Researchers are using functional magnetic resonance imaging (fMRI) to probe brain activity in search of the answer. According to a new fMRI study using a "diary" method to collect memories, it all depends on what we're thinking about!
Researchers have known for decades that thinking about autobiographical facts is different from thinking about autobiographical episodes that happened only once. Since both kinds of thoughts can occur at the same time when people talk about their past, researchers have struggled to find an effective way to separate them.
The new study, published in the current issue of the Journal of Cognitive Neuroscience (16:9), is the first brain imaging study of its kind to use diary-like memories collected by volunteers. It was led by The Rotman Research Institute at Baycrest Centre for Geriatric Care.
Over a period of several months prior to the brain scan, volunteers documented dozens of unique events from their personal lives on a micro cassette recorder (episodic memories). At the same time, they recorded statements about personal facts of their lives (semantic memories). The researchers played these recordings back to the volunteers while their brains were being scanned with fMRI.
The results showed that the two types of autobiographical memory engaged different parts of the brain, even when the memories concerned the same contents. For example, the semantic thought "Every Friday afternoon I take the dog for a long walk" produced brain activity in one set of regions, whereas the episodic thought "One Friday afternoon my dog got away and I spent 45 minutes running after him" produced brain activity in a different set of regions.
"Although both kinds of memory are autobiographical, they serve very different purposes," says lead investigator Dr. Brian Levine, a senior scientist with The Rotman Research Institute at Baycrest and associate professor in Psychology and Neurology, University of Toronto. "Factual autobiographical memory grounds us in time and gives continuity to our lives. Episodic autobiographical memory allows us to travel in time, to relive our past."
The ability to richly re-experience autobiographical memory is thought to be unique to humans and is important for advanced decision-making and quality of life. Dr. Levine and his team found that this form of memory more strongly engaged parts of the frontal lobes involved in self-awareness, as well as areas involved in visual memory. It's important to note that losing this ability to recall personal episodic memories as a result of serious brain injury can be tremendously devastating to an individual and their family. Even if the individual is able to retain autobiographical facts about their life, it is not the same as being able to recall and re-live personal episodes from their past, such as their wedding day.
A few brain imaging studies have already found differences in the brain between factual and episodic autobiographical memory. However, the participants in those studies were asked to recollect memories that were usually several years old, and it was impossible to tell how often they had been rehearsed over the years. This new study, on the other hand, used episodes from daily life that were only a few months old. The volunteers made dozens of event recordings within minutes or hours of the actual event. Only a fraction of these were selected at random for use in the study, so volunteers had no idea which ones they were until they heard them through the headphones in the scanner. The recordings created a very rich recollective experience, enabling scientists to tease out more easily the different brain regions associated with factual (semantic) and episodic autobiographical memories.
One problem researchers had was finding people willing to make the diary recordings over a six-to-eight month period. The scientists ended up using five intrepid volunteers drawn from research institute staff who were interested in memory research. Although the number of volunteers is small for this sort of study, the findings were robust and have been replicated in a larger sample.
"I got the idea for doing this one day when I came across a letter I had written about an event that I hadn't thought of since it happened," recalls Dr. Levine. "I felt like my brain was suddenly flooded with memory. At the time I was looking for ways to get at this feeling in the scanner. We needed a way to do it right on the spot, when the memory happened. That's where the idea of the recorders came in. I looked into it and found that a few memory researchers over the years had done this sort of diary study, but all of them used behavioral tests like reaction time and the like. No one had ever done a brain imaging study."
The study was supported by the Canadian Institutes of Health Research. Baycrest Centre for Geriatric Care is an academic health sciences centre affiliated with the University of Toronto.