From Train Station to Control Center—The Thalamus’ Role Gets an Upgrade

When you see something—a tree in your backyard, say, or the toy your toddler hands you—that visual information travels from your retinas to your brain. And like a train stopping at stations along its route, the information pauses at particular regions of the brain where it’s processed and sent along to its next location.

A region called the visual thalamus has been thought to be primarily a relay, simply directing visual information to its next area. But a new study published Aug. 13 in Neuron finds that the thalamus actually integrates additional information from other brain regions and reshapes the information it sends along to the brain cortex.

Liang Liang, PhD, assistant professor of neuroscience at Yale School of Medicine (YSM) and senior author of the study, suspected the thalamus might be doing more than it had been given credit for.

“Previous studies have found that information sent to the visual thalamus from the retina only accounts for around 10% of its total inputs,” she explains. “There are many from several other brain regions, but their roles have been largely mysterious.”

Liang wanted to know what those other inputs were doing, as part of her greater interest in understanding how the visual system works and the principles of information processing in the brain. So she and her lab investigated those arising from a midbrain region called the superior colliculus.

The retina sends information to several brain regions, but it has two main targets. One is the visual thalamus, part of a pathway responsible for image formation. The other is the superior colliculus, which is associated with reflexive visual responses, such as ducking when you see a ball flying towards you, or, importantly for animals, dodging a predator.

That the superior colliculus receives its own input from the retina and then talks to another region that also receives retinal information, made it especially intriguing to Liang.

“The fact that these two paths talk to each other at this early processing point is very interesting,” she says.

To see how these two information streams interact in the thalamus of mice, Liang’s team introduced genetically encoded indicators into the cells coming from the retina. Doing so caused the very ends of the cell, the boutons, to fluoresce green when they sent a signal to another cell. The researchers did the same for the superior colliculus boutons but instead had them fluoresce red.

“So then we could monitor the activity from each while the mice watched movies,” says Liang.

The researchers found that the inputs did not meet randomly; they were highly organized. “That means that the brain really makes an effort to wire them together during development,” Liang explains.

Further, inputs that shared similar properties clustered together, indicating that they delivered similar information to the same thalamic cell. To gain a deeper understanding of how this information convergence affected the thalamus and its signals, the researchers silenced cells from the superior colliculus.

“When we did that, it suppressed the amplitude of the thalamic cells’ visual responses,” says Liang. “And it especially reduced motion selectivity in cells tuned to specific motion directions.”

What this means, says Liang, is that inputs from the superior colliculus contribute to the computation of motion in the thalamus.

“There’s substantial computation going on to enrich and selectively enhance visual information before it even gets to the cortex,” she says.

The team is now investigating some of the other inputs into the visual thalamus to see if they shape information processing as well.

“We’re also looking at the cells that receive information from these inputs,” says Yue Fei, PhD, lead author of the study and a graduate student in Liang’s lab who recently defended his dissertation. “We want to see how they make use of that information.”

The research reported in this news article was supported by the National Institutes of Health (award R01EY034697)) and Yale University. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The work was also supported by the Smith Family Foundation, Whitehall Foundation, E. Matilda Ziegler Foundation, Klingenstein-Simons Fellowship Award, Lawrence Young Investigator Program, Yale College, and the Wu Tsai Institute.