[Epistemic status: Total wild speculation]

I.

The predictive processing model offers compelling accounts of autism and schizophrenia. But Surfing Uncertainty and related sources I’ve read are pretty quiet about depression. Is there a possible PP angle here?

Chekroud (2015) has a paper trying to apply the model to depression. It’s scholarly enough, and I found it helpful in figuring out some aspects of the theory I hadn’t yet understood, but it’s pretty unambitious. The overall thesis is something like “Predictive processing says high-level beliefs shape our low-level perceptions and actions, so maybe depressed people have some high-level depressing beliefs.” Don’t get me wrong, CBT orthodoxy is great and has cured millions of patients – but in the end, this is just CBT orthodoxy with a neat new coat of Bayesian paint.

There’s something more interesting in Section 7.10 of Surfing Uncertainty, “Escape From The Darkened Room”. It asks: if the brain works to minimize prediction error, isn’t its best strategy to sit in a dark room and do nothing forever? After all, then it can predict its sense-data pretty much perfectly – it’ll always just stay “darkened room”.

Section 7.10 gives a kind of hand-wave-y answer here, saying that of course organisms have some drives, and probably it makes sense for them to desire novelty and explore new options, and so on. Overall this isn’t too different from PCT’s idea of “intrinsic error”, and as long as we remember that it’s not really predicting anything in particular it seems like a fair response.

But I notice that this whole “sit in a dark room and never leave” thing sounds a lot like what depressed people say they wish they could do (and how the most severe cases of depression actually end up). Might there be a connection? Either a decrease in the mysterious intrinsic-error-style factors that counterbalance the dark room scenario, or an increase in the salience of prediction error that makes failures less tolerable?

(also, there’s one way to end all prediction error forever, and it’s something depressed people think about a lot)

II.

Corlett, Fritch, and Fletcher claim that an amphetamine-induced mania-like state may involve pathologically high confidence in neural predictions. I don’t remember if they took the obvious next step and claimed that depression was the opposite, but that sounds like another fruitful avenue to explore. So: what if depression is pathologically low confidence in neural predictions?

Chekroud’s theory of depression as high-level-depressing-beliefs bothers me because there are so many features of depression that aren’t cognitive or emotional or related to any of these higher-level functions at all. Depressed people move more slowly, in a characteristic pattern called “psychomotor retardation”. They display perceptual abnormalities. They’re more likely to get sick. There are lots of results like this.

Depression has to be about something more than just beliefs; it has to be something fundamental to the nervous system. And low confidence in neural predictions would do it. Since neural predictions are the basic unit of thought, encoding not just perception but also motivation, reward, and even movement – globally low confidence levels would have devastating effects on a whole host of processes.

Perceptually, they would make sense-data look less clear and distinct. Depressed people describe the world as gray, washed-out, losing its contrast. This is not metaphorical. You can do psychophysical studies on color perception in depressed people, you can stick electrodes on their eyeballs, and all of this will tell you that depressed people literally see the world in washed-out shades of gray. Descriptions of their sensory experience sound intuitively like the sensory experience you would get if all your sense organs were underconfident in their judgments.

Mechanically, they would make motor movements less forceful. Remember, in PP movements are “active inferences” – the body predicts that the limb it wants to move is somewhere else, then counts on the motor system’s drive toward minimizing prediction error to do the rest. If you predictions are underconfident, your movements are insufficiently forceful and you get the psychomotor retardation that all the pathologists describe in depressed people. And what’s the closest analog to depressive psychomotor retardation? Parkinsonian bradyphrenia. What causes Parkinsonian bradyphrenia? We know the answer to this one – insufficient dopamine, where dopamine is known to encode the confidence level of motor predictions.

Motivationally – well, I’m less certain, I still haven’t found a good predictive processing account of motivation I understand on an intuitive level. But if we draw the analogy to perceptual control theory, some motivations (like hunger) are probably a kind of “intrinsic error” that can be modeled as higher-level processes feeding reference points to lower-level control systems. If we imagine the processes predicting eg hunger, then predicting with low confidence sure sounds like the sort of thing where you should be less hungry. If they’re predicting “you should get out of bed”, then predicting that with low confidence sure sounds like the sort of thing where you don’t feel a lot of motivation to get out of bed.

