 So, the idea that memory is stored in the brain, the persistent physical changes, goes back to Plato. But the modern formulation of this hypothesis had to wait until the turn of the 20th century when Richard Zimmer coined the word engrams for these persistent changes. We have now identified a population of brain cells that hold specific memory. Not only that, we can now engineer these cells with light so that animals, memories, emotions, and even thoughts can be manipulated. So this is the idea that has existed only in the realm of science fiction until recently. When you encounter episodes, a population of cells deep inside your brain fires. And then this will be followed by persistent changes in these cells. You can recall this memory only when external stimuli will reactivate these cells. So these hypotheses have now been proven to be correct by using a technology called optogenetics. The key molecule of the optogenetics is a light-sensitive protein called chanelodopsin which is extracted from green algae. Scientists can insert chanelodopsin into memory cells. Subsequently, scientists can even activate these cells with a blue light which is delivered deep inside the brain with optic fibers. Now memory is usually quite reliable, but under certain conditions humans make incredible false memory. For instance, after arresting John Doe I or Timothy McVeigh following the Oklahoma City bombing in 1995, some of you may remember, the testimony based on the false memory caused nationwide manhunt for the second John Doe II who never existed. We can implant false memory in the brain of mouse. For that purpose you let the mouse stay in the blue box and let the mouse form the memory of the blue box and then you can label these cells with the chanelodopsin. Subsequently, when the same mouse receives the mild foot shock in the red box which is very different from a blue box, at the same time the animal is forced to recall the memory of a blue box, then if you let the animal return to the blue box, they will be scared. Now, this is the demonstration of formation of false memory because this mouse has never been shocked in the blue box. Depression is a terrible brain disorder which afflicts 350 million people worldwide. Depression is often caused by chronic stress which precipitates cities of negative memories. Now we know now that the negative and the positive memory compete each other in the brain network. Using this principle, we have recently made very exciting discovery that is to cure depression with optogenetic technology. For this purpose, male mouse form pleasure memory by playing with female mouse and then subjected to chronic stress treatment and fall into depression. We could cure the depression of this male mouse by activating the positive memory in the cell with light. This can also cure the memory problem of mouse which is an early stage of Alzheimer's disease. This mouse can form memory but cannot retrieve the memory. But we could restore the memory recall of this mouse by using optogenetic technology. So optogenetics has demonstrated proof of concept for a variety of diseases and for possible therapy. The big question is now can we convert or translate these findings made with animal models into therapy for human patients?