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A team of neuroscientists at the University of Toronto in Canada has discovered a reason why we often struggle to remember small details of past experiences.

Many events in our lives resemble experiences we have had before, without being identical to them.

Whenever you attend a party, for example, you may well take along a gift, such as a bottle of wine or a box of chocolates, but the gift will differ on each occasion.

Researchers believe that as our memories for such events become older, the incidental details unique to each event (such as the identity of the gift) are mostly forgotten.

However, the common underlying patterns (what parties are like in general) are retained. This allows us to accumulate knowledge to guide our behavior in similar situations in the future.

Studies in rodents and people have shown that a region of the brain called the medial prefrontal cortex (mPFC) stores long-term memories about experiences.

But to what extent do neurons in this region represent abstract generalized knowledge as opposed to the specific incidental details?

“Memories of recent experiences are rich in incidental detail but, with time, the brain is thought to extract important information that is common across various past experiences,” said Dr. Kaori Takehara-Nishiuchi, senior author of the study.

“We predicted that groups of neurons in the mPFC build representations of this information over the period when long-term memory consolidation is known to take place, and that this information has a larger representation in the brain than the smaller details.”

To test their prediction, Dr. Takehara-Nishiuchi and her colleagues studied how two different memories with overlapping associative features are coded by neuron groups in the mPFC of rat brains, and how these codes change over time.

Rats were given two experiences with an interval between each: one involving a light and tone stimulus, and the other involving a physical stimulus. This gave them two memories that shared a common stimulus relationship.

The researchers then tracked the neuron activity in the animals’ brains from the first day of learning to four weeks following their experiences.

“This experiment revealed that groups of neurons in the mPFC initially encode both the unique and shared features of the stimuli in a similar way,” said Mark Morrissey, first author on the study.

“However, over the course of a month, the coding becomes more sensitive to the shared features and less sensitive to the unique features, which become lost.”

Further experiments also revealed that the brain can adapt the general knowledge gained from multiple experiences immediately to a new situation.

“This goes some way to answering the long-standing question of whether the formation of generalized memory is simply a result of the brain’s network ‘forgetting’ incidental features,” Morrissey said.

“On the contrary, we show that groups of neurons develop coding to store shared information from different experiences while, seemingly independently, losing selectivity for irrelevant details.”

“The unique coding property of the mPFC identified in the study may support its role in the formation, maintenance, and updating of associative knowledge structures that help support flexible and adaptive behavior in rats and other animals,” he said.