Light-Triggered Genes Reveal the Hidden Workings of Memory

Susumu Tonegawa's presence announces itself as soon as you walk through the door of the Massachusetts Institute of Technology's Picower Institute for Learning and Memory. He had been using transgenic mice in his immunology studies, knocking out particular genes and observing the physical effects, and he used a similar approach to uncover the biological basis of learning and memory.

This past summer, along with Roy and other colleagues, he reported that - contrary to neuroscience dogma - the neural circuit in the brain structure called the hippocampus that makes a particular memory is not the same circuit that recalls the memory later.

In addition to revealing that different mechanisms control memory formation and recall, Tonegawa, Roy and their colleague Takashi Kitamura have shown that memory formation itself is unexpectedly complex. The road to that discovery started back in 2012, when Tonegawa's lab came up with a way to highlight brain cells known as engram cells, which hold a unique memory.

A day later, they returned each mouse to the cage and illuminated its brain to activate the brain cells storing the memory. The researchers called these cortical cells "Silent engrams" because they contained the memory but did not respond to a natural recall cue.