Conservation of hippocampal memory function in rats and humans

THE hippocampus is critical to declarative memory in humans(1). This kind of memory involves associations among items or events that can be accessed flexibly to guide memory expression in various and even new situations(2-4). In animals, there has been controversy about whether the hippocampus is specialized for spatial memory(5,6) or whether it mediates a general memory function(3,4), as it does in humans. To address this issue we trained normal rats and rats with hippocampal damage on non-spatial stimulus-stimulus associations, then probed the nature of their memory representations. We report here that normal rats demonstrated two forms of flexible memory expression, transitivity, the ability to judge inferentially across stimulus pairs that share a common element, and symmetry, the ability to associate paired elements presented in the reverse of training order. Rats with neurotoxic damage limited to the hippocampus demonstrated neither form of flexible expression, indicating that non-spatial declarative processing depends specifically on the hippocampus in animals as it does in humans.,