In the brain: the hardware is the software.

A few months ago, I made the somewhat controversial assertion that depression is not a chemical imbalance. In that post, I argued that depression is the result of dysfunctional neural circuitry. I’d like to expand a little more on that idea. (Warning, it’s going to get a little technical here, so continue at your own risk).

The brain is an incredibly powerful parallel processing machine. Each of the estimated 86 billion neurons is able to take multiple inputs and produce a stream of discrete signals that are relayed to connected cells. The sheer processing power of the brain is immense. However, like a microchip, the brain’s full speed is limited to a small set of hardware-encoded functions. For example, your brain can process high-definition stereoscopic video in real-time, while identifying faces and assigning emotions to each one, but it does not have an inbuilt method to perform simple addition*. The brain is incredibly powerful for the tasks it has evolved to do, while at the same time, it can appear remarkably limited compared to even the most basic computers.

Computers perform tasks described by their software. I see no reason to believe that the brain is any different in this regard. However, the brain does not have a separate storage unit like a hard disc drive that retains its software in non-volatile memory. The theoretical location where data is stored is called an engram**. The best evidence we have suggests that engrams are stored in neural circuits close to the brain regions that process them.

In other words, the software and the hardware are inseparable.

So when I assert that the cause of depression (and realistically, all psychiatric disorders) is dysfunctional neural circuitry, I am not throwing away every hypothesis on the subject. I am merely suggesting that the core problem–be it disruptive thoughts, relationship woes, past trauma, environment, diet, exercise, genetics, or even neurotransmitters–is manifest at the level of the circuit. The circuit doesn’t tell us the whole story, but it gives us a great model for how we try to identify the culprit***.

So I stand by my assertion that depression is caused by dysfunctional brain circuitry. But although this is a useful abstraction, it does little to define appropriate clinical targets. I just think that restructuring of neural circuits is much more accurate than any of the other high-level models of brain pathology that I’ve heard described.

* Thankfully, some of these brain functions can also performed at very high level of abstraction. But like high level, uncompiled programming languages, these functions will often be hit by a pretty hefty speed limit. Sadly, your arithmetic skills will always be a lot slower than JavaScript running in Internet Explorer 6 on your grandma’s old desktop.

** If you Google engram, you are likely to get lost on Scientology sites as they also believe in something called engrams. To my knowledge, this is a very different concept.

*** I doubt we’ll find a common circuit related to the Oedipus complex :).

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