Guyenet on motivation
Rereading The Hungry Brain, I notice my review missed one of my favorite parts: the description of the motivational system. It starts with studies of lampreys, horrible little primitive parasitic fish:
How does the lamprey decide what to do? Within the lamprey basal ganglia lies a key structure called the striatum, which is the portion of the basal ganglia that receives most of the incoming signals from other parts of the brain. The striatum receives “bids” from other brain regions, each of which represents a specific action. A little piece of the lamprey’s brain is whispering “mate” to the striatum, while another piece is shouting “flee the predator” and so on. It would be a very bad idea for these movements to occur simultaneously – because a lamprey can’t do all of them at the same time – so to prevent simultaneous activation of many different movements, all these regions are held in check by powerful inhibitory connections from the basal ganglia. This means that the basal ganglia keep all behaviors in “off” mode by default. Only once a specific action’s bid has been selected do the basal ganglia turn off this inhibitory control, allowing the behavior to occur. You can think of the basal ganglia as a bouncer that chooses which behavior gets access to the muscles and turns away the rest. This fulfills the first key property of a selector: it must be able to pick one option and allow it access to the muscles.
Many of these action bids originate from a region of the lamprey brain called the pallium…
All of this is standard neuroscience, but presented much better than the standard neuroscience books present it, so much so that it brings some important questions into sharper relief. Like: what does this have to do with willpower?
Guyenet describes high dopamine levels in the striatum as “increasing the likelihood of engaging in any behavior”. But that’s not really fair – outside a hospital, almost nobody just sits motionless in the middle of a room and does no behaviors. The relevant distinction isn’t between engaging in behavior vs. not doing so. It’s between low-effort behaviors like watching TV, and high-effort behaviors like writing a term paper. We know that this has to be related to the same dopamine system Guyenet’s talking about, because Adderall (which increases dopamine in the relevant areas) makes it much easier to do the high-effort behaviors. So a better description might be “high dopamine levels in the striatum increase the likelihood of engaging in high-willpower-requirement behaviors”.
But what is high willpower requirements? I’m always tempted to answer this with some sort of appeal to basic calorie expenditure, but taking a walk requires less willpower than writing a term paper even though the walk probably burns way more calories. My “watch TV” option generator, my “take a walk” option generator, and my “write a term paper” option generator are all putting in bids to my striatum – and for some reason, high dopamine levels privilege the “write a term paper” option and low dopamine levels privilege the others. Why?
I don’t know, and I think it’s the most interesting next question in the study of these kinds of systems.
But here’s a crazy idea (read: the first thing I thought of after thirty seconds). In the predictive processing model, dopamine represents confidence levels. Suppose there’s a high prior on taking a walk being a reasonable plan. Maybe this is for evo psych reasons (there was lots of walking in the ancestral environment), or for reinforcement related reasons (you enjoy walking, and your brain has learned to predict it will make you happy). And there’s a low prior on writing a term paper being a reasonable plan. Again, it’s not the sort of thing that happened much in the ancestral environment, and plausibly every previous time you’ve done it, you’ve hated it.
In this case, confidence in your new evidence (as opposed to your priors) is a pretty important variable. If your cortex makes its claims with high confidence (ie in a high-dopaminergic state), then its claim that it’s a good idea to write a term paper now may be so convincing that it’s able to overcome the high prior against this being true. If your cortex makes claims with low confidence, then it will tentatively suggest that maybe we should write a term paper now – but the striatum will remain unconvinced due to the inherent implausibility of the idea.
In this case, sitting in a dark room doing nothing is just an action plan with a very high prior; you need at least a tiny bit of confidence in your planning ability to shift to anything else.
I mentioned in Toward A Predictive Theory Of Depression that I didn’t understand the motivational system well enough to be able to explain why systematic underconfidence in neural predictions would make people less motivated. I think the idea of evolutionarily-primitive and heavily-reinforced actions as a prior – which logical judgments from the cortex have to “override” in order to produce more willpower-intensive actions – fills in this gap and provides another line of evidence for the theory.