The brain appears to have near-perfect bilateral symmetry, meaning it can be divided into two near-identical halves. While the overall external shape adheres to this principle, the reality is far more complex, revealing subtle structural and functional asymmetry. The two halves of the brain, the cerebral hemispheres, operate with a distinct division of labor. Exploring this duality shows that the brain is not a simple mirror image but an integrated system of complementary halves.
The Appearance of Bilateral Symmetry
The cerebrum, the brain’s largest part, is separated into left and right hemispheres by the longitudinal fissure. This visible division creates the impression of two mirror-image halves, similar to the external symmetry of the human body. The cerebrum’s wrinkled surface, composed of ridges (gyri) and grooves (sulci), appears generally duplicated across the midline.
The corpus callosum, a massive bundle of white matter fibers, connects these two hemispheres. This structure serves as the primary communication pathway, allowing the halves to exchange information and coordinate activities. Both hemispheres look structurally identical, each containing the same four major lobes—frontal, parietal, temporal, and occipital—in corresponding locations.
Anatomical Asymmetries: Beyond Gross Structure
Despite the outward appearance of symmetry, the physical architecture reveals consistent, non-mirror-image differences known as anatomical asymmetries. One frequently cited example is the Planum Temporale, a region in the temporal lobe associated with language processing.
The Planum Temporale is significantly larger in the left hemisphere in about 65% of individuals, compared to only 11% where it is larger on the right. This leftward bias makes it one of the most asymmetrical structures in the brain. The hemispheres also exhibit a slight “torque,” where the right frontal lobe projects further forward and the left occipital lobe further back. These structural differences extend to the cellular level, with variations found in cortical column spacing and dendritic structures.
Functional Asymmetry: Brain Lateralization
The most significant consequence of imperfect symmetry is the specialization of function, known as brain lateralization. This establishes a division of labor where certain cognitive processes are primarily handled by one hemisphere. For most people, the left hemisphere is dominant for language functions, including speech production and comprehension.
The left hemisphere typically manages analytical tasks, sequential processing, and controls motor function for the right side of the body. Conversely, the right hemisphere specializes in non-verbal and holistic processes, such as spatial reasoning, facial recognition, and processing the emotional context of speech. It controls the motor function for the left side of the body, demonstrating contralateral control.
Variations in Symmetry and Brain Function
The patterns of structural and functional asymmetry are not uniform across the population, showing variation particularly in relation to handedness. While language is lateralized to the left hemisphere in about 96% of right-handed individuals, this pattern is less rigid in left-handed people. About 70% of left-handers still show left-hemisphere dominance for language, while others show right-hemisphere dominance or a more bilateral organization.
This variation suggests that the degree of lateralization is not fixed and depends on individual developmental factors. Atypical patterns of brain symmetry have also been observed in individuals with certain neurodevelopmental conditions. Research suggests a complex genetic overlap linking handedness and brain asymmetry to conditions like schizophrenia and autism.