Know Your Brain: The Thalamus — Shifting Focus

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We tend to think that the world operates exactly how we perceive it. You really can hear birds chirping that loudly on a spring day outside your bedroom window — or find tulips a perfect shade of red. In another scenario, you can eavesdrop on gossip from neighbors, even over a loud air conditioner — or you can gradually fall asleep to the sounds of city traffic outside your apartment. This is why when you take a picture or video of a familiar place, you may find that you can’t help but wince at the quality — maybe even wonder what exactly it was that you were hoping to capture in a single photo.

The answer of course, is that we go through life with our brain focusing on the things that stand out. Our brains are trained to discern human voices from other noises and activity in the cortex acts as a filter, enhancing the parts of daily life that we find the most intriguing. For decades, the cortex was thought to do all the filtering in this process, linking sensory perception and memory. Consequently, it is one of the most extensively researched portions of the brain, but a new study decided to take a different approach.

You may wonder why you’re drawn to certain details of a painting, trying to explain why a great work of art moves you in the way that it does — but what about the things you miss? The brain plays an active role in suppressing things we see and hear every day, just as much as it does in drawing specific details to our attention.

This process involves a pathway between the basal ganglia of the brain to a region called the thalamus, which famed geneticist Francis Crick referred to as the “brain’s gatekeeper” back in 1984. No exact mechanism was found to support this theory, however, until neuroscientist Michael Halassa looked at sensory inputs in the brain traveling to the cortex back.

Halassa found a thin layer of inhibitory neurons that coated the thalamus, known as “thalamic reticular nuclei” (TRN). When mice in the laboratory were awake, Halassa found that this membrane allowed sensory inputs to pass through, and shut them off while the mice were sleeping — acting as a sort of gate.

In 2015, he investigated further — using flashing lights and audio tones to guide lab mice along a maze. The lights would guide them along one path, but the audio cues would appear suddenly to offset their paths. Halassa and his team would provide cues on which signals the mice should pay attention to. When required to follow the lights, neurons activated in the visual TRN interfered with their performance. If these neurons were silenced, the mice had more difficulty processing audio cues. It became clear to the researchers that the brain was narrowing their focus to the task at hand.

In their most recent research, they found that the brain processes information in the prefrontal cortex, moving towards the basal ganglia associated with motor control and a number of functions, and then into the TRN and the thalamus before it moves to other regions throughout the cortex. With a circuit this size, the gatekeeping mechanism is able to not only filter out the other senses to keep us on task, but it can filter what’s important to target with a single sense as well — and it’s all being sorted even before the information reaches the visual cortex.

There’s always the risk that we may miss something as our brain acclimates to new information — but the good news is that it has more than one try to pick it up. What seem like those occasional lapses in attention may actually just be the times that we catch our brain refocusing itself, and the brain is capable of shifting focus four times per second to reassess sensory information and take in new information.

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