The long and the short of it
Memories form when we retain, store and recall past experiences, there are two types of memory: temporary and long term. Temporary memories are events that are stored for a very short time – anything from a millisecond to a few minutes. Example include the name of a person you just met or looking up a telephone number. Long-term memories include those events that were stored a long time ago. For example, what you wore to your graduation ceremony or the colour of your first car.
Why do smells evoke memories?
"The sense of smell is the strongest part of the brain and is also directly connected to emotions," says Dr Opperman. That's by the nerve cells in the nose are directly connected to the cells in the cerebral cortex (the external surface of the brain, which processes information). All the other senses first send signals to the lower part of the brain before they're sent on to the cerebral cortex.
"Contrary to popular belief, long-term memory isn't stored only in one part of the brain. Memories relating to an event are scattered across out brain's sensory centres, in the brain areas that were active when we first experienced them. If one of the senses is stimulated to evoke a memory, memories that relate to other senses are also triggered. For example, the aroma of freshly baked bread would possibly transport you to your mother's kitchen, conjuring up many other happy images from the past."
When we can't help forgetting
Occasionally forgetting things is normal. However, if you start having trouble remembering how to do things you've done many times before, such as following a recipe, or how to get to your house, the problem could be more serious. Normal memory problems associated with ageing don't usually get worse over time. However, in the case of Alzheimer's disease, memory loss increases in severity. There is no cure for Alzheimer's disease – a disorder that gradually destroys cells in the brain – and Alzheimer's does not form part of the normal process of ageing.
"A person suffering from Alzheimer's cannot form new memories, but they can for instance remember things from their past", says Dr Opperman. A patient will not remember recent conversations, but will be able to remember what their spouse looked like on their wedding day. Symptoms of Alzheimer's disease include depression, anxiety, personality changes and loss of recent memory.
What about bad memories?
"We tend to suppress bad memories, especially if they are connected to violence. For example, witnessing a car accident could make someone suppress the memory because of its severity, and push it into the back of the mind. The memory isn't lost; the person is manipulating it by consciously avoiding it," says Dr Opperman. Such memories can be retrieved by subjecting the person to a process of hypnosis.