Cerebral dominance refers to the asymmetrical lateralization of language and perceptual functions in the human brain. Cerebral dominance, or hemispheric specialization, was initially applied to language functions that are served by the left hemisphere in most individuals. However, the term was later expanded to include cognitive functions of nonverbal reasoning and visual–spatial information processing that are associated with the right hemisphere. In short, functions associated with the left hemisphere involve processing linguistic, analytical, and sequential information while the right hemisphere is responsible for processing nonlinguistic or spatial information in a holistic fashion (see Witelson,).
Early reference to cerebral dominance can be traced back to Dax in 1836 and Broca in 1861; they found that damage to the left hemisphere results in disorders of speech and language. They believed that the left hemisphere is the dominant side for most people in that it controls the functions of language (Gaddes,). The notion of cerebral dominance was further delineated by the writings of Jackson, who postulated that the left hemisphere is the dominant or the leading side and right hemisphere is the automatic and minor side (Dean,). The emphasis in determining cerebral dominance for language was also noted by (Orton). He speculated that delayed or incomplete lateralization for linguistic functions by the left hemisphere results in the types of language disorders often seen in children.
Methods for assessing specializations of each hemisphere have employed invasive techniques such as direct electrical stimulation of the brain, hemispheric anesthetization, and split-brain studies. Noninvasive procedures have involved dichotic listening and split-visual field research.
Research using direct electric stimulation of the brain was pioneered by Penfield (Penfield & Roberts,). This technique was developed to map the centers of the brain that controlled specific functions prior to surgical procedures. Since the brain does not contain pain receptors, the patient was conscious when a small electrical current was applied to the surface of the brain to determine areas of the brain associated with such functions as vision, hearing, olfaction, or haptic sensations. Applications of electrical stimulation to areas believed to control speech would be verified by the patient’s inability to talk. These “aphasic arrests” would occur only when areas of the brain associated with speech were electrically stimulated. In this way, hypotheses about other functions of the brain could also be verified if responses associated with those functions were absent during stimulation.
Another invasive technique to study brain functioning has been to anesthetize one hemisphere by injecting sodium amytal in the carotid artery located on either the right or left side of the patient’s neck. This procedure, known as the Wada test, quickly anesthetized that side of the brain. For example, if the left side or the side dominant for language was infused, the individual would become speechless while the drug was in effect, while the functions of the right hemisphere would remain intact. (Wada and Rasmussen) hypothesized that the left hemisphere is dominant for processing verbal information and the right hemisphere for nonverbal information. To demonstrate this, Wada and his associate injected sodium amytal into the left hemisphere and asked the patient to sing “Happy Birthday”; the patient was able to hum the tune without producing the words. When the right hemisphere was anesthetized and the patient was required to perform the same task, the patient was only able to recite the words of “Happy Birthday” in a monotone without producing a tune. Using this procedure, (Milner) found that 95% of right-handed and 70% of left-handed individuals are left hemisphere dominant for language.
Split-brain surgery or commissurotomy is another invasive technique used to study cerebral dominance. A commissurotomy is a surgical procedure used to stop the spread of seizure activity from a focal point in one hemisphere to the other hemisphere via the corpus callosum. This procedure involves the severing of the corpus callosum, a large band of nerve fibers that connects the left and right hemispheres, thereby preventing any communication between the hemispheres.
Much research was conducted by Speery in the 1950s. Researchers were able to localize functions of language, motoric control of the same or opposite sides of the body, and visual discrimination (Hacaen,). In one study that examined visual perception, Levy and her associates (Levy, Trevarthen, & Speery,) used stimulus figures in which the left half of one face was joined with the right half of another. The patient was required to gaze at a dot on the center of the screen before a figure was flashed on the screen. The presentation was such that each half of the face would be projected to only one hemisphere. When the patient was asked to respond by pointing to the correct picture from available alternatives, the left sides of faces, which are processed by the right hemisphere, were correctly chosen more often than the right sides regardless of the hand used for pointing. However, when the patient was required to verbally identify the picture, the face on the right side (left hemisphere) was chosen, although the number of errors made by this response mode was much higher. These results were subsequently replicated using other stimuli, suggesting that the right hemisphere is superior in processing nonverbal visual stimuli.
A noninvasive technique in the study of brain–behavior relationships has been dichotic listening. This procedure involves the simultaneous presentation of verbal or nonverbal information to each ear. Similar but different information is presented to each ear and the subject’s task is to identify or recall what was heard. This technique was initially developed by (Broadbent) to study auditory attention and later adapted by (Kimura) to study cerebral lateralization. Studying normal individuals, Kimura found that subjects were more able to identify correctly verbal information when it was presented to the right ear (left hemisphere). If the information was nonverbal, however, a left-ear advantage (right hemisphere) was found. Kimura also showed that if patients having neurological disorders were found to be left hemisphere dominant for language (via the Wada test), a right-ear advantage was noted for verbal information. Similarly, if the patient was right hemisphere dominant for language, a left-ear advantage (right hemisphere) was found for verbal information. These findings suggested that superiority for each ear varies with the specialization in function for the opposite hemisphere.
Studies that have examined language lateralization for dyslexic children using a dichotic listening paradigm have found mixed results. Dyslexic or reading-disabled children are usually characterized by a significant lag in reading achievement despite average intelligence and an absence of any sensory-motor, neurological, or emotional difficulties (Hynd & Cohen,). Some studies (e.g., Witelson & Rabinovitch,) have reported that children with dyslexia show a left-ear advantage for verbal information. Other researchers (e.g., Leong,) have demonstrated a right-ear advantage for verbal information for both dyslexic and normal readers. Differential findings may be partially due to differences in methodology, criteria of subject selection, and age and attention.
Another noninvasive technique in studying cerebral dominance has been split-visual field research. This involves a tachistoscopic presentation of verbal or spatial information to either the right-half or left-half visual fields. The visual pathways are such that information perceived in the left-visual field is processed by the right hemisphere while right-visual field information is processed by the left hemisphere. Studies have demonstrated that while word recognition levels were lower for the dyslexic children when compared with normal readers, both readers showed a right-visual field superiority for words (Marcel & Rajan,). However, when pictures were presented to either visual field, (Witelson) reported that while normal readers had a significant left visual-field advantage, this difference was not significant for a dyslexic group. These results suggest that while dyslexic readers, like normal readers, have a left-hemisphere representation for language, the dyslexic group appears to lack right-hemisphere specialization for visual-spatial information.
In sum, invasive and noninvasive techniques have made significant contributions in mapping functions of the brain. However, our knowledge of hemispheric specializations is far from complete. Given the interindividual differences in cognitive processing, the brain’s ability to compensate for damage, and developmental factors, the assessment of hemispheric specializations remains a complex and sometimes chaotic (Reynolds, Kamphaus, Rosenthal, & Hiemenz,) endeavor.
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