What kind of chemical imbalance does the brain have when an individual suffers from severe depression?
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There’s no simple or established answer for this. We see some correlations between _various_ levels of _various_ neurotransmitters — sometimes there’s too much, sometimes too little. Basically, it can be a lot of things.
It’s important to note that, again, these are _correlations_, not the _cause_ of any mental disorder. We still don’t know the causes for many of them, including depression.
“Chemical imbalance” is a waaaaaaaaaaay simplified layman-accessible explanation for what’s going on. Our understanding of the brain’s chemistry is not good enough to trace most mental conditions to their physical/chemical causes except very speculatively.
VERY roughly speaking, the brain has two chemicals, dopamine or serotonin, that can be thought of as the ways the brain “encodes” reward and goals in its chemistry.
A release of dopamine, roughly, signals the brain that the current outcome is providing some important reward (or punishment). Eating, sex, and other simple pleasurable activities trigger the dopamine system. They tell the brain “okay, we got food or sex, remember how we got here so we can get here again”. The brain releases dopamine again when it anticipates one of these “remembered” rewards.
Serotonin, on the other hand, is what helps you actually act. It allows you to *pursue* the rewards that dopamine is supposed to have caused you to remember.
You can think of the combination as being sort of like a lazy dog. The dog knows “walk” means “good happy thing” (dopamine), and so it’ll get up and run to the door to go for one (serotonin). Again, I stress that this is **very very rough as an analogy and it is not as simple as “oh get more of X”**.
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Depression seems to involve both. Specifically:
* Depressed people have trouble enjoying or anticipating reward.
* Depressed people have more trouble initiating actions or pursuing goals towards those rewards.
But depression is also a thought disorder, so it includes all of the loops that involve these things. So, for example:
1. A depressed person has trouble enjoying going out. (Dopamine malfunction.)
2. Not enjoying going out means they’re more likely not to go out. (Behavioral.)
3. Not going out means they don’t get good social interaction. (Actual lack of reward.)
4. Lack of reward because they didn’t go out stops them from getting any further training towards that reward. (No dopamine training to increase the apparent reward.)
5. No training means higher levels of activity needed to go do it, i.e., more serotonin activity. (Normal brain function, but trained on less positive stimuli.)
6. …but they don’t have it because their serotonin system is also acting up. (Serotonin malfunction.)
7. Because they struggle to go out, they *don’t*, but they know they’re lonely. So they start blaming themselves for that. (Cognitive.)
8. That cognitive self-abuse further reduces the reward (‘why would I go out? i’ll just sit in the corner anyway’)
And so on. The brain is a complicated system capable of a lot of feedback loops, and such systems are very hard to model using only a few simple values.
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