Zachary Glass, BioNB222 Final Project
Place memory is the brain's ability to comprehend and recollect the position of the body in its environment. It is currently believed that place memory is stored in the hippocampus. This belief originates in the work of O'Keefe and Dovstrovsky in 1971. Their experiment consisted of implanting electrodes into different regions of the rat brain and applying various stimuli, such as light strobes, sound blips, or physical contact with the rat. They hoped observe electrical response to each stimuli at a given point in the brain, and thus to map the stimuli to particular regions. This was successful in most trials. However, they discovered that electrodes in the hippocampus could not be stimulated with any of the given stimuli alone. However, these locations were stimulated when the rat was in a particular area of the laboratory environment. This led O'Keefe and Dovstrovsky to hypothesize that the hippocampus controls some form of spatial map. Experiments to date have supported and expanded on this theory.
Currently, it is thought that the hippocampus contains specialized "place cells". Each cell corresponds to a particular space field in the environment. The cell fires when the animal is in the space field. The environment is completely divided into overlapping space fields, so the hippocampus contains a computational map. This map has a cell for each location in the environment. It is believed that this is the mechanism by which an animal can learn spatial tasks, such as navigating a maze. As more research is done, more details become apparent. Concepts such as path learning and optimization are now attributed to this pathway in the hippocampus.
The believed mechanism for learning in this system is Long-Term Potentiation (LTP). LTP is the concept that when two neurons repeatedly fire synchronously, the synaptic connections between them strengthen. If two place cells correspond to overlapping, successive space fields along an often-traveled path, these cells will frequently fire together. This can lead to LTP, and thus strengthening or learning of the pathway. Elucidating this process has been the goal of many current research projects. Some researchers have attempted to use computer modeling to simulate and clarify this mechanism.