One of the most unsettling symptoms of depression is how it affects your memory. Many studies, including one in 2018, have found that people with major depression find it harder to "retrieve" memories of recent events than people who don't have a mood disorder, and have a trickier time distinguishing between similar memories when they're asked about them. But for the first time, a study published in the journal PLOS One has tried to map how depression changes the way our brains produce new cells — and has discovered that when it comes to memory, people with depression may have a harder time creating new cells than we'd previously thought.
The scientists made a computational model of a brain of someone with depression to look at how patterns of depressive episodes, which tend to ebb and flow in many people with depression, affect memory cells over the long term. It turns out that one of the main ways in which it's thought that depression affects memory has to do with a controversial process called neurogenesis. For centuries, it was thought that humans stopped producing brain cells once they reached adulthood, but it's now thought by some scientists that our brains may actually keep growing and producing new cells — neurogenesis — as we age.
The "neurogenesis theory," as it's known, has been debated for a while, and isn't a sure thing among the scientific community. In 2015, a study pointed out that there's good evidence for it in animals, but that it's not actually been observed in humans. We know that the hippocampus, which is partially responsible for memories, shrinks in people with depression, but nobody has proven that has anything to do with neurogenesis. So it's not clear if this is actually what's happening when people suffer from memory issues in the depths of a depressive episode. But if we assume it's true, then the scientists behind the new study have created an interesting new explanation for what goes awry with someone's memory when they are experiencing depression.
The scientists came up with a mathematical model of a brain as it goes through depressive episodes and then recovers, seeing slower neurogenesis in various areas whenever the episodes hit. And then they tested how the brain would react to those lower neurogenesis rates. This is where something surprising happened. The brain in their model wasn't just worse at recovering recent memories; it also showed poor performance when it came to much longer-term memories, ones that were years old.
This is an interesting new idea. For many researchers, it's short-term memory that seems to be impaired the most by depressive episodes. Long-term memory is often thought to be "safer"; the brain has already processed that information and made it into something more stable, so it's seen as less vulnerable to the issues caused by depression, whatever they are. But this new model suggests that things are a bit more complex than that — and that people with depression might see issues with their older memories as well as their more recent ones.
This is another piece of proof that depression isn't just having a "low mood" for a bit: It's a complex mental health condition that affects a lot of different parts of the brain, and can interfere pretty radically with a lot of your cognitive functioning. Fortunately, antidepressants have been shown to be good for memory issues in animals, but they can also cause or exacerbate memory loss in humans. The tie between memory, the brain, and depression is a pretty knotty one, and we're still figuring out how everything connects.