Left or right? Apple or orange? Selma or Birdman? One way to make these decisions is precisely what intuition tell us it should be: we weigh up the pros and cons of each choice. Then, when we have sufficient evidence for one over the other then we go ahead and make that choice.
How this is represented in the brain is quite straightforward: the firing of neurons would go up or down as evidence for one choice or another becomes clear and, when the firing had reached some fixed threshold, when the neurons had fired enough, a decision would be made.
The difficulty has been in figuring out precisely where the information is being encoded; in determining which neurons were increasing their activity in line with the evidence. In fact, multiple regions seem to be participating in the process.
So let us say that you are a little rodent who hears sound from the left and the right; little click click clicks. And you need to decide which side the most clicks are coming from. Every click on one side gives you a smidgen of evidence that that side will have the most, while a click on the other side will make it less likely. You don’t know when the clicks will end – so you have to stay ready.
Now there are two interesting areas of the brain that we can look at: the frontal orienting fields (FOF) that probably guide how you will orient your little snout (to the left? to the right?), and the posterior parietal cortex (PPC), which integrates diverse information from throughout the brain. Here is what the activity of these neurons look like if you plot how fast the neurons are firing, separated out by ‘accumulated value’ (how much evidence you have for one side or another; I will refer to this as left or right but is actually more like ipsilateral or contralateral):
It looks like PPC, the cortical integrator, fires progressively faster the more evidence the animal has to go left, and progressively slower the more evidence it has to go right. In other words, it is exactly the evidence accumulator we had been hoping for. The orienting region (FOF) has a different pattern, though. Its firing is separated into two clusters: low if there is a lot of evidence to go left, and high if there is a lot of evidence to go right. In other words, it is prepared to make a decision any second, like a spring ready to be released.
It is interesting that this is implemented by sharpening how tightly tuned neurons in each region are for the decision, going from something like a linear response to something more like a step function:
This is consistent with an idea from Anne Churchland’s lab that the PPC is integrating information from diverse sources to provide evidence for many different decisions. This information can then be easily ‘read out’ by drawing a straight line to separate the left from the right, a task that is trivial for a nervous system to accomplish in one step – say, from PPC to FOF.
And yet – there are mysteries. You could test the idea that FOF provides the information for left or right by removing it or just silencing it. If it was standing ready to make a decision, you would only care about the most recent neural activity. Indeed, ablating the region or just silencing it for a couple hundred milliseconds has the same effect of biasing the decision to the left or right. But it is only a bias – the information for the decision is still in the system somewhere!
Even more baffling is that the FOF begins to respond about 100 milliseconds after hearing a click – but PPC doesn’t start responding until 200 millseconds after a click. So how is FOF getting the information? Is FOF actually sending the information to PPC?
Decisions are hard. It is not a “step 1. hear information, step 2. add up pro/cons, step 3. make decision” kind of process. A linear 1, 2, 3 would be too simple for the real world. There are many different areas of the brain getting information, processing it and adding their own unique twist, sending their evidence to other areas, and processing it again. Again: even simple decisions are hard.
Hanks, T., Kopec, C., Brunton, B., Duan, C., Erlich, J., & Brody, C. (2015). Distinct relationships of parietal and prefrontal cortices to evidence accumulation Nature DOI: 10.1038/nature14066
Brunton, B., Botvinick, M., & Brody, C. (2013). Rats and Humans Can Optimally Accumulate Evidence for Decision-Making Science, 340 (6128), 95-98 DOI: 10.1126/science.1233912