For the first time researchers at Case Western Reserve University School of Medicine have created stimulus-specific sustained activity patterns in brain circuits maintained in vitro.
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.
In the current study, Dr. Ben W. Strowbridge and Dr. Phillip Larimer wanted to identify 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.
"Seeing the memory deficits that so many people with epilepsy suffer from led me to wonder if there might be a fundamental connection between hippocampal mossy cells and memory circuits", Nature quoted Larimer as saying.
Mossy cells are unusual because they maintain much of their normal activity even when kept alive in thin brain slices.
The spontaneous electrical activity in mossy cells played a major role in the 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.
"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," said Strowbridge.
The scientists measured the frequency of synaptic inputs onto the mossy cells to determine whether or not the hippocampus had retained memory.
"Memory was not evident in one cell but it was evident in a population of cells," said Strowbridge.
The study will be published in the February 2010 issue of Nature Neuroscience.