I’m hesitant to take “low self-confidence” as a gimme – it seems relying too much on a trick of the English language. But I think there really is a connection. Suppose that you’re taking a higher-level math class and you’re really bad at it. No matter how hard you study, you always find the material a bit confusing and are unsure whether you’re applying the concepts correctly. Eventually you start feeling kind of like a loser, you decide the math class isn’t for you, and you move on to something else where you’re more talented. Your low confidence in your beliefs (eg answers to test questions) and actions (eg problem-solving strategies) create general low self-confidence and feelings of worthlessness. Eventually you decide math isn’t for you and decide to drop the class.

If you have global low confidence, the world feels like a math class you don’t understand that you can’t escape from. This feeling might be totally false – you might be getting everything right – but you still feel that way. And there’s no equivalent to dropping out of the math class – except committing suicide, which is how far too many depressed people end up.

One complicating factor – how do we explain depressed people’s frequent certainty that they’ll fail? A proper Bayesian, barred from having confident beliefs about anything, will be maximally uncertain about whether she’ll fail or succeed – but some depressed people have really strong opinions on this issue. I’m not really sure about this, and admit it’s a point against this theory. I can only appeal to the math class example again – if there was a math class where I just had no confidence about anything I thought or said, I would probably be pretty sure I’d fail there too.

(just so I’m not totally just-so-storying here, here’s a study of depressed people’s probability calibration, which shows that – yup – they’re underconfident!)

This could tie into the “increased salience of prediction error” theory in Part I. If for some reason the brain became “overly conservative” – if it assigned very high cost to a failed prediction relative to the benefit of a successful prediction – then it would naturally lower its confidence levels in everything, the same way a very conservative better who can’t stand losing money is going to make smaller bets.

III.

But why would low confidence cause sadness?

Well, what, really, is emotion?

Imagine the world’s most successful entrepreneur. Every company they found becomes a multibillion-dollar success. Every stock they pick shoots up and never stops. Heck, even their personal life is like this. Every vacation they take ends out picture-perfect and creates memories that last a lifetime; every date they go on leads to passionate soul-burning love that never ends badly.

And imagine your job is to advise this entrepreneur. The only advice worth giving would be “do more stuff”. Clearly all the stuff they’re doing works, so aim higher, work harder, run for President. Another way of saying this is “be more self-confident” – if they’re doubting whether or not to start a new project, remind them that 100% of the things they’ve ever done have been successful, odds are pretty good this new one will too, and they should stop wasting their time second-guessing themselves.

Now imagine the world’s most unsuccessful entrepreneur. Every company they make flounders and dies. Every stock they pick crashes the next day. Their vacations always get rained-out, their dates always end up with the other person leaving halfway through and sticking them with the bill.

What if your job is advising this guy? If they’re thinking of starting a new company, your advice is “Be really careful – you should know it’ll probably go badly”. If they’re thinking of going on a date, you should warn them against it unless they’re really sure. A good global suggestion might be to aim lower, go for low-risk-low-reward steady payoffs, and wait on anything risky until they’ve figured themselves out a little bit more.

Corlett, Frith and Fletcher linked mania to increased confidence. But mania looks a lot like being happy. And you’re happy when you succeed a lot. And when you succeed a lot, maybe having increased confidence is the way to go. If happiness were a sort of global filter that affected all your thought processes and said “These are good times, you should press really hard to exploit your apparent excellence and not worry too much about risk”, that would be pretty evolutionarily useful. Likewise, if sadness were a way of saying “Things are going pretty badly, maybe be less confidence and don’t start any new projects”, that would be useful too.

Depression isn’t normal sadness. But if normal sadness lowers neural confidence a little, maybe depression is the pathological result of biological processes that lower neural confidence. To give a total fake example which I’m not saying is what actually happens, if you run out of whatever neurotransmitter you use to signal high confidence, that would give you permanent pathological low confidence and might look like depression.

One problem with this theory is the time course. Sure, if you’re eternally successful, you should raise your confidence. But eternally successful people are rarely eternally happy. If we’re thinking of happiness-as-felt-emotion,itt seems more like they’re happy for a few hours after they win an award or make their first million or whatever, then go back down to baseline. I’m not sure it makes sense to start lots of new projects in the hour after you win an award.

One way of resolving this: maybe happiness is the derivative of neural confidence? It’s the feeling of your confidence levels increasing, the same way acceleration is the feeling of your speed increasing?

Of course, that’s three layers of crackpot – its own layer, under the layer of emotions as confidence level, under the layer of depression as change in prediction strategies. Maybe I should dial back my own confidence levels and stop there.