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.
Researchers from the University of Toronto, Canada, have discovered a
reason why we often struggle to remember the smaller details of past
‘Groups of neurons in the medial prefrontal cortex initially encode both the unique and shared features of the stimuli in a similar way. 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.’
Writing in the journal eLife
, the team found that there are
specific groups of neurons in the medial prefrontal cortex (mPFC) of a
rat's brain - the region most associated with long-term memory. These
neurons develop codes to help store relevant, general information from
multiple experiences while, over time, losing the more irrelevant, minor
details unique to each experience.
The findings provide new insight into how the brain collects and
stores useful knowledge about the world that can be adapted and applied
to new experiences.
senior author and Associate Professor of Psychology at the University
of Toronto, said, "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, the team 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 scientists then tracked the neuron activity in the
animals' brains from the first day of learning to four weeks following
"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," says first author Mark Morrissey, formerly a graduate
researcher at the University of Toronto. "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
"This goes some way to answering the long-standing question of
whether the formation of generalised memory is simply a result of the
brain's network 'forgetting' incidental features," Morrissey explains.
"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."
Morrissey adds that 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 behaviour in rats and other animals.