Identifying and Enginerring Memory-Holding Neurons and Their Circuits

Throughout the history of mankind, human beings have been fascinated by memory—how do we form, store, and remember memories? How do we lose them?

The idea that memory is stored in the brain as “persistent physical changes” goes back at least to Plato, but modern formulation of this hypothesis had to wait until the turn of the 20th century, when Richard Semon coined the word “engram” 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 so that an animal’s memories, thoughts, and emotions can be manipulated—ideas that have existed only in the realm of science fiction, until just several years ago. The hypothesis has been that when you encounter an episode, a small population of cells deep inside your brain, called the hippocampus, fires. This is followed by persistent physical changes in these cells. You will recall this memory only when external stimuli re-activate these engram cells. These hypotheses have now been proven to be correct by using a technology called optogenetics. The key molecule for optogenetics is a light-sensitive protein called channelrhodopsin, extracted from green algae. A scientist can insert channelrhodopsin into memory engram cells. Subsequently, a scientist can activate these cells with blue light delivered through an optic fiber. This results in retrieval of the specific memory without relying on natural recall cues.

Using such techniques, researches have been able to implant false memory in the brains of lab animals, with the hope that this will further our understanding of false memory in humans. We can also engineer memory engram cells to cure depression and restore memory from animal models of early stage Alzheimer’s.