New technique allows researchers to watch memory formation!

A team of researchers at Stanford University have developed a new technique that allows them to watch a population of hippocampal neurons firing in real time. Firing patterns observed in this way were associated with the processes underlying long-term memory formation.

Although CA1 neurons in the hippocampus are known to be important in spatial memory, up until now, studies have recorded from only small numbers of neurons and for no more than a week, so it has been difficult to find out how these neurons encode spatial information. The new technique uses a viral vector to express a calcium-sensitive green fluorescent protein in the neurons of mice brains. Calcium ions entering the neuron when it fires cause the protein to emit light, visualised in real time using a tiny microscope that is implanted just above the hippocampus. The camera can view around 700 neurons at once.

The group, lead by Mark Schnitzer, followed the activity of a total of 500-1000 hippocampal neurons while the mouse explored a small arena. They observed that different neurons fired when the mouse was in different regions of the arena and that these patterns were maintained in repeated trials over the course of 45 days. The mouse’s location could be predicted from the pattern of neurons. The results, published online last week in Nature Neuroscience, suggest that the hippocampus uses these different responses to build a virtual map of its surroundings.

While this technique cannot be used to study human subjects, the researchers hope that it will be useful for studying neurodegenerative diseases such as Alzheimer’s disease, where loss of normal hippocampal function leads to long-term memory loss.