Different signal paths for spontaneous and deliberate activation of memories

Imagine you are asked to remember what you were doing exactly one week ago. You would probably have to make quite a mental effort to sift through your memories. On another occasion, a smell, a picture, or a word might suddenly and unexpectedly trigger a vivid memory of something that happened to you. Science still does not fully understand why our brain sometimes automatically supplies us with a memory that we have done nothing to deliberately call to mind, whereas why, on other occasions, we cannot remember things even though we make efforts to recall them.

The studies in Kristiina Kompus's dissertation show that these two different ways of remembering things are initiated by entirely different signal paths in the brain. Efforts to retrieve a specific memory are dealt with by the upper part of the frontal lobe. This area of the brain is activated not only in connection with memory-related efforts but also in all types of mental efforts and intentions, according to the dissertation. This part of the brain is not involved in the beginning of the process of unintentionally remembering something as a response to external stimuli. Instead, such memories are activated by specific signals from other parts of the brain, namely those that deal with perceived stimuli like smells, pictures, and words. Sometimes such memories are thought to be more vivid and emotional; otherwise they would not be activated in this way. But Kristiina Kompus's dissertation shows that this is not the case -- memories do not need to be emotionally charged to be revived spontaneously, unintentionally. Nor do memories that are revived spontaneously activate the brain more than other memories.

The studies also reveal that our long-term memory is more flexible that was previously believed. There is not just one single neurological signaling path for reliving old memories but rather several paths that are anatomically separate. This discovery is important, since it helps us understand how we can help people who have a hard time remembering things, regardless of whether this is related to aging or to some disorder in the brain. It may also help people who are plagued by unpleasant memories that constantly haunt them. This can happen following traumatic experiences, but also in depression.

The dissertation uses a combination of two imaging methods for the brain: functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). The methods yield different information about the function of the brain. By combining them, Kristiina Kompus has been able both to determine what part of the brain is activated and how the activation proceeds over extremely brief time intervals, on the order of milliseconds.