Memory is a highly complex process, which involves several brain structures and the interaction of many different neurotransmitters. Although research has given us useful clues about how memories are formed, there is still a lot left to be discovered.

Memory is divided into two main types, short-term (ST) and long-term (LT). ST memory is described as a temporary state, in which a piece of information can be easily retrieved. This only lasts for a matter of seconds. For example if a man was told Queen Elizabeth II's birthday for the first time, and was asked to recall it 10 seconds later, he would be able to do so without difficulty. Thirty minutes later, however, having been distracted by an unrelated conversation, he probably would not remember the date.

Yet if having been told a piece of information, a person continuously repeats it in their head, or makes associations between it and something familiar, it is thought that the neurons holding the information as a ST memory start to undergo a structural change, which allows the information to be stored as a LT memory (for days, months, even years, depending upon the strength of the memory). In order to be recalled, LT memories must return to the ST state. This is known as retrieval.

Working memory is the term given to several aspects of ST memory, including planning, abstract thinking, rule acquisition, initiating appropriate actions and inhibiting inappropriate actions.

The hippocampus (a major memory centre in the brain) is a paired structure with left and right halves, and is found in the temporal lobes. It is often damaged in people with epilepsy, which is why many people with this condition experience memory problems. Memory difficulties can cause a lot of distress and significantly lower a person's quality of life.

Previous studies of epileptic brain tissue have revealed particular damage to a type of neuron in the hippocampus known as a mossy cell. This led Professor Ben Stowbridge and Dr Philip Larimer, at the Case Western Reserve University School of Medicine, Ohio, US, to wonder if there is an important link between mossy cells and memory.

Mossy cells are unusual because they maintain much of their normal activity even when kept alive in thin hippocampal brain slices (HBS). This makes them very useful for laboratory study.

The scientists prepared several rodent HBS and placed a set of stimulating electrodes into each. They then activated the electrodes in a random order, one at a time, for very brief moments, and studied the response of the mossy cells.

They found that the relevant mossy cells continued to fire after stimulation from a particular electrode was stopped. In essence, the mossy cells were remembering which electrode had been stimulated. The memory lasted for approximately 10 seconds, about as long as many working memories in people.

Professor Ben Stowbridge commented on this finding: "This is the first time anyone has stored information in spontaneously active pieces of mammalian brain tissue. It is probably not a coincidence that we were able to show this memory effect in the hippocampus, the brain region most associated with human memory,"

When the team studied the activity within the HBS in response to stimulation, they found that it originated in another type of cell called a semilunar granule cell (SGC). The external input from the electrodes caused the SGC to send a barrage of synaptic inputs to the mossy cells, which led to their persistent excitable activity. This suggests that it is the SGCs that 'remember' the initial stimulation and connect to the mossy cells.

The researchers then analysed the synaptic activity between different cells within the hippocampus, to see if the memories had been retained. They found that this was not so in any single cell or cell types, but was the case in populations of different neurons and cells.

Professor Stowbridge said: "Like our own memories, the memories we created in isolated brain slices were stored in many different neurons or cells, that's why we had to watch several different cells to see the stored information,"

These findings are exciting, because they highlight a specific brain circuit that is involved in ST memory. A lot more research is necessary, but an increased understanding of memory could allow people with memory difficulties, such as those with epilepsy, to be monitored more effectively in clinics. It could also lead to the development of therapies to slow / prevent memory loss in the future.

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