In one important way, we are capable of traveling back in time. Through memory, we can return to childhood, go back to the moment we first fell in love or recall our first professional triumph.
Vivid memories that call to mind an experience in deep sensory detail can seem to transport us back to another time and place. When we recall the particular bit of information we're looking for, we also recall many other surrounding details -- how we felt, what we were wearing, what came before and afterwards.
New research from neuroscientists at Vanderbilt University sheds light on how the brain performs this remarkable function.
The researchers analyzed brain activity of individuals performing a memory recollection task in order to show what's going on when we come up with elaborate memories that feel so real that they take us back to a time in the past. They were able to use brain activity patterns to predict the order in which study participants recalled information that they had previously received.
“It’s extremely important that we understand what different brain regions are doing as we search through our memories,” psychologist Dr. Sean Polyn, the study's lead author, said in a statement. “Diseases like Alzheimer’s and epilepsy are devastating to memory, and this information can help us develop treatments to preserve patients’ memories and identify adverse effects that new psychotropic drugs may have on people’s memory.”
The Vanderbilt research team sought to determine how memories are encoded with different levels of vividness and detail. They investigated both "high fidelity memories" -- those that are very well-preserved in our minds, even years later -- as well as bits of information that are remembered, but in isolation, without surrounding details and sensory information.
"In everyday life, when you have an experience, your brain constructs this rich neural code representing the details of that experience. Later, if you think back to that experience, the brain attempts to reactivate that neural representation," Polyn explained in an email to The Huffington Post. "Mental time travel is when the brain does a really good job reactivating that past state, which can feel like you are actually revisiting the experience, in your mind’s eye.
According to Polyn, if a person studies a list of items, and then searches their memory to try to report back those items in order, they'll often report a cluster of things that happened right around the moment of remembering that item, suggesting that their brain has mentally travelled back to that moment.
A brain region called the medial temporal lobe (MTL) has been known to play a role in memory, largely because amnesia often results when this region is damaged. The researchers devised a model to show how structures within the MTL support the retrieval of memories. Finding that the anterior region of the MTL signals when the memory is being retrieved, but doesn't suggest how detailed the memory is. However, they found that the posterior region becomes active when a highly detailed memory is being recalled.
"We find that activity in the posterior medial temporal lobe allows us to predict when the person is going to have a ‘mental time travel moment,’ we can tell when they are about to report a cluster of memories for things that happened nearby in time," Polyn said.
To test the model, fMRI scans were conducted on 20 participants between the ages of 18 and 35, while they were given a list of 24 names of common objects like 'horse' and 'boat.' After briefly concentrating on the words and then pausing, they were asked to recall the words they had just studied in the order the occurred.
The researchers found that when the participant's brain activity revealed that they had retrieved a memory with "high fidelity," their next response was likely to be the next item on the list -- suggesting that they also recalled the detail around the object. But when they did not recall the item with high fidelity, the next item was often not the next one on the list, which suggests that the information was recalled in isolation.
“This demonstrates that the brain stamps memories with a temporal code," Polyn said in the statement. "These time-travel recollections allow the brain to retrieve that temporal code, which makes memories for nearby things more accessible, in this case the next item in the list.”
Understanding what's happening in different brain regions when we're searching for memories may have some important implications for the study of memory-related disorders like Alzheimer's.
"People with Alzheimer’s disease and other forms of dementia, often have difficulty remembering things that just happened to them," Polyn told HuffPost. "A doctor could have a patient repeat back a series of numbers a few times, but then a few minutes later, the patient might not even remember that they even studied those things. If we can understand what different brain regions are doing during healthy memory retrieval, that can give us great insight into what’s going wrong when memory is damaged. It may also help us develop better tests for early detection of memory disorders, and give us ideas for how to better treat people with these disorders."
The study was published on February 18 in the Journal of Neuroscience.