Nov 06 2020

How the Brain Predicts Outcomes

This is a really interesting study trying to work out one of the brain regions involved in decision-making. The researchers are studying mice, and using a technique know as calcium imaging and optogenetics to view the activity of brain cells in a living animal in real time. The short version is that a brain region known as the anterior cingulate cortex (ACC) is necessary for model-based decision making. But let’s back up a bit and talk about decision making.

There appears to be two basic ways that vertebrates make decisions – model-based decision making and model-free decision making. The former involves creating an internal model of what is likely to happen in the world as a consequence of our specific actions. This approach also updates that model based upon experience. For example you may have a mental model of what is likely to happen if you slap people without provocation. If you ever decided to actually slap someone, their reaction would be used to update your model and inform future model-based decisions.

Model-based decision making is very effective, as it can take into consideration many variables and constantly updates itself with real-world experience. But this approach is also very labor-intensive. One of the basic principles of neuroscience is that brains are lazy, meaning that they tend to choose the pathway of least energy expenditure to get to a desired outcome. What this means on a neurological level is that the brain is generally wired with mechanisms to reduce energy expenditure, by which I mean thinking. As we learn, pathways form that make actions, thoughts, and behaviors more automatic. This often literally means laying down subcortical pathways that can carry out the procedure automatically without having to engage higher cortical processing, which is very energy intensive. For example, when you drive to work, which you have done hundreds or thousands of time, you will likely be on “automatic pilot”. You don’t have to think about your route, or when to turn, you just drive there from memory. It’s so easy, your brain can engage in other tasks, to the point that you may not even remember the drive.

This more automatic behavior involves model-free decision making, which does not involve calculating possible outcomes, but rather relies on simple learned rules. For example, you may simply know that it is wrong to hit other people without reason, and so you don’t have to make a specific model-based decision in each case. You know not to touch a hot stove, or pee in public, or a thousand other things that are now just baked into your memory of appropriate social behavior or basic survival skills.

This is all part of the “thinking, fast and slow” concept that Kahneman wrote about. Formally this is know as system 1 and system 2 thinking, or the intuitive automatic thinking vs detailed analytical thinking. As an aside, I need to dispel this notion that most people are dominantly intuitive or analytical (like the Myers Briggs nonsense). Rather, most people are a mix of both. These systems are not in competition, they generally work in parallel. Thinking fast is optimized for efficiency and speed, while thinking slow is optimized for complexity and flexibility. We need both to function well.

Model-based decision making is basically analytical, while model-free is similar to intuitive thinking. The model-free is not exactly or entirely intuitive – intuitive is more innate, while model-free decision making is learned, but it is learned simple rules that then become part of our intuition.

OK – back to the mice in this study. This is what the researchers did:

In this task, an animal first chooses one of two centrally located holes to poke its nose into. This action activates one of two other holes to the side, each of which has a certain probability of providing a drink of water.

“Just like in real life, the subject has to perform extended sequences of actions, with uncertain consequences, in order to obtain desired outcomes,” said Dr. Akam.

To do the task well, the mice had to figure out two key variables. The first was which hole on the side was more likely to provide a drink of water. The second was which of the holes in the center activated that side hole. Once the mice learned the task, they would opt for the action sequence that offered the best outcome. However, in addition to this model-based way of solving the puzzle, mice could also learn simple model-free predictions, e.g. “top is good,” based on which choice had generally led to reward in the past.

This paradigm successfully “disambiguated” model-based and model-free decision making by changing up the rules. The mice first use model-based decision making to learn the rules, then when the rules are stable, the mice rely more on model-free decision making. But then when the rules change they need to shift back to model-based in order to update their model. Because they could use this setup to distinguish model-based from model-free decision making, the researcher could look at brain activity to see if the area of interest, the ACC, was involved in either. They found that the ACC was active when using model-based decision making and when updating the model with new information.

They could also “turn off” the ACC in the mice and they lost their ability to update their model when the rules changed. This essentially demonstrates that ACC activity not only correlates with model-based decision making, it is necessary for it.¬†As is often the case, this is one piece of a complex puzzle, but it appears to be a crucial piece. Little by little we are building a more detailed model of how the brain works.

I also feel obligated to point out that research like this completely destroys any notion of dualism – that mental function exists somehow outside of or separate from the biological functioning of the brain. So far, the “neuroscience” hypothesis, that mental function is brain function, is working quite well. The brain is a complex biological computer, and we can figure out how it works by studying it. Even the most sophisticated cognitive processes, such as analytical decision-making, are demonstrably happening in the brain. Further, not only is there zero evidence for the dualist hypothesis, it is completely unnecessary, which is a fate in science even worse than being wrong.

No responses yet