Recent research has increased our understanding of the mechanics of memory and how the brain can improve memory when needed. The focus of the research was based around sharp wave ripples (SPW-Rs). These ripples can be thought of as waves that activate pathways that store and later form memories. SPW-Rs are found in the region of the brain named the hippocampus which plays an important role in spatial memory that is used for navigation. Two regions of the hippocampus responsible for the production of ripples are the CA1 and CA3 regions. These regions come together to cause an activation of pyramidal cells (neurons), which leads to the creation of ripples. Scientist hypothesized that the size of these ripples could be related to improved memory.

Using rats as subjects, it was found that tasks that require memory such as being introduced to a new maze led to increasing number of long ripples. They also found that as learning progressed there was a decrease in long ripples. This suggests that longer ripples happen when memory demands are high. Another finding was an increase in number of neurons that fired. This research showed longer ripples are present when memory is required but did not show the ripples are beneficial for memory. To test this, they optogenetically (a method used to control cell activation by light) prolonged ripples. They used differing light stimulation to create both prolonged ripples and a ripple that would act as control (normal ripple) that is triggered with a delayed light. They tested SPW-Rs prolongation in rats by getting them to perform a M-maze task. A M-maze is used to test memory and consists of three arms. It is then broken down into an outbound task which is the memory dependent task which requires the rat to wait 20 seconds and choose the opposite arm of the last arm chosen. It also contains an inbound task that requires the rat to return to the center arm (original location). Two groups of rats were studied one was the ripple prolongation and the other was a no stimulation group. The ripple prolonged group was found to have significantly increased performance for the maze. This showed that there is a connection between long duration ripples and increased memory.

They then examined the differences between normal SPW-Rs and prolongated ripples. It was found that pyramidal cells can activate both in the early and late parts of a ripple. Since the late part of a ripple is only found in the prolonged version, it was seen to include more cells or in other words more cell diversity. The cells activated in longer duration ripples were more selective and contain more information. They also looked at when neurons preferred to fire. It was found that when they fired in the late part of a long ripple it was related to the side arms of the M-maze. This is the outbound task described earlier. These findings demonstrate extended SPW-Rs help with recall in memory tasks. It was found that that prolongated ripples caused activity in a wide range of different cells. This diversity of cells covers a large area thus having a greater chance of being along a network route which can trigger memories.

This study concluded that SPW-Rs depending on size alter memory. The shorter ripple leads to an impairment in memory while a longer duration ripple improves memory. It was also found that the longer duration caused an activation of more cells which creates a stronger network to activate memories. The study could only assume that a diversity of cells is the reason behind the increase in memory. Further research must be conducted on why longer duration ripple improve memory and if this research on rats carries over to research on humans.

Reference:

Fernández-Ruiz, A., Oliva, A., de Oliveira, E. F., Rocha-Almeida, F., Tingley, D., & Buzsáki, G. (2019). Long-duration hippocampal sharp wave ripples improve memory. Science, 364(6445), 1082-1086.