Ben W. Strowbridge, PhD, associate professor of neuroscience and physiology/biophysics, and Phillip Larimer, PhD, a MD/PhD student in the neurosciences graduate program at Case Western Reserve University School of Medicine were involved in the study and were the first to create stimulus-specific sustained activity patterns in brain circuits maintained in vitro.Their work entitled “Representing information in cell assemblies” will be published in the upcoming edition of Nature Neuroscience
The aim of their study was to identify the specific circuits that could be responsible for working memory and henceforth create a type of working memory in vitro.Working memory is a type of short term memory. Larimer isolated pieces of brain tissue from rodents and and focused his study on a particular type of brain nerve cell known as mossy cells. Mossy cells are found in the hippocampus region of the brain and are the ones damaged in epilepsy patients. Epilepsy patients often suffer from memory deficits due to this.Mossy cells have a special property to retain much of their normal function even when isolated in invitro conditions.The spontaneous electrical activity found in these cells was the key to the discovery of memory storage in this region of the brain. 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. 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 neurons and hence they had to study many different cells to look for stored information..
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.The semilunar granule cells have an unusual form of persistent activity, allowing them to maintain memory and connect to the mossy cells. 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.Semilunar granule cells are the third type of brain cell that Strowbridge’s group has uncovered.
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.”