The brain operates not as a collection of isolated regions, but as a dynamic set of interconnected teams known as functional networks. These networks are formed by distinct brain areas showing synchronized activity over time, and their coordinated efforts give rise to our cognitive abilities. Think of it as a social network where groups of regions collaborate to perform specific functions.
How Brain Networks are Identified
Scientists primarily identify these networks using functional Magnetic Resonance Imaging (fMRI). This technology tracks a proxy for brain activity: changes in blood flow. When a brain area is more active, it requires more oxygen delivered by the blood, and the fMRI machine detects the magnetic differences between oxygenated and deoxygenated blood to map brain activity.
A powerful method for this is “resting-state fMRI,” where a person lies still in the scanner without performing a specific task. By analyzing this resting state, researchers observe which brain regions show synchronized fluctuations in activity. These correlated patterns reveal the brain’s underlying networks that exist even when we are not engaged in a goal-oriented activity.
While fMRI provides excellent spatial resolution, other technologies offer complementary information. Electroencephalography (EEG), for instance, measures the brain’s electrical activity directly with high temporal resolution. This captures the rapid timing of neural communication, and combining methods like EEG and fMRI provides a more comprehensive picture of network dynamics.
Major Functional Brain Networks
Among the dozens of networks researchers have identified, a few are consistently observed and well-studied. These core networks form the foundation for a wide range of mental processes, from daydreaming to intense focus.
One of the most studied is the Default Mode Network (DMN). The DMN is most active when the brain is at rest and not focused on the outside world. It is associated with internal processing, such as self-reflection, recalling autobiographical memories, imagining the future, and mind-wandering. The DMN involves regions like the medial prefrontal cortex, posterior cingulate cortex, and the hippocampus.
In contrast to the inward focus of the DMN, the Central Executive Network (CEN) is the brain’s “task-master.” This network is engaged during cognitively demanding, goal-oriented tasks that require externally directed attention. When you are solving a puzzle or making a decision, your CEN is hard at work. Areas involved include the dorsolateral prefrontal cortex and the posterior parietal cortex.
Acting as a moderator between these networks is the Salience Network (SN). The primary role of the SN is to detect and filter important internal and external stimuli to determine what deserves our attention. Anchored by the anterior insula and the dorsal anterior cingulate cortex, the SN constantly monitors the world for noteworthy information.
Networks and Everyday Cognition
The brain dynamically shifts resources between networks depending on the demands of the moment. This interplay allows for fluid and adaptive cognition, ensuring the most appropriate network takes the lead for a given situation.
A common example is the relationship between the Default Mode Network (DMN) and the Central Executive Network (CEN), which often have an antagonistic relationship. When you are daydreaming, the DMN is highly active while the CEN is suppressed. If an external event suddenly requires your attention, like a car horn honking, the Salience Network (SN) detects this sensory input. The SN then acts as a switch, signaling for a decrease in DMN activity and an increase in CEN activity, allowing you to shift your focus to the external environment.
When Brain Networks Go Awry
The balance of functional brain networks is fundamental to healthy cognition. Disruptions in their connectivity are increasingly linked to neurological and psychiatric disorders, as alterations in communication can manifest as the symptoms seen in a wide range of conditions.
For instance, an overactive Default Mode Network (DMN) is frequently associated with depression, which is thought to underlie symptoms of rumination and excessive self-focused thought. In Alzheimer’s disease, the DMN is also affected, but in this case, the network shows degeneration and reduced connectivity, which correlates with memory impairments.
Dysfunction is not limited to a single network, as conditions like schizophrenia and ADHD are characterized by poor coordination between multiple networks. In schizophrenia, there is evidence of abnormal communication between the Salience Network, DMN, and Central Executive Network, which may contribute to difficulties distinguishing internal thoughts from external reality. Research also suggests that ADHD may involve atypical interactions between these same core networks, affecting attention and cognitive control.