Clues On Short-Term Memory in Hippocampus

Researchers Ben Strowbridge and Phillip Larimer at Case Western Reserve have studied thin slices of living rodent hippocampus, and discovered some potentially important new clues about short term memory.

Their study will be published in the February 2010 issue of Nature Neuroscience and is currently available online.

Neuroscientists often classify human memory into three types: declarative memory, such as storing facts or remembering specific events; procedural memory, such as learning how to play the piano or shoot basketballs; and working memory, a type of short-term storage like remembering a phone number. With this particular study, Strowbridge and Larimer, were interested in identifying the specific circuits that could be responsible for working memory.

Using isolated pieces of rodent brain tissue, Larimer discovered a way to recreate a type of working memory in vitro. He was studying a particular type of brain neuron, called mossy cells, which are often damaged in people with epilepsy and are part of the hippocampus....

...Mossy cells are unusual because they maintain much of their normal activity even when kept alive in thin brain slices. The spontaneous electrical activity Larimer and Strowbridge found in mossy cells was critical to their discovery of memory traces in this brain region.

When stimulating electrodes were inserted in the hippocampal brain slice the spontaneous activity in the mossy cells remembered which electrode had been activated. The memory in vitro lasted about 10 seconds, about as long as many types of working memories studied in people....

..."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," said Strowbridge.

Larimer and Strowbridge also found the brain circuit that enabled the hippocampus to remember which input pathway had been activated. The memory effect occurred because of a rare type of brain cell called semilunar granule cells, described in 1893 by the father of neuroscience, Ramón y Cajal. The semilunar granule cells have an unusual form of persistent activity, allowing them to maintain memory and connect to the mossy cells. That was the foundation for this paper. The semilunar granule cells remained an obscurity for more than a century until Strowbridge's group rediscovered them in a paper they published in 2007....

...Strowbridge's group is now looking into how much information they can store in the hippocampus.

"It took us four years to be able to reproducibly store two bits of information for 10 seconds" says Larimer. "Our findings should progress faster now that we know what to look for and have found the brain circuit that actually holds the memory." _SD

The semilunar granule cells (as a group) appear to be specially adapted via evolution to allow a persistent actively maintained memory for several seconds -- up to 10 seconds in the rodent hippocampal slices. Short term memory in the human brain can last longer.

How short term memories were stored -- and how they are transferred to long-term memory -- are important problems in neuroscience, and the science of consciousness formation.

We have seen in the previous post that the cortex can maintain a type of ultra-short term "echo memory" or "liquid mind", that allows for sophisticated integration and combinations of sensory input (and almost certainly of a wide range of mental images). As discussed previously, this echo memory is likely to be an important mechanism for maintaining moment to moment continuity of thought, and a sense of subjective reality.