Existence of Hemispheric Dominance

Abstract

Traditionally, the left hemisphere is considered dominant for language and speech, while the right hemisphere is associated with spatial processing. However, studies on split-brain patients, such as J.W., challenge this notion. Initially unable to produce speech using his right hemisphere, J.W. gradually developed this ability over time, improving from naming 25% of images shown to his left visual field (right hemisphere processing) to 60% over the span of a year. This improvement highlights the role of neural plasticity in language development. Additionally, findings suggest that while the right hemisphere is limited in phonetic processing, it can recognize whole words visually. These observations raise questions about why the right hemisphere shows language abilities after commissurotomy but not in cases of left hemisphere damage. One explanation is that in an intact brain, the left hemisphere suppresses the latent abilities of the right hemisphere. When the connection between the two is severed, the right hemisphere is freed to function independently, revealing capabilities that were previously overshadowed. Furthermore, studies indicate that the right hemisphere excels in spatial processing compared to the left, reinforcing the idea that each hemisphere has distinct but complementary cognitive functions.

Split-brain patients have a surgery called a callosotomy, where doctors cut the corpus callosum—the bundle of nerves that connects the left and right sides of the brain. This surgery is usually done to help people with severe epilepsy in the past.

Some split-brain patients, who at first cannot speak using their right hemisphere, sometimes develop this ability over time. J.W., the patient in this study, is one such case. When he had surgery to cut the connection between his brain hemispheres, his left hemisphere controlled language. His right hemisphere could understand spoken and written words but could not speak. (1, Link)

Fourteen years after his surgery, J.W. could name about 25% of the images shown to his left visual field (processed by the right hemisphere). A year later, his ability improved to 60%. This improvement seems to be due to the brain’s ability to change and adapt over time (neural plasticity). However, when J.W. gave longer verbal responses about these images, it looked like both hemispheres were working together, with the left hemisphere helping to interpret and explain what he saw.

For most people, the left side of the brain controls language and speech. It is also responsible for reading and understanding written words. The right side of the brain can recognize whole words and their meanings but is not as good at sounding out letters and forming words. The left hemisphere is much better at this because it is the main center for language.

The right hemispheres (RH) of two patients who had their brain hemispheres separated could understand spoken words and read printed words by matching them with pictures of the named objects, even though they could not speak. These findings suggest that the right hemisphere reads words as whole visual images (like recognizing a symbol) rather than sounding them out letter by letter, which is how the left hemisphere typically processes written language. (2, Link)

After commissurotomy (surgery that cuts the connection between brain hemispheres), researchers discovered that the right hemisphere could do things like reading, which raised a big question: Why can the right hemisphere do these tasks after surgery, but not when the left hemisphere is damaged?

One possible explanation is that previous studies of left hemisphere damage were misleading. Normally, when the brain hemispheres are connected, they work together as a team, with one side (usually the left) taking the lead in language. If the left hemisphere is damaged, it disrupts the whole system, making it seem like the right hemisphere has no ability at all.

However, when the connection between the hemispheres is completely severed (as in a commissurotomy), the right hemisphere is freed from the damaged left hemisphere’s influence. This allows the right hemisphere to show abilities that were always there but were previously suppressed by the stronger left hemisphere. (3, Link)

Studies with split-brain patients show that the left hemisphere is actually capable of complex visual and spatial processing, and the differences between the two hemispheres are more subtle than previously thought.

Research on how we perceive visuals suggests that these differences appear later in the process of interpreting visual information, particularly in areas of the brain that process both sides of the visual field.

The right hemisphere is more skilled at visual tasks than the left. Some scientists even suggest that the right hemisphere has a “visual interpreter” that helps create a complete and meaningful picture of the world. (4, Link)

Although the two sides of the human brain look similar, they have different functions. Some differences, like language and movement control, are well known. Others, like how each side processes visual and spatial information, are less understood.

Many researchers believe that the right hemisphere is better at understanding space and locations. The results showed that the right hemisphere was much better at judging spatial relationships (such as distances and positions). On the other hand, the left hemisphere was slightly better at recognizing objects and patterns.

These findings suggest that the right hemisphere specializes in spatial thinking, while the left hemisphere focuses more on identifying objects and patterns. (5, Link)

Many studies over the years have shown big differences between the two sides of the brain. The most well-known differences are in language and movement control, but how each side processes visual and spatial information is also different. The left hemisphere can handle complex visual tasks, it is not as precise as the right hemisphere when it comes to understanding spatial relationships. This means that both hemispheres recognize patterns, but the right hemisphere is better at processing spatial information.

These results support the idea that the left hemisphere focuses more on recognizing patterns, while the right hemisphere is better at understanding spatial details.

Conclusion

The study of split-brain patients, particularly J.W., provides valuable insights into hemispheric specialization and neural plasticity. His ability to gradually develop speech through his right hemisphere suggests that the brain can adapt and reorganize functions over time. The findings challenge the traditional view of strict hemispheric dominance for language and highlight the right hemisphere’s ability to recognize whole words visually. Additionally, research confirms that the right hemisphere is superior in spatial processing, while the left hemisphere excels in pattern recognition and phonetic decoding. These results indicate that both hemispheres play critical roles in cognition, and their interaction is essential for a complete and functional understanding of language and space. Further research into neural plasticity and interhemispheric communication may provide deeper insights into how the brain compensates for injuries and adapts to structural changes.