Direct measurements in the brain of mice, looking for their way in a maze, show that memory information is sent with another frequency to the mental map than sensory information is. The brain area representing the mental map synchronizes with these frequencies like a radio receiver: it is only tuning into the information that is important at a given time, an international team of researchers led by Francesco Battaglia from the Donders Institute for Brain, Cognition and Behaviour at Radboud University Nijmegen show. This research sheds light on the intriguing question how brain parts choose relevant information from the constant scattering of neurons going on in the brain.
Information transfer within the brain
Using implants in the brains of mice, the neuroscientists have found the first direct proof of the way the brain tuning knob works. The tiny implanted electrodes can collect and send information about individual brain cells at work. Because the devices -- that weigh only two grams and don't seem to bother the free roaming rodents -- measure several cells at a time, the network activity can be followed as well, Francesco Battaglia explains.
The researchers used a maze in the shape of the Pentagon, with five corridors in which a treat was hidden or not. The mice were trained to do their search for the reward from a fixed starting point and started to take the shortest route to the treat quite routinely. To test how the animals are able to navigate to the goal, researchers challenge them by starting them, every once in a while, from a different maze arm than they are used to. Then, mice headed to the usual location of the treat, probably using landmarks to orientate, but other times they just relied on the memorized sequence of left and right turns and ended up in the wrong maze arm.
Data analysis challenge
Meticulous analyses of the brain data revealed that when the mice used their memory the place field cells oscillated in the same frequency as the memory cells (at 35 Hz), but tuned into the vibe of the sensory cells when they appeared to be using landmarks (60 Hz).
To further clarify the different roles of memory and sensory input to the mental map, the researchers repeated the experiment with knock out mice in which the gene coding for the NMDA-receptor was blocked. Battaglia says: 'We know NMDA is important for well functioning synapses, and for oscillations. To our surprise, the knock out mice weren't able to send information from their sensory system to the mental map at all.'