The world floods us with sensation and stimulation. At every moment we are experiencing colors, temperatures, sounds, smells, and tastes. A new study from Brown University finds that our brains have a unique way of paying attention to only some of this ever-present noise. Our brains, researchers say, change frequencies to control the bandwidth of the region responsible for sorting through all the stimulation.

If the neocortex is the brain’s chief executive officer, responsible for making decisions based on raw information gathered by the senses, then the thalamus is the communications officer, tasked with relaying sensory and motor signals to the cortex. However, scientists say it isn’t just a one-way street with information flowing from the thalamus into the cortex. In fact, cortical neurons project 10 times more axons — nerve fibers that conduct electrical impulses — into the thalamus than vice versa.

Axon Action
The cortex sends 10 times as many axons (green) into the thalamus as the thalamus sends into the cortex (red). Connors lab/Brown University

What’s going on? One hypothesis is that the cortex controls the bandwidth of the thalamus. It does this, the theory goes, by using these axon connections to open frequencies of particular interest from the many channels of incoming information. The new study adds support to this idea, while explaining in detail how the cortex manages this.

Alternating Current

To begin their exploration of the brain, the researchers obtained genetically engineered mice that possessed cells in their cortexes that could be turned on and off by simple flashes of light, a technique called “optogenetics.” The researchers decided to focus on the circuits running between the neocortex and the thalamus, the tag team of brain cells that process sensory information gathered by the whiskers of the mice. After they stripped down some brain tissue in the mice to highlight this circuit, the researchers electrically stimulated the cells in the thalamus to mimic how they report sensory information.

Once the thalamus cells were activated, the researchers used light flashes to operate the cells in the cortex of the mice. They wanted to see whether (and how) cortical cell activity affected thalamic cell activity.

They discovered when the cells in the cortex fired at low frequencies (less than one spike per second), the thalamic cells became stifled. The cortical cells, then, had the power to silence the thalamic cells. Next, when the researchers forced the cortical cells to fire faster — 10 times a second — the thalamic cells increased their own activity in turn.

Many neuroscientists predicted the cortex had some easier, more blunt way to mute the thalamus. Instead, these results suggest frequency variations delivered from the cortex suppress or enhance individual thalamic cells.

Having discovered this, the scientists ran other experiments to measure how physical properties of the circuits changed with the different frequencies. They also examined the receptors on the thalamic cells involved in this process. The researchers also generated brainwaves in the mice at the gamma frequency, which occurs both naturally and regularly in real life, and this also stimulated greater activity in the thalamus.

Source: Crandall SR, Cruikshank SJ, Connors BW. A Corticothalamic Switch: Controlling the Thalamus with Dynamic Synapses. Neuron. 2015.