If you develop Alzheimer’s disease, you not only lose your sense of time, you also lose your sense of place. Could time and place be two sides of the same coin?
About 55 million people worldwide are currently living with dementia, such as Alzheimer’s disease, a number that is expected to triple by 2050. At the Institute of Basic Medical Sciences at the University of Oslo, researchers are working to get closer to understanding what happens in the brain so that we can prevent or slow down the development of such diseases.
All memories are made up of different elements. You not only remember what you had for dinner yesterday, but also the time and place. We often think of time and space as separate categories, a distinction created by philosophers and physicists that is incredibly practical for organizing our lives. But our brain cells don’t see it that way.”
Koen Vervaeke, Professor, University of Oslo
These cells do not distinguish between a step forward in space or a second passing in time. Instead, they simply record an ever-changing stream of information from our senses, tracking events as they unfold. For the brain’s internal network, time and place are essentially two sides of the same coin, he explains.
“In Alzheimer’s disease it is not surprising that both are affected; when the neural network is damaged, the sense of ‘where’ and ‘when’ begin to unravel together.”
The scent of silence: Monitoring the brain’s internal relay
Remembering where, when, and how something happened is called episodic memory. In your brain, billions of nerve cells form large networks, passing signals like a relay race to process information from your senses, the sounds, smells and sights of your life.
We already know that cells that link memories to time and space are in the hippocampus. But Vervaeke and his research team had a theory that another area of the brain was also involved, namely the retrosplenial cortex. Located at the back of the cerebral cortex near the hippocampus, this region was previously known only for associating memories with place.
To test whether this region also keeps track of time, the team designed a memory challenge for mice. The task required them to hold a specific smell in their “working memory” for a short period. The article was recently published in Cell Reports.
“We presented the mice with a simple sequence: one odor, a five-second pause, and then a second odor. If the two odors were different, banana followed by peppermint, the mice licked a tube for a sugar water reward. If the odors were the same, they had to remain still,” explains Vervaeke.
At first, the mice were impatient and constantly licked the tube. But after a week of training, they mastered the task.
“They learned to use that five-second silence to hold the first memory in their mind, waiting to see if the next scent would match or change,” explains Vervaeke.
The universal script: A unified theory of experience
To see what was happening inside the brain, the researchers used a microscope to monitor nerve cells in the retrosplenial cortex while the mice performed the task.
“Under the microscope, we saw two distinct groups of nerve cells coming to life,” says Vervaeke. “The first group acted as smell specialists, active only when a specific smell was present, such as banana or peppermint. But the second group was even more fascinating: these cells were active in a precise order during the five-second silence.
Like a relay race, one cell passed the signal to the next, “holding” the memory of the first scent until the second arrived.
“Working together, these cells did two jobs at once: they recognized the smell and kept track of exactly how much time had passed.”
The most striking discovery was that the retrosplenial cortex uses the same “neural script” for both space and time. “We found that the sequence of neuronal activity in the retrosplenial cortex looks almost identical whether a mouse runs naturally across a room or simply holds a memory in its mind for five seconds,” says Vervaeke.
Redefining the Dementia Treatment Reality
This discovery brings us back to the tragic reality of Alzheimer’s disease, where sufferers struggle to anchor both time and place. By showing that the brain uses the same “neural script” for both, our research explains why these two senses often fail together, says the professor.
This work also questions the way we perceive the world around us. While we use the concepts of time and space to organize our lives, this distinction is largely a human construct. In fact, some modern theories in physics move away from using time and space as the fundamental building blocks of the universe. It appears that the brain’s internal wiring reflects this deeper reality, he says.
“We still have a lot to learn about how a healthy brain works,” explains Vervaeke. “It’s almost impossible to fix a car if you don’t first understand how the engine works when it’s running smoothly. Similarly, we need to understand the ‘blueprints’ of a healthy brain, how it builds and stores these episodic memories, before we can really understand what goes wrong in dementia. These findings bring us one step closer.”
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