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Lateralization of Brain Functions
If you watched a team sport, you might have noticed that all the players on the team had some forte. To put it another way, they all have some strengths and are better at them than others who have their strengths, and they work well together to achieve wins or losses in a game. Similarly, several systems in our body have a specific function, such as the respiratory system, which takes in oxygen and expels carbon dioxide. What about our brains? The brain is the most complex organ and the control unit of our body. How does it carry out so many different tasks despite its small size? Are there certain centers that are responsible for various functions of our body?
What is "Lateralization of Brain Functions?"
Our brain is primarily divided into two hemispheres. These are the right, and the left hemispheres, connected by a bundle of nerve fibers called the corpus callosum. Although the two hemispheres' macrostructures appear almost identical, the varied neural network compositions enable each hemisphere to have a unique set of specialized functions. It refers to the basic principle of the brain's organization. This lateralization of brain functions helps the brain carry out different functions at the same time, for example, looking for food while keeping an eye on predators at the same time. One of the major goals of lateralizing brain functions is to avoid redundancy or duplication of cognitive capacity.
Research on Lateralization of Brain Functions
Several studies have been conducted on split brains. These studies were conducted in humans and animals where the corpus callosum was severed. They have helped us understand the lateralization of brain functions better. These studies have highlighted the specialized functions of the right and left brain hemispheres.
It was found that within a species, the capacity or strength of lateralized responses can differ. Some individuals from the species have strong laterality, while others are weak in it. However, weak or missing laterality manifested in motor behavior may not necessarily indicate that the brain is less lateralized for processing cognitive information. It is more of a question of whether one hemisphere solely regulates a specific behavior, as in strong laterality, or whether the other hemisphere is partially involved.
As mentioned earlier, the brain's two hemispheres are connected via the corpus callosum. In earlier times, neurosurgery was performed to slice the corpus callosum to halt communication between the two hemispheres. This surgery was performed on patients with severe epileptic attacks. People who underwent this surgery were called split-brain patients. A renowned neurobiologist, Roger Sperry, undertook several studies and experiments to study the split-brain and the localization of function in the hemispheres. In his experiments with the split-brain patients, he projected words in the right and left fields of vision. He covered one eye of the participant to understand if they could say the word he had displayed in front of them. He found that participants could verbally say the word displayed on their right visual field. The left hemisphere controls the right visual field, where language centers are situated. The words displayed on the left side could not be verbally spoken. The right hemisphere controls the left side.
What are the Benefits of a Lateralized Brain?
The right and left hemispheres participate in the behavior's production and control via interhemispheric connections. However, cognitive capacity is increased when the brain's functions are lateralized. This is when both hemispheres perform different computations of the given information. When both hemispheres perform the task of processing the same information in the same way, then duplication occurs. Hence, no enhancement of cognitive capacity takes place.
Lateralization of Different Functions in the Body
Lateralization and the sensorimotor system
The brain consists of many cortices responsible for the body's motor and sensory functions. The sensory and motor cortices are contralateral. This means that the hemispheres control the opposite side of the body, and the right hemisphere controls the left side of the body and vice versa. Studies have found that damage to the right occipital lobe (which is the area that controls vision) can lead to loss of sight in the left field of vision.
Lateralization and Language
Wernicke's and Broca's areas are responsible for language comprehension and speech production, respectively. Wernicke's area is situated in the left hemisphere of the brain. The Broca's Area produces speech. The premotor area aids this area in choosing and ordering speech sounds. The Wernicke's Area's linguistic data can be sent to the Broca's Area to produce speech. One of the key functions of Wernicke's area is to name objects. When an object is placed in the right visual field, it is easily identified by people with or without a split brain. However, split-brain subjects could not identify objects placed in the left visual field as well as their counterparts who did not have a split brain. This demonstrates that naming recognized objects is the known linguistic function of the left hemisphere. When an object is placed in the left visual field, identifying the object verbally was difficult for individuals who had undergone split-brain surgery. However, they accomplished this task by touching the object.
Lateralization and handedness
Most right-handed people have left-hemisphere language dominance in terms of handedness. Thus, their propensity for logical and analytical thinking tends to outweigh their propensity for creative and expressive thinking. Contrary to popular assumptions, most left-handed people do not exhibit right-handed verbal dominance. Instead, 70% of left-handed adults still exhibit dominant language abilities in the left hemisphere.
The left and right sides of the brain have distinct functions for processing sensory inputs, attending to different information, and controlling various motor behaviors. This is known as brain lateralization or hemispheric specialization. Currently, lateralization is well-documented in many different species, from the tiny brains of insects to the large brains of vertebrates, birds, and mammals. This widespread lateralization of the brain implies that, despite the brain's actual size, its capacity and efficiency may be enhanced by avoiding duplicating all functions on the left and right sides. However, both hemispheres do work in coordination, and both are not independent of each other.
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