Brain lateralization

Type of psychology: Biological bases of human behavior; Cognition; Neuropsychology; Psychopathology

Brain lateralization refers to the anatomical asymmetry and the functional division of labor between the left and right cerebral hemispheres. Diverse research methods, including studies of split-brain subjects who have had connecting tissues between the hemispheres severed, have revealed that the hemispheres function differently. Most prominent among these differences are that the left hemisphere is usually dominant in language production and comprehension, whereas the right hemisphere is typically dominant in visuospatial processing. Degree of lateralization varies among individuals, however, and both hemispheres contribute to most thinking functions.

Introduction

In 1861, Paul Broca performed an autopsy on a man who had spent the last thirty years of his life in an asylum because he would not speak. Broca discovered a lesion in the posterior frontal lobe of the left, but not the right, hemisphere and concluded that this location was the speech area of the brain. Subsequent research confirmed his conclusion and the area was named after Broca. Broca's discovery was the first tangible evidence that a function of the brain was localized to one cerebral hemisphere. It would be nearly a century, however, before the extent of cerebral lateralization would be more fully explored with individuals known as split-brain people.

Ronald Myers and Roger Sperry had demonstrated in the 1950s that severing the corpus callosum, the large band of fibers that connects the cerebral hemispheres, and smaller hemispheric connecting bands known as commissures caused behavior changes in animals. However, similar operations performed on humans in the 1940s by William Van Wagenen in order to reduce epileptic seizures appeared to result in no discernible psychological changes in these split-brain patients. This seeming contradiction between the animal and human findings was resolved in the 1960s by Sperry and Michael Gazzaniga's research with split-brain people. Knowing that stimuli presented to the right visual field of each eye go to only the left hemisphere and that stimuli presented to the left visual field only go to the right hemisphere, they presented visual stimuli briefly to one visual field or the other. Because the corpus callosum and commissures were disconnected, the brief visual presentations were available to only one hemisphere. Just as the findings in animal research, they demonstrated that there were striking differences in the functions of the two hemispheres.

Other research methods have expanded upon Sperry and Gazzaniga's findings. The Wada test, in which one hemisphere is briefly anesthetized by an injection of amobarbital into the carotid artery that supplies the hemisphere, has been used to identify the functions of one hemisphere when the other is dormant. A less invasive approach is the dichotic listening task in which competing auditory information is presented to both hemispheres simultaneously. Because information presented to one ear reaches more to the contralateral hemisphere, dichotic listening tasks can reveal which hemisphere is more active in auditory processing. Additionally, a variety of brain imaging techniques can elucidate anatomical and functional differences between the two hemispheres.

Anatomical lateralization

The right hemisphere typically protrudes more toward the front of the skull and is larger in the frontal lobe than the left hemisphere. Conversely, the left hemisphere typically bulges more toward the back and is larger in the temporal (particularly in areas associated with language) and parietal lobes than the right hemisphere. The Sylvian fissure, a deep groove between the temporal and frontal lobes, is normally more horizontal in orientation in the left hemisphere but has a prominent upward curl in the right hemisphere.

The muscles and senses of the body are controlled by the contralateral hemisphere. However, there are some differences in the degree of control. The left hemisphere is engaged more closely in the copying of motor movements than the right hemisphere. Conversely, damage to the right hemisphere parietal lobe is more likely than damage to the left hemisphere parietal lobe to lead to sensory neglect of the opposite side of the body. The senses of smell and taste are ipsilateral; sense organs on the right side of the body provide input to the right hemisphere and vice versa.

Lateralization is affected by handedness, sex, and experience. About 96 percent of those primarily right-handed are left hemisphere dominant for language; of those primarily left-handed, approximately 70 percent show the same pattern, 15% show the opposite pattern, and little to no lateralization is found in 15 percent. Males tend to have a greater right hemispheric cell development than females, particularly the right side of the corpus callosum, whereas females often have more extensive cell development in left hemisphere language areas. Overall, males and those more dominantly right-handed appear to have a higher degree of lateralization than females and those primarily left-handed or ambidextrous. Regarding experience, research has demonstrated that practicing music increases the size of the left, but not the right, planum temporale, a temporal lobe area crucial for language comprehension.

Functional lateralization

Language. The most prominent hemispheric difference for the majority of people is that most aspects of language expression and comprehension are primarily the domain of the left hemisphere. For example, Broca's area, which controls speaking, is usually located in only the left hemisphere. Likewise, grammatical ability is based heavily on the functioning of the left hemisphere. When people listen to different conversations in their right and left ears in dichotic listening tasks, they tend to favor listening with the right ear (especially for consonant sounds). The right ear providing more input to the left hemisphere demonstrates a larger role of the left hemisphere in language comprehension. Damage to the left hemisphere is much more likely than the right to disrupt linguistic abilities, including reading and writing.

