Washington, Aug 23: University of Oregon researchers have revealed that seeing and exploring are both necessary for stability in a person's episodic memory when taking in a new experience.

The human brain continuously records experiences into memory. Seeing provides the big picture, but exploration burns it into memory.

In experiments in the UO lab of Clifford G. Kentros, researchers have been studying the components of memory by recording how neurons fire in the hippocampus of rats as they are introduced to new activities.

As in humans, brain activation in rats is seen in particular locations called "place cells." It has been believed that these cells together form a mental map of the environment.

To differentiate between simply observing a new environment and exploring it more deeply, researchers injected rats with a drug that destabilized newly formed place fields in the hippocampus.

They measured the firing of place cells as the animals either observed or directly experienced an environment. The rats were then placed in two concentric boxes.

Initially in an inner box, they could only observe the outer box. The rats then either were injected with saline or a chemical that blocked the NMDA receptor, which binds with glutamate and is needed for memory formation, and allowed the rats to explore the outer box.

The place fields representing the outer box were significantly different in the NMDA receptor-blocked animals, and resembled those of a new environment. Blocking that receptor destabilized the place fields of areas extensively observed but not experienced.

"We found that the place cells active in the outer box area behaved as if the area was completely novel," said lead author David C. Rowland, a postdoctoral researcher with Kentros in the Institute of Neuroscience.

"That is, their spatial preference developed only as the animal directly experienced the environment, echoing the autobiographical nature of episodic memory.

"We found that the construction of this hippocampal representation of space -- the map's construction -- is also self-centered.

"The place cells therefore appear to help create an autobiographical record of experience. Our results help to align the hippocampal `place cell' phenomenon with the hippocampus's well-described role in episodic memory, a connection that has been elusive," he added.

The study was published online in the Proceedings of the National Academy of Sciences. (ANI)