ScienceDaily reports on the dissertation of Kristina Kompus at Umea University on the entirely different signal paths for spontaneous (just happened to remember) and deliberate (try to remember) activation of memories. (here)

…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….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. …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.