Their findings, published in the Journal of Neuroscience, overturn the established view of the hippocampus and offers insight on how the brain forms and recalls memories by piecing together related bits of experiences.
"For over 40 years, the chief paradigm has been that the hippocampus was important for creating long-term memory but not short-term or working memory," said Ingrid Olson, a member of Penn's Department of Psychology and researcher at Penn's Center for Cognitive Neuroscience. "However, our data show that one type of working memory, working memory for the relationship between bits of information, is dependent on the hippocampus.
According to Olson, how much time has elapsed - or, in other words, the age of the memory -- is less important to the hippocampus than is the requirement to form connections between pieces of information to create a coherent episode of memory.
"I can remember what my keys look like, and I can remember where the coffee table is located, but the critical test of my memory is if I can remember that I left my keys on the coffee table," Olson said.
To study the role of the hippocampus in forming short-term memories, Olson and her colleagues used visual memory tests to study the ability of nine amnesiacs to recall images presented to them on a screen. These subjects all suffered from damage to their hippocampi and related brain structures, and their lives are ruled by the fact that they can no longer form long-term memories, much like characters from the movies "Memento" or "Finding Nemo."
The task required amnesiacs and controls to remember three objects, locations or objects in locations over delays of one or eight seconds. The results show that working memory for objects or locations alone was at normal levels, but that memory for object-location conjunctions was severely impaired at eight-second delays.
"While 'long-term' memory and 'short-term' memory have been useful distinctions for us, they may not exist in the same way for the brain," Olson said.
The researchers believe that a more useful distinction would be between feature memory and conjunction memory - the ability to remember specific things versus how they are related. In that regard, the hippocampus serves like the brain's switchboard, piecing individual bits of information together in context.
"The hippocampus is another part of our evolving view of the nature of memories and consciousness," Olson said. "Our memories are not the static, permanent things we would like to think and, even in healthy people, these connections can erode or become muddled, leading to false memories or illnesses like post-traumatic stress disorder."
Contributing authors of this study are Katie Page and Mieke Verfaellie from the Boston VA Healthcare System and Boston University School of Medicine; Katherine Sledge Moore of the University of Michigan; and Anjan Chatterjee of Penn's School of Medicine.
Funding for their research was provided through grants from the National Institute of Mental Health and the Medical Research Service of the Department of Veterans Affairs.