By Sara Reardon, ScienceNOW
Snuffling around in a Plexiglas box that it knows well, a black and white rat catches a whiff of chocolate cookies. It scampers toward them—but suddenly, it finds itself teleported into another, equally familiar box. One could hardly blame the poor rat for being confused and disoriented for at least a fraction of a second, and researchers have now figured out why: cells in the memory center of its brain compete over where it is for exactly one-eighth of a second.
The “teleportation” effect in rats is similar to the momentary disorientation you feel when elevator doors open and you step out onto the wrong floor. It occurs because the place you expect to see and the place you actually do are “mutually exclusive,” says Edvard Moser, a neuroscientist at the Norwegian University of Science and Technology in Trondheim. Normally, the brain orients itself gradually as you move. The hippocampus, the brain’s memory center, contains neurons known as place cells, which record both your environment and your movement within it in order to form memories that ensure you always know where you are. To update the brain on your position, place cells fire in a rhythm called a theta oscillation, which repeats itself every 125 milliseconds and is especially prominent when you’re moving.
To teleport rats, Edvard Moser and his wife, neuroscientist May-Britt Moser, built two rat boxes connected by a tunnel. One box had a circle of white light-emitting diodes shining up through the clear floor, and the other had a row of green LEDs around the ceiling. The researchers let a rat run back and forth between the two boxes and forage for food until it became familiar with both. They also implanted an electrode array into the rat’s hippocampus and recorded firing patterns from individual neurons while the rat was in each box.
Then the researchers played a mean trick. They put the rat into the white box and placed some cookie crumbles at one end. While the rat was running toward the treat, they switched the light pattern, fooling the rat into thinking it was suddenly in the green box.
The rat still managed to find its cookies, but when the researchers looked at the recordings from the place cells, they saw a war going on. At the moment of “teleportation,” one group of cells was firing with the pattern that it had used in the white box, but another group fired with the pattern corresponding to the green box. The neurons sorted it out eventually: Within 125 milliseconds, they were all firing together, which is the amount of time that a theta cycle takes to complete, the researchers report online today in Nature. The fact that the two distinct patterns stuck around to fight it out rather than slowly drifting from one pattern to the other suggests that the brain puts memories into discrete, 125-millisecond packages, preventing itself from mixing them up.
The conflict, Edvard Moser says, likely occurs because although the light change was instantaneous, the illusion wasn’t. The visual system told the rat that it was in a new place, but the number of footsteps it had taken, the smell of the box, and other variables said it was still in the old one. The researchers hope to look at other oscillation patterns next, such as the gamma wave, which repeats itself several times faster than the theta wave.
“It’s certainly interesting,” says Neil Burgess, a cognitive neuroscientist at University College London. It would be difficult, he says, to pin down similar phenomena in humans, for whom theta oscillations are more difficult to measure, or to know what the findings might say about memory disorders such as Alzheimer’s disease. But he expects it to attract a lot of interest from other researchers. “Theta, whatever it’s doing,” he says, “is very pervasive” in neuroscience.
This story provided by ScienceNOW, the daily online news service of the journal Science.
Image: Klara Kim/Flickr