hasn’t been done and actually should be tested. The immediate neighbouring regions to the supplementary eye field are other regions that are involved in the control of arm movements or for example in humans, they are known to involve in speech – control of the tongue, and the lips and support. There's an entire map of all the different parts of the body in this region and you can think of a particular region that we recorded as the subpart related to the control of eye movements. And so, you could imagine that we would have found very similar findings if we had recorded in this neighbouring region. But let’s say, train the animal to indicate his choice by making an arm movement which is going to the left or to the right and pressing a particular button to indicate his choice.
Chris - Where does this lead us then? How does this leave the field now? We started this interview with, there are two models. One of them is based on sizing things up first. The other is basically, you keep all these things in parallel in your head at once and then make a decision. Where do we now stand based on what you found?
Veit - We suggest a different model in our paper. According to our new model, every decision that you make really involves two different steps. One step would be to select the option and another step is to select the action that is most likely to bring that option about. So the idea would be that two different parts of the brain are responsible for the first selection step and another network would be responsible for the selection of the action.