A longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. The two sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each cortical hemisphere are distinct.

The hemispheres of the cerebral cortex

The human brain is divided into two hemispheres–left and right. Scientists continue to explore how some cognitive functions tend to be dominated by one side or the other; that is, how they are lateralized.

Broad generalizations are often made in popular psychology about one hemisphere having a broad label, such as "logical" for the left side or "creative" for the right. But although measurable lateral dominance occurs, most functions are present in both hemispheres. The extent of specialization by hemisphere remains under investigation. If a specific region of the brain or even an entire hemisphere is either injured or destroyed, its functions can sometimes be taken over by a neighboring region even in the opposite hemisphere, depending upon the area damaged and the patient's age. When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

While many functions are lateralized, this is only a tendency. The implementation of a specific brain function significantly varies by individual. The areas of exploration of this causal or effectual difference of a particular brain function include gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having a hemispherectomy (removal of a cerebral hemisphere), no one is a "left-brain only" or "right-brain only" person.

Lateralization and Handedness

Brain function lateralization is evident in the phenomena of right- or left-handedness, but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere dominance for language, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of left-handed people have bilateral language functions. Even within various language functions (e.g., semantics, syntax, prosody), degree and even hemisphere of dominance may differ.

Language functions such as grammar, vocabulary and literal meaning are typically lateralized to the left hemisphere, especially in right-handed individuals. While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral or even right-lateralized in approximately 50% of left-handers. In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.

Further Lateral Distinctions

The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show right-hemisphere dominance. Numerical estimation, comparison, and online calculation depend on bilateral parietal regions. Exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing. Dyscalculia is a neurological syndrome associated with damage to the left temporoparietal junction. This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to understand or apply mathematical concepts.

Lateralization and Evolution

Specialization of the two hemispheres is general in vertebrates including fish, frogs, reptiles, birds, and mammals, with the left hemisphere specialized to categorize information and control routine behavior. The right hemisphere is responsible for responses to novel events and behavior in emergencies, including the expression of intense emotions. Feeding is an example of a routine left-hemisphere behavior, while escape from predators is an example of a right-hemisphere behavior. This suggests that the evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain.

Split-Brain Phenomenon

Patients with split-brain are individuals who have undergone corpus callosotomy, a severing of a large part of the corpus callosum (usually as a treatment for severe epilepsy). The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other.

The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere. The evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain. In a 2011 study published in the journal of Brain Behavioral Research, lateralization of a few specific functions as opposed to overall brain lateralization was correlated with parallel tasks efficiency.