Depression affects memory by making it harder to differentiate between similar objects or experiences, according to a new study from Brigham Young University.
The skill is called pattern separation, and people suffering from depression lose it in proportion to the severity of their symptoms.
The link between depression and poor memory is well documented, but psychology and neuroscience professor Brock Kirwan decided to examine which aspects of memory are affected. His study is appearing in the November edition of the journal Behavioral Brain Research.
"What we think is going on is there are physiological changes in the brain that are underlying both depression and these memory problems," Kirwan said, though not everyone with a bad memory has depression.
The effect isn't limited to people with severe, or even clinical depression. Kirwan looked for subjects by surveying a Psychology 101 class, and found many people who had symptoms of depression but weren't on medication.
For the study, Kirwan and former graduate student D.J. Shelton asked people to take a computer memory test. They viewed a series of objects on a screen, then were asked whether they'd seen the object before, had seen something similar to it, or had never seen anything like it. The depressed subjects were able to differentiate between old and new just fine, but faltered on the similar-object questions, most often saying they'd seen something before when they had not.
Diminished pattern separation can make it harder for people to find a car in a parking lot, for example, or remember which friends and family members they've told about something personal.
It happens because the hippocampus, the area of the brain involved with memory, grows fewer new cells in people suffering from depression.
"Because of the way new cells are integrated, they seem to be really important in helping you in distinguishing between similar things," Kirwan said. Anti-depressant medication does help with the linked memory problems.
The results could have a particular impact in Utah, which has been ranked as having the highest rate of depression in the country.