Depression is incredibly common, and its most notorious symptoms—loss of interest, changes in appetite, differences in sleep habits—are a familiar refrain found on doctors’ office questionnaires and black and white antidepressant commercials everywhere. But depression can manifest as changes in memory, a serious but lesser-known symptom that deserves some attention. Depression impacts memory in three distinct ways, which help explain why depression changes our outlook and can be hard to overcome.
A 2018 study published in Trends In Neuroscience suggests that besides making it harder to recall specific, detailed memories, depression also makes it easier to access negative memories and harder to create and recall positive ones. “[These findings are] obviously very clinically relevant because if you're constantly remembering the negative things in life and not the positive things, it's an obvious way to perpetuate or maintain a depressive episode,” says Daniel Dillon, MD, an associate professor in the department of psychiatry at Harvard Medical School.
The association between depression and negative memories sounds intuitive, albeit a little abstract. But it’s not just a pessimistic attitude that keeps people with depression feeling poorly. Depression has tangible, measurable effects on the body and brain that Dr. Dillon and his colleagues believe are responsible for memory deficits. Dr. Dillon launched the Motivated Memory and Learning Lab at McLean Hospital’s Center for Depression, Anxiety, and Stress Research to learn more about the physiological phenomenon behind these symptoms. So far, his findings suggest that depression and memory loss are a two-way street; not only does depression affect memory, the memory problems may exacerbate and sustain depression.
The connection between depression and memory fog
When someone’s dealing with depression, their memories become vaguer. Psychologists call this “over general autobiographical memory phenomenon.” In a lab setting, researchers test for this by asking participants to tell them about a particular memory, like a time they were happy. Dr. Dillon says that while someone might typically say, “seeing U2 at Yankee stadium when I was a teenager,” a depressed person might say something more general, like “seeing concerts when I was young.” Some theorize that this is because of the way depression dampens, among other things, the ability to focus. “If someone is not paying attention to what's going on around them, they're not going to learn that information for them to be able to recall it later,” explains Lydia Cho, PhD, a neuropsychologist at Mclean Hospital and an instructor in psychology at Harvard Medical School. But Dr. Dillon thinks this symptom might also be related to another common feature of depression: a smaller hippocampus, the structure deep in the brain that plays a starring role in memory.
“The most prominent structural finding in depression, when we look at structural MRIs, is that the hippocampus tends to be smaller in depressed adults,” Dr. Dillon says.
Dr. Dillon believes that stress is primarily to blame. The hippocampus, he explains, is home to tons of glucocorticoid receptors, which accept stress hormones. Chronic stress, a known cause of depression, tends to over-activate the glucocorticoid receptors, disrupting the brain’s ability to produce new neurons. Dr. Dillon and his team have observed that old neurons seem to be involved with forming more general memories versus detailed, specific ones. Their theory, then, is that the lack of new neurons, and subsequent reliance on old ones, means that depressed people rely on more general memories and fewer specific ones.
Besides making memories vaguer, depression also biases the brain towards negative memories. Psychologist Jennifer Sanderson, PsyD, gives the example of having a work supervisor plan an unexpected meeting. While a healthy person might wonder about this meeting calmly, a depressed one is more likely to assume they’re in trouble. They’ll remember every mistake and disagreement but forget any successes. If the meeting goes well, a depressed person likely won’t remember it. But if it ends up going poorly, they won’t forget it, Sanderson says. Dr. Dillon’s research points to a few reasons why this happens.
Each day we form thousands of short-term memories, but only a few are stored long-term. One way that memory gets upgraded from short-term to long-term is via dopamine, the neurotransmitter that controls pleasure and is released when something surprising or positive happens. “Dopamine tells your brain, hey, something important could have happened here. We should build a strong memory for this,” Dr. Dillon says. So (Dr. Dillon’s theory goes that) when a good thing happens to a depressed person, since their dopamine system isn’t working correctly, their brain doesn’t get the signal to build a strong memory of it.
On the other hand, depression actually trains the brain to get better at storing and recalling negative memories. “Your brain gets into a state where it’s just really easy to pull up negative memories,” Dr. Dillon says. His lab has a study in review right now in which 4,500 people were shown positive and negative words and later asked to recall which ones they’d seen before.
Previous research shows that healthy people exhibit what’s called a positive memory bias; they can remember more positive words than negative words. In depressed people, this effect is weak or nonexistent. In this study, when given a mix of words they’d seen before, plus new ones, depressed participants not only remembered more negative words, they were more likely to incorrectly identify new negative words as ones they’d seen before while mistakenly identifying old positive words as new ones.
Dr. Dillon suspects that the connection between the amygdala, a part of the brain that processes negative emotions like fear, and the cortex, which plays a significant role in recalling memories, is stronger in depressed people, making it easier to recall negative memories. But Dr. Dillon thinks depression may also allow the brain to create stronger negative memories in the first place. Chronic stress, a known cause of depression, sensitizes the amygdala, which Dr. Dillon theorizes could create an exaggerated emotional response to negative events, causing the brain to create stronger than usual memories for them.
How to manage depression and memory-related symptoms
The lab’s research has highlighted several things that can counteract the physical effects of depression. Exercise seems to signal the brain to grow new neurons and may slow or even reverse the hippocampus shrinkage characteristic of depressed people. Antidepressants can do the same. In 2013, researchers at Columbia University performed autopsies on 42 people. About a third had no history of depression, a third had been taking antidepressants, and a third had died depressed and unmedicated. They found that while the unmedicated and depressed group had significantly fewer neurons in their hippocampus, the antidepressant group had comparable numbers to otherwise healthy adults.
Dr. Dillon is also excited about emerging research on something called memory therapeutics, in which depressed patients are guided through exercises with a therapist where they practice recalling specific memories and noting down specific events each day. An early randomized controlled trial on a group of depressed Afghan refugees found it helped make their memories more specific, which then improved other symptoms of depression.
Dr. Sanderson also says that journaling is a great tool, but for depressed patients, adding another item on their to-do list can be overwhelming. Instead, she recommends using the cellphones notetaking app to jot down accomplishments, positive thoughts, praise, or even just nice interactions when they happen. Then, when negative thoughts strike, that list can provide counterevidence.
No matter what course of action one takes, Dr. Cho urges those struggling to seek support as early as possible because the effects of depression, like reduced memory and lower cognitive functioning, can exist (to a lesser degree) even after someone comes out of a depressive episode, and may increase the risk of future bouts of depression.
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