Now that we’re in the dog days of summer, I find myself imagining the perfect summer vacation – long trips to Greece, or maybe Fiji… somewhere I have never been. My imagined trips would not be complete without my memories from past trips – the feel of the breeze from the shore, happy times spent with family. But do my imagined scenarios and memories come from the same place? New research finds that while imagining and remembering draw on shared regions of the brain, the two processes activate those regions in different ways.
Often when we imagine something, it feels as vivid as a memory. That got Stefania Ashby of the University of California, Davis, thinking about whether pure imagination exists or “if imagining was merely just a recombination of memories into a novel scenario,” she explains.
Scientists have long been interested in how our brains distinguish remembering, planning, and imagining. Although we look ahead to the future to imagine events and look back to the past to remember, our imaginations may feel limited by what we have experienced. And imagining the future is a more vivid experience than planning one. “Just planning that you are going to go to the beach tomorrow is a much different cognitive experience than actually imagining yourself sitting on a beach tomorrow, feeling the sand between your toes, and smelling the sea salt carried in the breeze,” Ashby says.
Past research has identified the hippocampus as a critical area involved in constructing novel potential scenarios based on past experiences. Researchers have found, for example, that patients with hippocampal damage also have deficits in imagining future scenarios.
Ashby, with her adviser Brock Kirwan of Brigham Young University and colleague Michelle Nash, wanted to further explore the role of the hippocampus in imagining and remembering. They asked participants to provide 60 personal photographs from the past 5 years to help them recollect past memories. The researchers also surveyed participants about travel and leisure activities so that they could select novel experiences. They then showed the participants who were in an fMRI scanner either a personal photo or a photo representing a novel experience, gave them time to elaborate on their memory or imagination, and then asked them to rate the degree of their remembering or imagining.
“Because of the individualized nature of the experiment, we received and used a wide variety of photographs for the ‘remember’ portion of the study,” Ashby says. “Some of the photos provided depicted participants engaged in travel, playing sports, family reunions, and parties.” The imagined photos tended to depict high-adventure activities, such as skydiving, bungee jumping, and zorbing, as well as landscape photos of never-before-seen travel locations.
As published last month in the journal Cognitive Neuroscience, the research team found distinct patterns of activity in the hippocampus, and the brain in general, for imagining compared to remembering. Importantly, the results varied based on which of two different fMRI analysis techniques were used – univariate, the traditional method used in most past studies, or multivariate, a relatively newer method that statistically accounts for more variables. Only the multivariate analysis revealed the differences in hippocampal activation between imagining and remembering.
“I was surprised that we were able to observe differences in activation within just the hippocampus using the multivariate analysis technique when we didn’t see any differences using the traditional univariate approach,” Kirwan says. “I thought this difference would come through when we looked at activation all over the brain – and it does – but I was surprised when it held when we looked at just the activation in the hippocampus.”
This study is consistent with others that have revealed differences between the two analytic approaches. It is also, Ashby says, the first to use personal photographs to cue past memories to explore this research question. They wanted to use personal photos, she says, to limit the potential for false memories. Past studies used generic cue words to trigger memories (for example, words like Christmas, presents, and snow) that may or may not have actually happened.
One challenge of using the personal photos, however, is that it pitted familiar stimuli versus novel ones. “Thus, the differences in activation might have been due to novelty processing – which is something we’ve known the hippocampus to do for a good long time – and not due to the different tasks we asked subjects to engage in,” Kirwan says. So the researchers tried to control for this aspect by excluding from their analysis brain activity that respond to novelty.
Another potential concern that Ashby would like to better control for in future studies is how the hippocampus processes emotional responses. The personal photos tended to represent pleasant memories, while some of the imagined scenarios represented aversive activities. “For example, showing someone who is afraid of heights a skydiving picture, or looking at a tsunami disaster photograph, would elicit a much different emotional reaction than remembering a fun family vacation,” she says. “Future studies may wish to ask the participants to rate the emotional valence of each photograph and then only use similarly rated trials for comparison between conditions.”
One idea that Ashby and Kirwan have for future experiments is to recruit potential subjects following public events, like a basketball game, and to then ask questions about that event. “That way, we would be able to verify that the event actually occurred and thus control for subjects fabricating or mis-remembering things in the remember condition,” Kirwan says. “We like to think that our memories are like a literal play-back of past events, but research going all the way back to Frederick Bartlett has shown that much of the remembering process is actually a reconstruction, which is why we think that imagining the future and remembering the past look so similar in the brain.”
The ultimate goal, Kirwan says, is to understand how the brain forms and retains long-term memories. “If the hippocampus is also involved in imagining the future, then that gives us important information on what the functions of this structure are and how it performs those functions.”
-Lisa M.P. Munoz