New evidence emerges that memory is finite

The idea that the brain might have a finite capacity for storage, replacing old memories with new ones, might be intuitive for many. But scientists have long puzzled over the limits of memory, working hard to identify which parts of the mind might be responsible for how strongly we remember certain events.

Now, recent experiments in mice reveal that strong memories can prevent weaker ones from forming.

Universities often use genetically-engineered mice to test for brain activity, injecting them with a chemical that lights up parts of their brains: a field of study known as optogenetics.

The idea was to give these mice something to remember — like a delicious meal — and then introduce new things into their living space to see if new memories would form as easily, tracking their brain activity to see where new neurons had formed.

Researchers from the University of Oxford fed mice fatty pellets — which they prefer over other foods — and tracked which parts of their brains activated, finding that stronger memories were forming.

The scientists then separated the mice into two groups, feeding the first group the high-fat foods and then leaving the rest of the mice with normal food to compare brain activity.

After some time, the scientists dropped new items into the living space of the mice and measured their reactions.

The mice who ate the fatty food didn’t pay much attention to the new items and showed little neural activity compared to the mice who had the normal food.

When scientists dissected the brains of the mice, they found that the mice that had regular normal food had been able to form new memories more easily, whereas the mice who had eaten the fatty food had not. The stronger memories of the tasty food had made their memory less flexible.

“Previous memory is restricting the pace in which other memory can be built on,” says Giuseppe Gava, a postdoctoral researcher at the University of Oxford and the lead author of the study. “What we showed in our study is that if mice form this very strong association between space and fat food, once they are placed in different environments, [they struggle] to actually experience something new.”

Could the fatty food itself have affected the way the mouse reacted to new environments? Perhaps the now-satiated mice were just tired after their meal? Gava said no, and they were able to perform multiple controls to make sure the mice’s neural pathways had physically changed. There was no chance the less-hungry mice didn’t react to the new objects because they were full, he said.

The scientists genetically engineered the mice so neurons that activated during the study reflected were visible under a yellow light, allowing the scientists to compare brain activity between rodents.

On closer inspection, the researchers saw spikes of activity in a region of the hippocampus known as CA1. The neurons involved with consolidating memory in that part of the brain are known as superficial cells, which appear to fire when strong memories are formed.

The study intentionally provided the mouse brain with conflicting priorities: It should be evolutionarily useful to remember every detail about an area if food was plentiful there, Gava reasoned, but on the other hand, such behavior could be damaging if it led to missed opportunities to find new sources of food.

But the results showed that the brain prioritizes what’s familiar and often ignores what’s new, the same way a person rushing out the door for work might miss a newly parked van outside their house, said Gyorgy Buzsaki, a neurology professor at NYU.

Although the tests were done on mice, Buzsaki said the results were likely to be applicable to humans. Humans have a larger brain capacity than mice, though, so we should be able to learn new things more easily — meaning that our strong memories shouldn’t interfere with our new memories as much.

But our capacity for new memory is always limited. “This is why forgetting is important, right?” said Antonio Fernandez Ruiz, a neurobiologist at Cornell University, who was not involved in the study. “Because if people never forget anything, then they [don’t have enough neural resources] to learn new things.”

“The brain is a zero-sum game,” said NYU’s Buzsaki. “No matter how much you learn, the total amount of synaptic strengths won’t change.”