Depression and Control Theory, part 1

For a while now I have been thinking about explaining depressive behaviour using control theory. I only have a rough idea for now, and it will never be a very good model, but I think it can help me explain what I go through. Given that most of my friends are scientists or engineers, it will give them better understanding than me just saying "but I can't".

However, because not all of my readers are, and to help myself along, I'm going to start by talking about control theory. This is mainly so the idea is out, rather than just bouncing in my head, and that will mean I have an extra motivation to keep thinking about it.


Everybody ready? No need for those calculators, don't worry. If you know about the basics of control theory, you can skip this, or you can keep reading and point out my glaring mistakes :)

Control theory is all about achieving a desired output. So you have a control "box" that takes an input, processes it, and spews an output. That is, however, very unexciting. The cool bit is feedback, so that, for your next input, the previous output is also an input. The output of your system plays a role. Let's say you are driving: you step on the accelerator to go faster. However, the faster you go, the harder you have to push to go any faster, because the state your output is in (your speed) also affects the input (the force on the accelerator).

Everybody still with me?

This feedback is very important, and can be of two types: positive feedback or negative feedback. Don't get lured by what positive and negative feedback mean in normal life. In this context, if applies to whether they are added or subtracted.

A positive feedback loop amplifies the effect of your input, while a negative feedback loop dampens the effect of your input to a stable state.
For example, imagine a Trivial Pursuit drinking game. If you are playing with a positive feedback mechanism, you drink every time you get a question wrong. The more you drink, the more likely you are go get a question wrong, which leads you to drink more, etc. The effects of the input stack, and you end up in hospital.
However, if you are playing with negative feedback, you drink every time you get a question right. So you'll be drinking more at the beginning, but as you drink, you get more questions wrong, which doesn't make you drink more, and thus kind of slows down your drinking rate as you get drunker. This doesn't lead to alcohol poisoning, which is a Good Thing.

Negative feedback mechanisms are what are used in control theory, because you can get your system into a stable state. A positive feedback look will get the system out of control, and we don't want that.


People that knew about Control Theory can join us again now.

My idea is that the brain functions in a negative feedback loop. You get sad for whatever reason, but after a while your brain processes that, and you go back to your normal state. However, a depressed brain has changed to be in a positive feedback loop. You get a bit sad for whatever reason and your brain amplifies that, and can't bring you back to your normal state.

Duh, I can hear you say, we *knew* that! But I don't want to just know that, I want to detail it more. I want to model it. I want to add switches to filter the happy thoughts when depressed, and how different contributions modify the control mechanism. I want to model normal brains and see how the structure of the control loop differs from that of a depressed brain.

Of course, this has no actual use beyond explaining how my brain works, and it might not even be transferable to other brains, but I like it. If I can cajole my depression into an engineering model, it will make me feel closer to understand it. If I can obsess about the modelling instead of the depression, it will distract me with it. It appeals to my engineering mind to deal with it in this way, as I can't seem to make any sense of the emotions I feel, or why.


Feel free to tell me that I'm completely insane (now it's Zoidberg's turn to say duh!). I probably won't work on this very often, or very seriously, but it is a nice little project to have, and it might help to explain depressive behaviour to other people. Suggestions, comments, yarn welcomed!