Memories Shown to Reside in Small Numbers of Brain Cells

Researchers at RIKEN-MIT Center for Neural Circuit Genetics with help from Stanford’s optogenetics guru Karl Deisseroth have shown that individual memories reside in a very small number of brain cells. Activating just a few cells responsible for holding onto specific memories can bring back rather complex recollections, a finding that may help lead to interventional treatments for conditions like post traumatic stress disorder.

The researchers performed their experiments on mice who were conditioned to fear a certain environment they found themselves in.  After identifying where in the brain that fear resided, they were able to use light, thanks to optogenetics, to activate small groups of cells and bring back the fear even though the environment was not fear-inducing.

Some details from MIT’s announcement:

The researchers first identified a specific set of brain cells in the hippocampus that were active only when a mouse was learning about a new environment. They determined which genes were activated in those cells, and coupled them with the gene for channelrhodopsin-2 (ChR2), a light-activated protein used in optogenetics.

Next, they studied mice with this genetic couplet in the cells of the dentate gyrus of the hippocampus, using tiny optical fibers to deliver pulses of light to the neurons. The light-activated protein would only be expressed in the neurons involved in experiential learning — an ingenious way to allow for labeling of the physical network of neurons associated with a specific memory engram for a specific experience.

Finally, the mice entered an environment and, after a few minutes of exploration, received a mild foot shock, learning to fear the particular environment in which the shock occurred. The brain cells activated during this fear conditioning became tagged with ChR2. Later, when exposed to triggering pulses of light in a completely different environment, the neurons involved in the fear memory switched on — and the mice quickly entered a defensive, immobile crouch.

Press release: Researchers show that memories reside in specific brain cells…

Abstract in Nature: Optogenetic stimulation of a hippocampal engram activates fear memory recall