Researchers at the UC Davis Center for Neuroscience and the Department of Psychology have successfully used light to erase specific memories using a new technique called "optogenetics."

In doing so, the researchers proved a 40 year-old theory that retrieving episodic memories (or memories about specific events and places) has to involve coordinated activity between the cerebral cortex and the hippocampus.

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The hippocampus is a small structure deep in the brain that plays important roles in the consolidation of information from short-term memory to long-term memory.

Pioneered by Karl Diesseroth at Stanford University, optogenetics is a technique that uses light to manipulate and study nerve cells. Optogenetics techniques are becoming the standard method for investigating brain function.

In 2010, optogenetics was chosen as the "Method of the Year" across all fields of science and engineering by the research journal, Nature Methods.

Kazumasa Tanaka, Brian Wiltgen and their colleagues at UC Davis recently applied optogenetics to test the 40 year-old theory about memory retrieval.

Wiltgen said the theory is learning involves processing in the cortex. The hippocampus reproduces this pattern of activity during retrieval, allowing a person to re-experience the event. If the hippocampus is damaged, patients can lose much of their memories.

Researchers trained genetically modified mice and placed them in a cage where they received a mild electric shock. Mice will usually explore a new environment. But when placed in a cage where they had received an electric shock, the mice froze and refused to move in a "fear response."

Researchers were able to switch off specific nerve cells in the hippocampus, and show the mice lost memories of the unpleasant event. This after labeling the cells involved in learning and demonstrating they were reactivated during memory recall.

Researchers demonstrated that turning off other cells in the hippocampus didn't affect retrieval of a memory, and also follow fibers from the hippocampus to specific cells in the cortex.

Wiltgen said the cortex can't do it alone. It needs input from the hippocampus.

"This has been a fundamental assumption in our field for a long time and Kazu's data provides the first direct evidence that it is true."