Scientists have now found how the brain remembers one-time events like a marriage proposal, a wedding toast, a baby's birth--events always hold that special place in our memory, without them being repeated over time.
In a study on rats, UC Irvine scientists have found that a single brief experience was as effective at activating neurons and genes associated with memory as more repetitive activities.
Neuroscientist John Guzowski and colleagues say that understanding how the brain remembers one-time events can help scientists design better therapies for diseases like Alzheimer's, in which the ability to form such memories is impaired
"Most experiences in life are encounters defined by places, people, things and times. They are specific, and they happen once. This type of memory is what makes each person unique," said Guzowski, UCI neurobiology and behaviour assistant professor.
In the study, the researchers focussed on how neurons in the brain's hippocampus area react to single events - particularly in the CA3 region, which is thought to be most critical for single-event memory.
For their research, the scientists put groups of rats on a rectangular track. Some rats took one lap, while others did multiple laps.
After inspecting the brains of rats that took one lap, the researchers found that 10-15 percent of neurons in the CA3 region activated. The same percentage of CA3 neurons fired in the brains of rats that walked multiple laps.
While previous computer simulations predicted that brief and repetitive experiences would activate CA3 neurons similarly, this is the first study to actually show that it's true.
The researchers arrived at the percentages by examining the activation of a gene called "Arc" within hippocampal neurons.
Earlier studies have shown that turning on Arc is required to convert experience into long-term memory.
"Together with our past findings, this study provides key insight into how fleeting experiences can be captured by the brain to form lasting memories," said Guzowski.
Arc activation is disrupted in mouse models of mental retardation and Alzheimer's disease.
"Our findings on Arc regulation in CA3 neurons should prove useful to researchers testing new therapies for Alzheimer's disease. If you understand how the hippocampus works, it is much easier to understand and potentially treat diseases that affect memory," said Guzowski.
The study has been published in the Journal of Neuroscience.