Finding ourselves racking our brains first thing on a Monday morning, trying to remember where we put the car keys is not un uncommon frustration. When those keys are eventually found, we have the hippocampus to thank for.
The hippocampus is a region in the brain which is responsible for storing and retrieving memories of different locations, including that unusual spot where those car-keys were hiding.
Researchers at the Salk Institute for Biological Sciences have discovered how the brain is able to store and retrieve data from all the amazingly rich and complex environments we navigate every day.
Fred H. Gage and team have found out how the dentate gyrus, a sub-region of the hippocampus, helps store memories of similar but distinct events and environments separately. They reported their findings in eLife, March 20th, 2013 issue.
Their findings, which give us a better understanding of how the brain stores and distinguishes between separate memories, may also help experts identify how Alzheimer’s disease and other neurodegenerative diseases undermine these abilities.
The hippocampus (seen from bottom) has two main parts: the Dentate Gyrus and Ammon’s Horn
Fred Gage, senior author on the paper and the Vi and John Adler Chair for Research on Age-Related Neurodegenerative Disease at Salk, said:
“Everyday, we have to remember subtle differences between how things are today, versus how they were yesterday – from where we parked our car to where we left our cellphone. We found how the brain makes these distinctions, by storing separate ‘recordings’ of each environment in the dentate gyrus.”
Pattern separation is the process of taking complex memories and transforming them into representations that are less likely to be confused.
- What should happen in theory – According to computational models of brain function, the dentate gyrus helps humans perform memory pattern separation by firing up different groups of neurons when an animal is in distinct environments.
- What happens in laboratory experiments – However, prior laboratory studies showed that the same populations of neurons in the dentate gyrus are active in different environments. They distinguish new surroundings by changing the rate at which electrical impulses are sent.
This discrepancy between theoretical predictions (what computational models suggest) and laboratory experiments has baffled neuroscientists and made it much more challenging to understand memory formation and retrieval.
Gage and team set out to get to the bottom of this discrepancy. They compared how the dentate gyrus of a mouse functioned in comparison to CA1, another region of the hippocampus. They tracked the activity of neurons at several time points using laboratory techniques.
- Episode 1 – the scientists removed the mice from their original chamber and placed them in a new one so that they could learn about a new environment. They recorded which neurons in the hippocampus were active as the mice responded to their new environment. The mice were then placed back in their original chamber.
- Episode 2 – later on, the mice were either:
– placed back in that same new chamber, and their memory recall was measured
– placed in a slightly different chamber to the new one, to measure discrimination
The scientists also labeled the active labels in Episode 2 to determine whether the neurons that were activated in Episode 1 were used in the same way for recall and discrimination of tiny differences between the two environments.
The researchers found that the dentate gyrus and CA1 sub-regions did not function the same when they compared the neural activity during Episode 1 and Episode 2.
- In CA1 – the same neurons became active in Episode 1 (the learning episode) and Episode 2 (when the memories were retrieved).
- The dentate gyrus – different groups of cells were active during Episode 1 and Episode 2. The team also found that different cells were active when they were exposed to the slightly different new chamber.
Wei Deng, a Salk postdoctoral research and first author on the paper, said “This finding supported the predictions of theoretical models that different groups of cells are activated during exposure to similar, but distinct, environments. This contrasts with the findings of previous laboratory studies, possibly because they looked at different sub-populations of neurons in the dentate gyrus.”
According to the Salk researchers’ findings, recalling a memory, like the one on that Monday morning when we are trying to remember where our car keys are, does not always involve the same neurons being reactivated.
The authors wrote:
“More importantly, the results indicate that the dentate gyrus performs pattern separation by using distinct populations of cells to represent similar but non-identical memories. The findings help clarify the mechanisms that underpin memory formation and shed light on systems that are disrupted by injuries and diseases of the nervous system.”
Last year, MIT researchers reported in the journal Nature that memories reside in specific brain cells. By merely activating a tiny fraction of these cells, a human can recall an entire memory event. This would explain why we can, for example, recall childhood memories from certain smells we loved as a child.
Written by Christian Nordqvist