For sufferers of PTSD, bad memories can severely interrupt day-to-day life; episodic memories of specific places, people, or events can trigger insurmountable fear. Scientists though have found a way to target specific memories in mice and erase them using light, making it conceivable that conditions like PTSD might be thing of the past one day. Additionally, they proved a 40-year-old theory about how episodic memories are stored and retrieved in the brain. The research was led by Kazumasa Tanaka and Brian Wiltgen of University of California, Davis and the results were published in the journal Neuron.
Since the 1970s, neuroscientists believed that episodic memories are stored and retrieved in different parts of the brain. Interactions between the cerebral cortex and hippocampus need to work in concert in order to bring the memories out of mental storage to be re-experienced by the mind. However, it has been difficult to test due to insufficient technology.
"The theory is that learning involves processing in the cortex, and the hippocampus reproduces this pattern of activity during retrieval, allowing you to re-experience the event," Wiltgen said in a press release.
To understand how this is accomplished, scientists used optogenetics, which allows nerve activity to be studied with light. The study used mice with nerve cells that had been genetically modified to emit green fluorescence when activated. This fluorescence allowed them to track memory formation in the mouse's brain.
Additionally, the nerve cells could be switched off and on through light from a fiber optic cable directed into its brain.
Next, the scientists needed to give the mice some bad memories to measure. While mice are generally curious and love to explore in new places, they lightly shocked the mice in a particular cage in order to associate that location with fearful memories. Rather than happily running around the cage where the electric shocks were administered, the mice would essentially shut down and remain still.
The fluorescent cells allowed the researchers to identify which specific cells within the cerebral cortex and hippocampus were involved in learning and remembering those fearful experiences. When those particular cells in the hippocampus were turned off with the light, the mice weren't able to remember being afraid of the shock cage. However, turning off unrelated cells in the hippocampus did not affect the memory. This was incredibly important to support their claims.
"The cortex can't do it alone, it needs input from the hippocampus," Wiltgen continued. "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."
While it's not likely that this will be used to create an Eternal Sunshine of the Spotless Mind-like memory eraser, verifying how memories are stored and retrieved could open up possibilities for treatments for those experiencing memory loss or PTSD at some point down the road.