The first thing that comes to my mind when I hear people talk of memories is that they are speaking about something that presents itself as an image to their mind. An image retrieved from the recent to distant past. The image may be of me at about five years of age sitting on the California desert sand watching black ants busily running in and out of an entrance surrounded by a pile of sand miraculously extracted from the ground to make way for their tunnels. Introspection and research makes clear, however, that many types of memories exist including many that are not presented as images to the mind.
A remarkable study reported this summer in the research article “Creating a False Memory in the Hippocampus” (published July 26, 2013 in Science) received a lot of attention in the popular press. The study deserves our attention because of the elegant experimental techniques the research team developed to help observe and understand memory formation and its effects on behavior.
The type of memory the research team created in mice is known as contextual memory. It’s like memory that makes you feel uneasy when you walk to that corner of the garage where some years ago you stepped on a nail that punctured your foot. You formed a contextual memory with the result that you avoid that particular corner of the garage.
Some mice in this study formed contextual memories associated with active neurons labeled red in a part of the brain known as the dentate gyrus. The same contextual memories were activated in the same set of mice at a later point in time but this time the active neurons were labeled green. Mostly the same neurons were labeled with both red and green (emitting yellow) which indicated that the same contextual memory activated the same set of neurons at different points in time (see Figure 1 above). When mice formed different contextual memories, different sets of neurons were active.
Now here is the how they do the false memory trick. Using fiber optic cables, the scientist can activate just the red subset of neurons using red light or the green subset of neurons using green light. Imagine if the neurons that were active in your brain when you stepped on that nail were labeled in red and then, at some later date, reactivated using red light. That’s what was done in this study. While the mouse was exposed to events that resulted in contextual memory associated with the green labeled neurons, the research team used red light to activate the red labeled neurons. It’s as if you felt uneasy and avoided the sunflower growing in the front yard because, while you stood next to the sunflower, the neurons in your head associated with stepping on the nail in the garage were activated using red light.
These are powerful techniques developed to help us understand the brain. However, notice that we remain a long way from understanding how exactly an active set of neurons results in us feeling uneasy. How it is that this uneasiness is brought to mind? Many have speculated but the step from correlation to experience remains a mystery.