Nevertheless, the right hemisphere contributes to language comprehension and expression in a number of ways. It is necessary for the comprehension of the emotional aspects of language, prosody (tone of speech), figures of speech, and the interpretation of non-language sounds. The non-verbal aspects of language, such as understanding gestures and facial expressions, usually involve the right hemisphere more than the left hemisphere. If a person has damage to the temporal lobe of the right hemisphere, he or she typically has problems understanding sarcasm and humor, detecting lies in others, and speaking in an inflected voice. Additionally, vocabulary is represented in both hemispheres, though more so in the left hemisphere.

Music. When people listen to music, they tend to favor the left ear. The left ear inputting more to the right hemisphere demonstrates a dominant role of the right hemisphere for music perception. Many aspects of music perception, such as melody, pitch, and intensity, depend more upon the right hemisphere. However, lyric recitation, keeping a rhythm, and musical ability in singing or playing an instrument are more dependent upon the left hemisphere.

Visuospatial Processing. The ability to draw, understand spatial layout, comprehend maps, give and follow directions, and think in abstract spatial ways depends on the right hemisphere. Research has also shown that the right hemisphere is better, overall, at facial recognition and enables one to follow the gazes of others. However, while the right hemisphere is more active in the perception of others' faces, the left hemisphere plays the dominant role in the recognition of one's own face. Studies with composite faces (half the face is yours; the other half is someone else's) shown briefly to each hemisphere has demonstrated that the right hemisphere interprets the composite face as someone else while the left hemisphere interprets the composite as the self. Other research suggests that the right hemisphere emphasizes processing same sex faces, whereas the left hemisphere focuses more on opposite sex faces. Additionally, the left hemisphere is more involved in voluntary facial expressions (e.g., “Smile for the camera!”); the right hemisphere plays the primary role in spontaneous facial expressions.

Perceptual Style. Researchers have presented a visual stimulus consisting of a large letter (e.g., “P”) composed of small letters (e.g., “q”) briefly to each hemisphere's visual field and then tested to determine what each hemisphere would interpret. For most people, the left hemisphere reports the small letters and the right hemisphere reports the big letter. From such research, it has been deduced that the left hemisphere focuses on details whereas the right emphasizes the big picture. In other words, the right hemisphere has a more holistic and global processing framework; the left has a more reductionistic and sequential processing approach.

Split-brain people cannot divide their attention efficiently (i.e., one hemisphere attending to one event; the other hemisphere attending to a different event). In other words, despite the disconnection between their hemispheres, they maintain a unitary focus toward perceiving the world. However, the left hemisphere is more involved in conscious decisions while the right hemisphere plays a greater role in automatically attending to events. In general, the right hemisphere plays a bigger role in attentional focus than the left hemisphere.

Cognitive Style. The left hemisphere usually plays the dominant role in major problem solving, such as solving mathematical problems, testing hypotheses, and determining the specific course of action in implementing personal plans. It is adept at slower, more deliberate reasoning and determining explanations. Michael Gazzaniga has called the left hemisphere the “interpreter” due to its role in making inferences, deriving interpretations, understanding the self, finding order, and forming beliefs. It appears that the left hemisphere plays a bigger role in self-consciousness.

In contrast, the right hemisphere tends to favor a fast, simplest possible approach toward solving problems. It makes quick judgments about others and is of crucial importance in understanding the motives and beliefs of others.

Feelings and Emotions. In the 1990s, Richard Davidson suggested that the left hemisphere is specialized for approach behaviors whereas the right hemisphere is designed for withdrawal actions. Introverts, who tend to shy away from social stimulation, have some areas in their right frontal lobe larger than the left frontal lobe; conversely, extraverts, who gravitate toward social events, show the opposite pattern. Anesthetizing the left hemisphere leaves most people feeling sad and socially withdrawn; anesthetizing the right hemisphere results in most people feeling happy and socially engaged. Overall, however, the right hemisphere plays a greater role in processing emotion, especially humor. When people look at composite faces where half the face is smiling and the other half is not, they typically interpret a happier face when the left half is smiling – the half of the visual field that goes to the right hemisphere.

Conclusion

The cerebral hemispheres are specialized for different functions; however, for most tasks both hemispheres are active contributors. Thus, the idea that people are either right-brained or left-brained is an overstatement of the facts and a dubious assertion. The functions of the two hemispheres are more complementary than oppositional, and this fosters a more efficient use of the brain's resources.

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