Light, the form of energy that allows us to see, might also cause us to ‘un-see’ certain events of our lives by making us forget memories of the past. A group of scientists have demonstrated that when specific nerve cells of the brain of mice are switched off, certain memories of theirs are erased.
Want to forget? Light does the trick!
We all have snippets of memories that we wish we had never made; memories of moments of our lives we regret. If only, we could have them erased. If only, we could press a button for them to be completely out of our minds and hearts. This might sound like something from fantasy and sci-fi movies, but, a new research work has brought us to the point where science might have an answer to this. A team of scientists from the UC Davis Center for Neuroscience and Department of Psychology have shown how light can be used to erase certain fragments of memories of mice. They have, in so doing, provided evidence that several parts of the brain synchronise in function to help us retrieve specific memories.
Cerebral cortex working in concert with hippocampus to ‘relive’ memories
Scientists have in the past formulated the theory that coordinated activities linking the cerebral cortex and the hippocampus allow for us to retrieve episodic memories. The latter define memories about particular places and incidents.
“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,” Brian Wiltgen, one of the co-authors, said.
This long-held ‘belief’ has not been easily tested though. However, fortunately, a new technology involving optogenetics allowed for the researchers to put the theory under the microscope. The scientists set out to determine which nerve cells in the cortex and hippocampus were activated during the processes of learning and memory retrieval using mice.
Optogenetics: playing with nerve cells with light
A new branch in science called optogenetics, which is a new technique of manipulating nerve cells with the help of light, was exploited by the researchers. Studying the brain and its functions has been promoted to another level by the use of the methods of optogenetics.
Playing with the memories of mice
The memories of genetically modified lab mice were thus tested. The animals were first placed into a cage where they received a mild electric shock. Normally, if they are put in the same cage again, they would remember the previous episode and stay in a fearful state, having learnt from their past experience. However, if they are put in a new environment, they will tend to explore it. The scientists observed different reactions from the mice when they were caused to forget the previous episodes entailing the mild electric shocks.
Nerve cells: on & off
To test for which cells of the mice’s brains were involved, the animals were made to show specific signs when the nerve cells were activated: they would fluoresce green and express a protein that would allow the cells to be switched off by light. Hence, the scientists could map the nerve cells that were being activated, and switching them off using light which was provided by a fiber-optic cable.
The authors observed the specific nerve cells that were reactivated during memory recall (the mice remembering that they had experience electric shocks in the cage in the past). Thereafter, they switched off those cells in the hippocampus, demonstrating that the memories of the past traumatic experience were erased from the mice’s brains.
As a control, the scientists also showed that switching off other cells of the hippocampus did not lead to the memories being erased, thereby showing that only specific cells are responsible for the retrieval of memory.
“The cortex can’t do it alone, it needs input from the hippocampus,” Wiltgen said. “This has been a fundamental assumption in our field for a long time and the data provides the first direct evidence that it is true.”