Did the title make you recall the phrase “The link between you and me, this is the sound within us,” once shouted in a popular hip-hop music competition show? If so, it’s working! Let’s imagine together what happens in our brains when we watch a performance of our favorite music. As we listen to the music, watch the artist's expressions and dance moves, and reflect on the lyrics, sometimes feeling as if the words speak directly to our own experiences, all these processes are the result of the brain's external information processing and internal modeling. The human brain is an incredible system that processes diverse sensory information from the outside and combines it with what we already know, building a profound understanding of the world.

What is the Brain's Information Processing System? External Processing and Internal Modeling

The fundamental principle of brain function is external information processing and internal modeling. External processing is the rapid analysis of sensory stimuli from our eyes, ears, and touch. Internal modeling, on the other hand, is a more complex process where the brain forms inferences, meanings, intentions, and causal relationships based on the external information. These two systems of information processing are crucial for humans and higher primates to adapt and survive in changing environments.

Our brain’s large-scale functional networks are responsible for these two processes, primarily located in the cerebral cortex. The cerebral cortex is the brain’s outermost layer, playing a vital role in perceiving the external environment, solving problems, and performing higher-level cognitive functions. Therefore, understanding how the connections between the cerebral cortex and other brain regions form and develop is key.

The Key Mechanism of Brain Development: Thalamus-Cerebral Cortex Connectivity

One of the central mechanisms of brain development is the connectivity between the thalamus and the cerebral cortex. The thalamus, located at the brain’s center, processes various sensory inputs such as vision, hearing, and touch and relays them to the cerebral cortex, helping us perceive and respond to our surroundings. Recent research has revealed that the thalamus not only handles sensory information but also plays a role in higher cognitive functions like internal modeling, suggesting that the connection between the thalamus and the cerebral cortex is closely linked to the brain’s functional specialization.

To investigate the impact of thalamus-cerebral cortex connectivity on brain development, researchers analyzed functional MRI (fMRI) data from infancy to adulthood. They tracked how this connectivity changes with age and examined gene expression related to this connection through genomic analysis. The study revealed that thalamus-cerebral cortex connectivity serves different roles throughout development from infancy to adulthood.

In infancy, there was a strong connection between the thalamus and the sensory-motor network, a period when genes related to brain development are highly active. During this phase, the thalamus processes external sensory information and relays it to the cerebral cortex, laying the foundation for brain function. However, as individuals reach adulthood, connectivity with the salience network becomes dominant, reflecting the separation between external processing and internal modeling systems (refer to Figure 1). This shows how brain function becomes more specialized as high-level cognitive functions mature. These findings were published in Nature Neuroscience, a leading journal in brain science, in June of this year.

[Figure 1] Changes in Thalamus-Cerebral Cortex Connectivity Across Brain Development

(a) In infancy, thalamus-cerebral cortex connectivity reflects the initial differentiation of the sensory-motor network. (b) During childhood and early adulthood (ages 8-22), connectivity with the salience network establishes and differentiates between internal and external networks. (c) A schematic of the distinction between internal and external axes in the brain and the critical role of the salience network in children and adults.

Computational Simulations and Causal Analysis

Researchers used computational simulations to verify the effect of thalamus-cerebral cortex connectivity on functional network formation (refer to Figure 2). When the thalamus-cerebral cortex connectivity rules were disrupted in simulations, the separation between networks responsible for external information processing and internal modeling failed to occur. This highlights the critical role of thalamus-cerebral cortex connectivity in the brain's functional network differentiation. Notably, disruptions occurring after the age of 12 were particularly hard to recover from, underscoring the significance of the 12-18 age range for brain development.

[Figure 2] Computational Simulation of Thalamus-Salience Network Connectivity Disruption Model

(a) The disruption model for thalamus-salience network connectivity across developmental ages. Four disruption models were tested: for ages 8-12, 12-18, 18-22, and all age groups, compared to an undisturbed model. (b) The separation index (salience-external, salience-internal) was calculated for each model and expressed as a percentage difference from the undisturbed model. (c) Visual representations of the functional network distributions highlight how these disruptions affect brain connectivity development.

Contribution to Brain Disease Research and Treatment

This study marks the first time the role of thalamus-cerebral cortex connectivity in brain development has been revealed, an important breakthrough. It is expected to provide valuable insights into understanding the mechanisms behind various brain disorders, such as autism spectrum disorder and schizophrenia. These conditions can arise due to problems in functional connectivity between specific brain regions, and understanding thalamus-cerebral cortex connectivity could offer vital clues for early diagnosis and personalized treatment of brain disorders.

Today, we explored how external information processing and internal modeling are functionally separated in the brain and the critical role that thalamus-cerebral cortex connectivity plays in this process. So next time you're watching a concert or listening to music, why not cheer on your brain, which is working hard to process external information and update its internal model?