Different types of memory are formed in different parts of the brain. The repetitive drive to work or to the supermarket requires well-learnt place memory and involves different brain mechanisms than returning to your car in a car park which requires rapidly-learnt memory of a novel place.
Author Tobias Bast of The University of Nottingham teamed with Iain Wilson and Richard Morris at the University of Edinburgh and Menno Witter at the Norwegian University for Science and Technology to investigate how such rapid place-learning is translated into appropriate behavior.
They focused on the hippocampus - an elongated, banana-shaped structure beneath the brain's temporal lobe. The hippocampus contributes to conscious memory. It is especially important for the rapid learning of the ever-changing aspects of our everyday experiences. How the hippocampus mediates such rapid learning has received a lot of attention. A much-studied property of individual hippocampal neurons in rats is their striking ability to hone activity to certain places – known as place-cell firing. In other words, when rats move about in an environment, electrophysiological recordings from the hippocampus show that within seconds to minutes, many hippocampal neurons come to fire when — and only when — the animal passes a specific place. This means that the hippocampus rapidly 'learns' and then codes for specific places. But, until now, the way this rapid place learning is translated into behaviour has received less attention.
In the new study, the researchers identified the part of the hippocampus that is responsible for this learning-behaviour translation. They found that the critical part is the 'intermediate' or middle part of the hippocampus, which combines links to accurate visuo-spatial information — like the position of a car in a car park — with links to behavioural control necessary for returning to that car after a period of time.
To do this the researchers tested rats in a water maze experiment. The rats located and then returned to a platform in the water, with the platform location changing every day. Different parts of the rat's hippocampus were selectively 'lesioned,' or disabled, using a neurotoxin. The effects on the rats' behaviour were then measured.
The study found that if roughly 30-40 percent of neuronal tissue in the middle of the hippocampus — the intermediate region — was spared by the neurotoxin lesions, the rats could carry out the task with similar efficiency as with a fully intact hippocampus. But when the intermediate hippocampus, or a substantial part of it, was disabled, sparing 30-40 percent of tissue at the two ends of the hippocampus – the so-called 'septal' and 'temporal' hippocampus – the rats struggled with the task.
The researchers also found that the septal end of the hippocampus, featuring links to precise visuo-spatial information, can still rapidly form an accurate place memory — as reflected by the place-related firing of neurons in this region after the rest of the hippocampus was disabled. However, it cannot translate this memory into behaviour because without the intermediate hippocampus, it lacks the relevant links to behavioural control.
Dr. Bast plans to expand on these discoveries with research into how aberrant hippocampal activity that characterises many neuropsychiatric conditions, such as schizophrenia, contributes to symptoms.
"People often focus on memory deficits when thinking about the significance of aberrant hippocampal function," he said. "But our new findings highlight the important hippocampal links to behavioural control. We plan to build on these findings and examine the possibility that aberrant hippocampal function — depending on where in the structure it occurs and to which extent — may give rise to selective memory deficits, as well as to more profound disruptions of behavioural control."
Citation: Bast T, Wilson IA, Witter MP, Morris RGM (2009) From rapid place learning to behavioral performance: A key role for the intermediate hippocampus. PLoS Biol 7(4): e1000089.doi:10.1371/journal.pbio.1000089