Memories may be sorted by the thalamus before being stored long term

The thalamus – a structure in the centre of the brain that relays information from the senses – may be responsible for sorting memories before they are stored long term, a study in mice suggests. Previously, the thalamus has been largely overlooked in research on memory processing.

Priya Rajasethupathy at the Rockefeller University in New York and her colleagues studied the brain activity of dozens of mice navigating a virtual maze. When the mice’s thalamus was stimulated while learning the maze, the animals were able to recall the routes they learned weeks later – by which point they would ordinarily have forgotten.

The process of forming and storing memories involves multiple regions of the mammalian brain. Neuroscientists have largely focused their attention on the hippocampus, a region earlier studies identified as critical for forming new memories, and the cortex, where long-term memories are stored. But researchers weren’t sure where memories travelled between the time they formed in the hippocampus and when they ended up in the cortex.

To watch memories being formed in real time, the researchers used a new imaging technology that allowed them to see the electrical activity of individual neurons in the hippocampus, thalamus and cortex simultaneously. They combined brain imaging with a virtual reality screen, a Styrofoam ball and a sugar-water reward system.

By running atop the rotating Styrofoam ball, the mice could navigate the virtual maze to access three different real-world outcomes: a lot of sugar water, a few drops of sugar water or an unpleasant puff of air to the face. At the end of the maze training, the mice had more activity in their thalamus, suggesting to the researchers that the region may be playing an important role in memory.

“The thalamus hasn’t been appreciated really as a cognitive structure that’s important for memory and memory-guided processes, so when we first found this, we were kind of just struck,” says Rajasethupathy. After three weeks, they tested the rodents’ memory while either stimulating or curbing the function of the thalamus using a technique called optogenetics, in which researchers can turn neurons on or off with light.

Mice that received a thalamus boost during the virtual reality game were able to recall memories weeks later that would have otherwise been forgotten, like the path to a meagre few drops of sugar water. Mice that had their thalamus suppressed could learn and recall routes in the short term but could not form long-term memories. “That was a very clear indication to us that they really need the thalamus to consolidate memories to long term,” says Rajasethupathy.

Because mice are used as model organisms to illuminate characteristics often shared across mammals, this memory pathway could hold true for humans, too. “This study only looks at mice at the moment, but it is definitely interesting and will undoubtedly prompt much interest in the [thalamus] of humans in relation to memory,” says Dorothy Tse at Edge Hill University in the UK.