Laterality
Laterality refers to the preference of certain functions being linked to one side of the brain, manifesting in behaviors such as right- or left-handedness. Approximately 90% of people are right-handed, while around 10% are left-handed or ambidextrous. Historically, left-handed individuals have faced stigma, and the study of laterality has evolved significantly over the years. Early observations in the 19th century highlighted the relationship between brain hemispheres and cognitive functions, particularly in language processing, which is predominantly associated with the left hemisphere in most individuals. Notable research, such as Dr. Roger Sperry's work on split-brain patients, illustrated how each hemisphere can operate independently, leading to insights about their specialized roles. For instance, the left hemisphere typically manages language and speech, while the right hemisphere is involved in interpreting emotional nuances and music. Current research is increasingly focused on the connections between brain regions, known as the connectome, emphasizing the collaborative nature of brain function rather than strict lateralization. Understanding laterality is essential for grasping the complexities of human cognition and neural organization.
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Laterality
Laterality refers to the tendency of some functions to be linked to one side of the brain and for creatures to have a preference for one side over the other—such as right- or left-handedness. For instance, people have wondered for centuries why about 10 percent of humans favor their left hands or are ambidextrous while the majority are right-handed. People who are left-handed have occasionally been negatively stigmatized. Brain lateralization, or hemispheric specialization, is found in humans and other living creatures. While some brain structures exist in both sides of the brain, they are different sizes and often perform different functions.
A great deal of research has looked at the functions of the two cerebral hemispheres. While ideas about right-brain and left-brain functions have permeated popular culture, the reality is that laterality and hemispheric specialization are much more complicated. Although overall structure is the same, every human’s brain is wired differently based on a combination of factors including early childhood experiences and prenatal development.
Background
Lateralization in humans was noticed during the early nineteenth century. Marc Dax, a French physician, noticed in 1836 that some types of injury were more likely to be accompanied by a loss of speech. Those whose injuries caused paralysis of the right side of the body more often were unable to speak than those whose injuries led to left-side paralysis. Researchers had already come to realize that the left hemisphere of the brain controlled the right side of the body, so Dax recognized that the part of the brain that controlled speech was in the left hemisphere. He also narrowed down the area of the brain where damage left patients unable to speak. This part of the brain was later named for another French doctor, Paul Broca. During the 1870s and 1880s, German neurologist Carl Wernicke discovered that another part of the left hemisphere was the center for language comprehension. In his research he linked a variety of nerve diseases and conditions, such as Wernicke’s aphasia, to areas of the brain.
These and other researchers concluded that the left hemisphere was usually dominant and processed higher cognitive functions. The right hemisphere was seen as less sophisticated. Studies progressed over the next century as researchers mapped out more areas of the brain.
Major leaps in brain lateralization studies occurred in the late twentieth century. Dr. Roger Sperry was working with patients who had severe epileptic seizures. He used a recently developed treatment, severing the main connection between the left and right hemispheres, called the corpus callosum. This is a bridge of wide neural fibers that carry signals between the hemispheres. The procedure stopped the electrical impulse that resulted in seizures from crossing from one hemisphere to the other. Sperry found that once the corpus callosum was severed, the two hemispheres of the brain functioned as if the other side no longer existed. The patients were able to resume their lives with no apparent negative effects, and they no longer had seizures.
Sperry had earlier performed the procedure on cats to study how the hemispheres of the brain process information. He severed the corpus callosum and the optic nerves of both eyes of the animals. Then he covered the cats’ right eyes and taught them to distinguish squares from triangles using their left eyes only. He covered the left eyes and taught them to distinguish between triangles and squares using their right eyes. They learned one shape with one eye and the other with the other eye. Later, Sperry covered one of the cats’ eyes at a time. The cats recognized each shape only when seen with the eye that had been trained to recognize it. The two hemispheres of the brain were operating without knowledge of one another.
Sperry later performed the same learning experiment using volunteers whose corpus callosums had been severed by other means. In this study he showed the subjects words rather than shapes. The results were different, however. Patients could not remember the word if they saw it only with their left eyes. If they saw it with their right eyes, most patients remembered it. Sperry concluded that the left hemisphere, which is responsible for the right eye, processes articulation of speech and remembering and pronouncing words. The right hemisphere was not capable of these processes.
Overview
Each brain hemisphere is divided into four major lobes. Roughly from back to front, they are the occipital, parietal, temporal, and frontal. The primary visual cortex is located in the occipital lobe, which processes and interprets visual data. Sound and some memory are processed in the temporal lobe. Sensation and some spatial processing take place in the parietal lobe, which is just above the ear. The frontal lobe is the largest and most complex. Higher processes including reasoning, planning, and voluntary movement take place in the frontal lobe.
Most cognitive or thought processing activity takes place in both hemispheres of the brain. In most humans, however, the language centers are located in the left side of the brain. The two most important language areas are Broca’s Area, which handles language grammar and syntax, and Wernicke’s Area, which processes meaning and content of language. While both hemispheres are about the same size, these two areas in the left brain are slightly larger in most people. In about 30 percent of left-handed people, and about 3 percent of right-handed people, these areas are in the right hemisphere instead of the left.
While language centers are in the left hemisphere, interpretation seems to require engagement of the right brain as well. Patients with right-hemisphere damage often have difficulty understanding language nuances such as irony and questioning speech. Certain emotions are also linked to specific hemispheres: happiness is in the left and fear is in the right (although this may not be true in all cases, or for left-handed people). Similar whole-brain approaches are necessary when the brain processes music. The right hemisphere is generally engaged in creating and responding to music, but processing melody, pitch, and tempo involves multiple areas of both hemispheres.
Research of brain function has moved beyond localities to connections. The network of circuits is known as the connectome. It includes the corpus callosum as well as other lesser neural fiber bridges that link the two hemispheres. Scientists are looking at how the various areas of the brain communicate and work together.
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