The thickness of the outer layer of the brain influences the way neurons treat information

Researchers at the University of Queensland have discovered that the thickness of the outer layer of the brain influences how neurons deal with information.

The results challenge understanding of how brain circuits function throughout the brain.

The outer layer of the brain (the cortex) is made up of many "microcircuits" of smaller brain cells, each consisting of about 10,000 interacting neurons in a space as small as 1 square millimeter. There is a long-standing theory that the neurons of these microcircuits work and interact in a very standard way. Thus, by extension, all microcircuits must operate substantially identically.

If this is the case, if you can understand the operation of a microcircuit, then you can understand the rest.

Professor Stephen Williams and researcher Lee Fletcher of the Queensland Brain Institute (UBI) at UQ wanted to know if this is really true: does a defined class of neurons work the same way, no matter what? Where they are in the cortex?

Using high resolution MRI imaging of the rodent brain, they examined the structure of the cortex and found that the thickness gradually increased up to 3 times. back of the brain to the front.

They also discovered that gradual changes in thickness can be observed even within a single functional area of ​​the brain. For example, the front of the visual processing area is thicker than the back.

When Williams and Fletcher took a closer look, they discovered that the thickness of the cortex is directly related to the length of individual neurons: a neuron located in a thicker area of ​​the cortex is more elongated than a neuron located in a thinner area. This, in turn, raises a critical question, says Williams, "do they always work the same way?"

He explains that to understand this, we need to step back and see how the brain deals with information.

An important comparison

In addition to treating sensory inputs, the brain also generates an internal model of the world, he says. In addition, the brain constantly compares sensory information with its internal model.

In fact, such comparisons are thought to occur even at the level of individual neurons, says Williams.

For a neuron to be able to compare information from two very different sources – the internal model and the senses – it needs two "integration zones" that interact.

One area of ​​the neuron integrates information from the internal model and another integrates information from the senses. Because they are distinct and interact, it is possible to compare the two types of information.

Williams and Fletcher examined the function of longer neurons located at the thicker end of the visual cortex. They found that these neurons were functioning as expected: they had two zones of integration.

But this was not the case for shorter neurons.

"It turns out that they work quite differently," Williams says. "Electrically, they are even more distinct than we thought."

Indeed, the shortest neurons only have an integration zone for the processing of sensory information and the internal model. Therefore, they can not compare the two types of entries.

"One can perform a comparative integration, the other not," Williams explains.

Thus, depending on the part of the cortex where these neurons are located (thicker or thinner areas), these neurons work in a very different way.

"Our work demonstrates that the thickness of the neocortex governs not only the anatomical structure of neurons, but also their electrical properties," says Fletcher.

"The results reveal the complexity of the computer strategies used in the neocortex and suggest that it is composed of flexible circuits in terms of calculation."

"These findings challenge the fundamental theories of how the neocortex works and pave the way for future research on the functioning of the networks of these brain circuits."

The next step is to explore how these differences lead to cognition and behavior, says Fletcher.

After all, given the importance of maintaining and updating our internal model of the world, why do some neurons decide not to participate?

A high speed system for fast reactions?

Williams explains that there are clues about how rodents react to certain visual stimuli.

Different areas of the visual cortex explore distinct areas of the visual space, he explains. The front of the visual cortex treats the visual information coming from the space in front of the rodent, while the back part of the visual cortex treats the visual information from above and slightly behind the rodent .

In addition, rodents have been shown to be extremely sensitive to visual stimuli from above because this is where predators are found. If a shadow passes over, they freeze or run.

"The treatment of information may not be useful in such a system, but rather an instinctive reaction," says Williams. "To detect and react to predators, a wired system that works efficiently and very quickly can be a survival value."

"So, an idea is that these little neurons are hypersensitive to inputs," says Williams, "this information will spread directly to the output of the potential for action without requiring additional integration, thus forming a high-powered system. speed."

Indeed, the area of ​​the visual cortex sensitive to stimuli from above is the thinnest area, exactly where the shortest neurons are.

In other words, flexible computer circuits that allow the brain to make comparisons between an internal model of the world and the senses, while providing shortcuts when needed, are probably very useful for allowing animals to interact with a complex environment, while reacting quickly survive.