The cerebrum is the largest part of the brain, making up about 80% of total brain volume and weighing roughly 2 to 2.5 pounds in an adult. It sits above and in front of the other major brain structures, filling most of the skull, and handles everything from conscious thought and movement to language, memory, and sensory processing.
The average adult brain weighs between 2.6 and 3.1 pounds total, so the cerebrum alone accounts for the vast majority of that mass. By comparison, the cerebellum (the second-largest structure, tucked beneath the back of the cerebrum) takes up only about 11% of the cerebrum’s volume. The brainstem, which connects the brain to the spinal cord, is smaller still.
How the Cerebrum Is Organized
The cerebrum is split down the middle into two halves, called the left and right hemispheres. Each hemisphere is further divided into four lobes, each with its own primary responsibilities. A thick bridge of nerve tissue called the corpus callosum connects the two halves. It contains roughly 200 million nerve fibers that constantly shuttle information between hemispheres, allowing the two sides to coordinate sensory input, movement, and complex thinking in real time.
The outer layer of the cerebrum is the cerebral cortex, a thin sheet of densely packed neurons (gray matter) responsible for most higher-level processing. If you could flatten the cortex out, it would cover about 1,570 square centimeters, roughly the size of a large newspaper page. About 70% of that surface area is hidden from view, folded into grooves called sulci. The raised ridges between the grooves are called gyri. This folding is what gives the brain its wrinkled appearance, and it exists for a practical reason: it dramatically increases the amount of processing tissue that fits inside the skull.
Beneath the cortex lies white matter, made up of long nerve fibers coated in a fatty insulating layer. These fibers act as the brain’s internal wiring, connecting different cortical regions to each other and to deeper structures. Deeper still are clusters of gray matter called subcortical structures, which play roles in emotion, habit formation, and relaying sensory signals.
The Four Lobes and What They Do
Frontal Lobe
The frontal lobe is the largest of the four lobes, occupying the front portion of each hemisphere behind the forehead. It is central to personality, decision-making, planning, and voluntary movement. It also contains a specialized area (Broca’s area) that coordinates the muscle movements needed to produce speech. Parts of the frontal lobe process smell as well. When people describe someone’s personality changing after a brain injury, frontal lobe damage is often involved.
Parietal Lobe
Sitting behind the frontal lobe, toward the top and middle of the head, the parietal lobe processes touch and pain sensations from the body. It also helps you understand spatial relationships, like judging the distance between your hand and a coffee cup or navigating through a room without bumping into furniture. The parietal lobe houses Wernicke’s area, a region critical for understanding spoken language. While Broca’s area in the frontal lobe helps you produce words, Wernicke’s area helps you comprehend them.
Temporal Lobe
The temporal lobes sit on either side of the brain, roughly behind the ears. They are essential for hearing, language comprehension, and memory formation. Damage to the temporal lobes can make it difficult to form new memories or to recognize familiar faces and sounds.
Occipital Lobe
The occipital lobe occupies the back of the brain and is almost entirely dedicated to processing visual information. Signals from the eyes travel here first, where the brain begins interpreting shapes, colors, and motion. Even though your eyes are at the front of your head, the core work of “seeing” happens at the very back.
Left Hemisphere vs. Right Hemisphere
The two hemispheres of the cerebrum look nearly identical, but they handle certain tasks differently. In most people, the left hemisphere is more heavily involved in language, logical reasoning, and sequential thinking. The right hemisphere tends to take the lead on spatial awareness, emotional processing, and recognizing nonverbal cues like facial expressions and tone of voice. Vision and attention also lean toward the right hemisphere in many tasks.
That said, the popular idea that people are either “left-brained” or “right-brained” is a significant oversimplification. Both hemispheres are active during virtually every task, constantly sharing data through the corpus callosum. The specialization is real but relative: one side may contribute more to a given function, but neither side works in isolation.
Why Cortical Folding Matters
The degree of folding in the cerebral cortex varies from person to person, and those differences are linked to differences in cognitive ability and behavior. Research shows that alterations in cortical folding patterns are strongly correlated with changes in brain function and, in some cases, with neurological and psychiatric conditions. The major grooves (primary sulci) tend to line up reliably with specific functional zones across individuals. The smaller, shallower grooves are more variable and correspond to higher-order association areas involved in complex cognition, making them less predictable from one person to the next.
Different grooves even handle different subtypes of the same broad function. In the frontal lobe, for instance, distinct sulci manage different aspects of action control, while specific grooves in the lower frontal surface process different types of value-based decisions. The landscape of ridges and grooves is not just decorative; it reflects the underlying architecture of how the cerebrum organizes its work.
How the Cerebrum Compares to Other Brain Regions
The cerebellum, often called the “little brain,” sits beneath the cerebrum at the back of the skull. Despite containing more neurons than the cerebrum, it accounts for only about 11% of the cerebrum’s volume. Its primary jobs are coordinating movement, balance, and motor learning, though it also contributes to some cognitive processes.
The brainstem, connecting the brain to the spinal cord, is even smaller, roughly 2.3% of the cerebrum’s volume. It controls automatic functions you never have to think about: heart rate, breathing, blood pressure, and sleep-wake cycles. Together, the cerebellum and brainstem are essential for survival, but the cerebrum is where conscious experience, language, reasoning, and voluntary action originate. Its sheer size reflects the outsized role it plays in making human cognition possible.