In both mice and men the parts of the brain involved in memory formation are the hippocampuses. These, however, contain millions of nerve cells, and the formation of any given memory involves but a few of them. In order to study false-memory formation, Mr Ramirez and Dr Liu had therefore to isolate and manipulate only those cells involved in storing a particular memory they had deliberately created.
They did this by fitting as many hippocampal cells as possible with a clever genetic switch. To work at all, this switch has first to be primed by a process (known as an action potential) which a nerve cell undergoes during learning—meaning that it can operate only in those cells that have learned something. Thus did Mr Ramirez and Dr Liu confine their experiment to the relevant cells.
Once primed, the switch can be activated by shining a light at it. This activation mimics an action potential—fooling the cell into learning whatever is happening to the animal at the time, in addition to what it had already learned, thus linking the two things in the animal’s mind. The researchers could therefore choose which events they would try to conflate into a single false memory.
Having smuggled their switches into the animals’ hippocampuses using viruses, Mr Ramirez and Dr Liu were ready to start the experiment proper. On the first day, they put their mice, one at a time, into a cage. As the animals explored this safe environment, they formed memories of what they saw and the switches in the relevant nerve cells were primed.
On the second day, the researchers put the mice in another cage, whose interior looked different. Here, they shone light into the animals’ brains, to activate the primed switches. At the same time they shocked each animal’s feet with electricity.
Mr Ramirez and Dr Liu knew from previous experiments that such shocks are so traumatic that a mouse will usually freeze if it is put into a cage where it has previously been shocked. But they also found, as they had hoped, that the mice froze when put back in the first, safe cage. The process had, in other words, created a false memory that this cage was dangerous by superimposing the memory of the shock on the memory of the cage.
The experiment did not stop there. To show just how powerful the new false memory was, Mr Ramirez and Dr Liu then put each mouse into a completely new safe cage and, once again, shone a light into its brain to throw the switch and activate the nerve cells. In response, the mice froze just as if they had had their feet shocked. Once a false memory is formed, it seems, it will be remembered whenever and however the nerve cells supporting it are activated.
Just as Dr Loftus implanted false memories in her volunteers, then, Mr Ramirez and Dr Liu have implanted them in mice. They have not shown how the process happens naturally, since they had to stimulate the memory cells directly to achieve the effect. Natural false memories would require some as-yet-undiscovered analogue of this process. Nevertheless, by shining a real light into the brains of their mice, they have shone a metaphorical one onto the whole, vexed question of why it is that some people so firmly believe in things that actually did not happen.