" One of the main symptoms is the low mood, so you may feel hopelessness or lose interest in activities you were normally very interested in, like meeting with friends or going for walks. There are three main symptoms of depression, and if you experience them for weeks or months, then it’s likely you have depression. There are also more changing and varying symptoms like loss of concentration and energy, loss of appetite or sleep problems, and psychological symptoms like losing your ability to do certain things. There are different things that are impaired and they vary from person to person. However, they’re all signs that something is wrong with your brain. It’s very difficult to say whether it’s depression or whether some underlying processes in the brain are putting people at risk of experiencing these symptoms. Research has shown that there are certain regions of the brain particularly affected in people with depression. Amygdala is a region that is particularly important in regulating our moods, and in relation to our question about depression and cognitive decline, the hippocampus plays an important role in storing our memories. Most research related to depression suggests that when you’re depressed, it’s more likely you have an impaired functioning with regulating your mood and with many cognitive functions including your memory, which could be directly linked to the hippocampus. And if you experience depression for a long period of time, it could also be stress (chronic stress), which could cause your brain to shrink and cause loss of physical and psychological abilities. That’s one of the possible biological mechanisms that may underline the link between depression and the cognitive disability of dementia. "