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when someone is clinically depressed, their symptoms not only distort reality in a way, but their brain neurons and receptors are thrown off in some way. this ultimately leads to a decrease in some cognitive abilities

Can those cognitive abilities return? And what or moments were they return but only for a short period of time?

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this is such a difficult question to answer. some research suggest that it’s possible for permanent damage to be made from depression, while other’s suggest it is not permanent. this differs from mania, the opposite of depression, where we know that permanent damage is made with each manic episode

Part of the pathology of depression is not just in the connectivity and function of the ‘emotional’ parts of the brain, but also in regions responsible for the basic cognitive functions involved in reading. E.g. the prefrontal cortex (PFC) is implicated in depression because of lacking regulation of the amygdala (which at this eli5 level is responsible for mood). But, that dysfunction of the PFC is also responsible for impairments in things like decision making, thinking, or, in your question, reading.

Depression, in many ways, is a situation in which the brain becomes untethered from its positive reward systems. It's not just absence of happiness or the presence of unhappiness, it's largely a loss of the desire be happy.

In this situation, the brain function in "low power mode". How much effort your brain expends on processing information is proportional to a variety of complex brain systems that measure how worthwhile what you're doing is. Doing something worthwhile results in positive reward.

No positive reward systems, no ability to detect the worthwhilness of activities, no proportional increase in effort expended.

Turns out, remembering, reading, understanding, doing math, general normal function, requires a lot of effort. If you're sufficiently depressed, it can be more ffort than the brain is calibrated to expend.

Keep in mind, there is a lot more to depression than I explained. For example, the serotonin system is just one of many system that calibrates threat sensitivity, mood stability, and as a result partially mediates systems like aggression and exploratory behavior. But that's not the whole picture. The important thing to understand is that the brain more or less shuts down

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