Scientists have been searching for ways to better understand how memories are formed in the brain and to potentially weaken harmful or distorted memories in order to treat conditions such as schizophrenia and post-traumatic stress disorder. Understanding how the activity of the brain leads to fearful representations of the world may explain what goes wrong in these disorders.

Memories can be triggered experimentally by stimulating various regions of the brain. Mark Mayford and researchers at The Scripps Research Institute in La Jolla, CA, in collaboration with scientists at the University of Oregon in Eugene, have developed a new method of triggering memories by creating transgenic mice with a genetic 'on/off switch.'

These transgenic mice were first put into a box, activating a group of neurons involved in forming a memory of the box. The researchers next put each mouse into a different box and injected it with a drug to turn on the neurons involved in forming the memory of the first box. This induced the mice to involuntarily remember the first box while in the second box. They then gave the mice a small shock. Normally, the shock would condition the mouse to fear its current environment, the second box, but instead, the mouse was conditioned to fear a combination of the two contexts. Only when the mouse had been injected with the drug to induce the memory of the first context and was present in the second context did it exhibit a fear response (Science 335, 1513–1516; 2012).

In a similar study, a team of scientists led by Susumu Tonegawa (Massachusetts Institute of Technology, Cambridge) used a new technology called optogenetics to genetically engineer mice so that when a memory was formed, the neurons involved became responsive to light. In this experiment, a mouse was placed in a box and given a shock. The mouse was then put in a different box, and pulses of light were delivered through optic fibers implanted in the mouse's brain in order to activate the neurons involved in forming the memory of the first box. In response to the activated memory of the box where it had been shocked, the mouse froze even though it was in a totally different context (Nature doi:10.1038/nature11028; published online 22 March 2012).

These experiments demonstrate that artificial perceptions produced when memory-associated neurons are activated can seem just as real as the actual environment. In the future, researchers hope to find ways to deactivate these neurons in order to disrupt harmful thought processes.