Why does lateralization happen?

CurioCity
23 January 2012

The human brain is separated into left and right hemispheres by the longitudinal fissure but remains in contact via over 200 million nerve fibers called the corpus callosum. Though the brain is specialized in each hemisphere we do not notice the asymmetry since the two hemispheres remain in close communication. Information that is received by one hemisphere is readily shared across the corpus callosum and is able to be processed by the opposite or same side of the brain.

Did you know? The corpus callosum is sometimes severed in epilepsy patients to prevent seizures through a process called “split-brain” surgery.

Interpretive, speech and motor areas are generally more highly developed in one hemisphere termed the “dominant hemisphere” and for 95% of people this is the left hemisphere. The motor areas for hand control tend to be dominant in the left hemisphere which explains why most people are right handed (nearly 90%).  In general, the left hemisphere is usually better at tasks involving verbal processing, such as language, speech, reading, and writing. The right hemisphere tends to be better at tasks involving spatial, musical and visual recognition tasks.

Did you know? The origins of brain lateralization are yet to be understood within scientific literature, although brain lateralization can also be found among fish, birds, reptiles and amphibians.

Each side of the brain delivers and receives motor and sensory information from the opposite side of the body. The left side of the brain will control the right arm, the right leg, right hand, etc. Vice versa, the right side of the brain will communicate with the left side of the body. 

However, not all senses are controlled by the opposite side of the body. Auditory input from the environment is delivered to both sides of the hemisphere but will register stronger in the opposite hemisphere. For example, sound presented to the right ear will be registered in the left hemisphere first. 

Visual information is relayed to both hemispheres of the brain. 

The fibres closest to the nose referred to as the “nasal fibres” will cross to the opposite side of the brain while the outer “temporal fibres” remain connected to the same side of the brain.  The nasal fibres cross at a point referred to as the optic chiasm on the lower portion of the brain.

Language appears to be specialized in the left hemisphere of the brain by two special areas called “Broca’s area” and “Wernicke’s Area”. Broca’s area, located in the frontal lobe controls a role in the production of speech and Wernicke’s area, located in the temporal lobe controls the comprehension of language.

Studies of the brain in the past have been conducted on patients that have undergone a brain infarction such as a stroke or aneurysm, wounds to the brain, or patients that have had a severed corpus callosum.  The researcher are able to determine the function of parts of the brain by determining what function has been lost and which area has received trauma.  

Did you know? The beginning of research on cortical specialization began on soldiers from WWI who suffered bullet wounds to different areas of the brain. A study of these areas allowed researchers to discover function related to these areas.

Learn More:

Handedness and brain lateralization

Brain lateralization

Neurons, hormones and the brain

This answer was researched and written by Darren Kelly who currently has an H.BSc from the University of Toronto specializing in Biology and Biological Anthropology. Darren will be attending a B.Ed program next year with the goal of being a high school biology teacher.

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