Brain parcellation is the process of dividing the brain into distinct, measurable regions to understand its complex organization. This mapping is necessary because the brain is not a uniform mass but a collection of specialized territories, much like a large country is organized into states or provinces. Each of these regions, or “parcels,” is assumed to have a unique set of characteristics, whether based on physical structure, cellular makeup, or functional activity. By creating these precise anatomical and functional maps, scientists can systematically investigate how different parts of the brain contribute to thought, emotion, and behavior.
The Core Purpose of Brain Parcellation
Scientists undertake brain parcellation primarily to gain a foundational understanding of functional specialization within the brain. The division of the brain into parcels allows researchers to localize specific mental processes, answering the question of where a particular task or function resides. This approach moves the analysis of complex imaging data from millions of individual points, or voxels, to a manageable number of distinct regions.
Parcellation is also performed to establish how different brain regions communicate, a concept known as connectivity. By defining the boundaries of each parcel, researchers can measure the flow of information between them, which is essential for understanding how networks are formed and how they coordinate complex behaviors. These maps also create a common coordinate system and lexicon, allowing scientists worldwide to compare findings accurately across different individuals and studies.
Traditional and Structural Parcellation Approaches
Historically, brain parcellation relied on examining static, physical differences in post-mortem brain tissue. The primary method was Cytoarchitecture, which involves staining and observing the distribution, density, and structure of neuron cell bodies under a microscope. This microscopic observation revealed that the six layers of the cerebral cortex vary systematically in their cellular composition across the brain.
This technique led to the creation of the famous Brodmann areas in the early 20th century, which divided the human cortex into 52 distinct areas based on these cellular differences. Although based purely on structure, these areas often correspond closely to functional regions, such as area 17 being the primary visual cortex. Another structural approach uses the brain’s macroscopic folding pattern, where the visible ridges, or gyri, and grooves, or sulci, serve as landmarks. While the boundaries of primary sensory and motor areas often align well with these major folds, the relationship is less predictable in higher-order association areas.
Modern Functional Mapping Techniques
Modern parcellation shifts the focus from static structure to dynamic activity and communication, relying on advanced non-invasive neuroimaging. One powerful approach is Resting-State Functional Connectivity (RSFC), which measures the spontaneous, synchronized fluctuations in the blood oxygen level-dependent (BOLD) signal while a person is at rest. Regions that show highly correlated BOLD signals are grouped into the same parcel, as they are assumed to be communicating and part of the same functional network. This method has been instrumental in identifying large-scale functional networks, such as the Default Mode Network, which are active when the brain is not focused on an external task.
Another method is Diffusion Tensor Imaging (DTI), which maps structural connectivity by tracking the movement of water molecules along white matter tracts. Since water diffuses more easily along the direction of the nerve fibers, DTI allows scientists to reconstruct the pathways, or “highways,” that connect different gray matter regions. By identifying which regions are physically connected by dense fiber bundles, researchers can define parcels with similar connection profiles, creating a connectional fingerprint for each area.
Task-based fMRI maps functional regions by measuring the BOLD signal change as a person performs a specific action, like tapping a finger or speaking. This technique highlights areas that are actively recruited during a given task, with areas showing similar activity patterns often grouped together into a single parcel.
How Parcellation Informs Research and Medicine
The resulting brain parcellation maps transition from a methodological tool to a practical resource with significant real-world applications. In a clinical setting, these maps are used to guide neurosurgery, particularly for tumors or epilepsy, by helping surgeons identify and avoid eloquent cortex—regions responsible for motor control, language, or sensation. Using individualized parcellations, surgeons can determine the precise location of functional boundaries, which can shift significantly due to a disease process, thereby maximizing the removal of diseased tissue while preserving function.
In research, parcellation maps are foundational for understanding neurological and psychiatric disorders. By applying these maps, scientists can identify which specific networks are disrupted in conditions like Alzheimer’s disease or schizophrenia, which are often characterized by altered connectivity between regions. For instance, a disease may not simply affect one area but instead weaken the communication between two distant, functionally defined parcels.
Parcellation also contributes to personalized medicine by acknowledging that the precise location and size of functional areas vary between individuals. This allows for the development of highly specific interventions, such as guiding personalized targets for non-invasive brain stimulation therapies for conditions like depression.