Aaron J. Louie
Human Neuropsychology (PSY 449)
Dr. M. I. Posner
12/2/96

Cerebral laterality and atypical dominance: A critical review of a case study

Introduction

Among all the questions about the human brain and its processes, the question of localization of function is one of the most enigmatic. It has been shown in numerous studies that the left hemisphere of the brain is responsible for speech function (Kolb & Whishaw, 1996). However, in a case study by Padovani, Pantano, Frontoni, Iacoboni, Di Piero, and Lenzi (1992), it was found that a patient exhibited reversed laterality. F.A., a left-handed neurological patient that suffered a left hemisphere stroke, showed no signs of aphasia, or deficit in speech.

By using a method known as a dichotic listening task, clinicians have indicated the dominance of the left hemisphere for perception of speech in the majority of normal persons and a predominance of the right hemisphere in the processing of melodic patterns and other nonverbal sounds (Kimura & Folb, 1968). It also has been pointed out that the right hemisphere plays a dominant role in a subject's perception of their environment (Kimura, 1973). However, in the case of F.A., it is apparent that these functions of dominance are reversed, since an insult to the left hemisphere did not cause any deficit in speech perception. Since F.A. was left-handed, the reversal of cognitive functions may have resulted from or be the cause of F.A.'s hand preference. Witelson and Goldsmith (1991) suggest that there is a continuum of hand preference that varies as a function of diversity in brain structure instead of a simple dichotomy between right- and left-handed groups.

Many studies have demonstrated that left-handers, or sinistrals, display more bihemispheric representation of cognitive functions than do right-handers, or dextrals. As a result, Witelson and Goldsmith (1991) hypothesized that this variation in functional asymmetry was a result of the size of the corpus callosum. The corpus callosum, which consists of fibers connecting the two hemispheres (Kolb & Whishaw, 1996), provides for greater interhemispheric communication and may have a role in the reversal of laterality in cases such as F.A. (Witelson and Goldsmith, 1991). It is also possible that the reversed laterality seen in F.A. was a result of genetic, developmental, or hormonal factors correlated with his sinistrality (Alexander, Fischette, & Fischer, 1989; Kolb & Whishaw, 1996).

Although many left-handers exhibit what Goodglass and Quadfasel (1954) termed "anomalous dominance", Levy and Nagylaki (1972) address the fact that there "are dozens of cases of crossed aphasia both in right handers with right-hemisphere lesions and in left handers with left-hemisphere lesions, as well as asymptomatic patients with lesions in the hemisphere contralateral to the preferred hand" (117). The manner in which behavioral functions within the human brain develop differentially occurs during childhood and may be altered at this stage by either damage to the brain in one hemisphere or by forced training (Carmon, Harishanu, Lowinger, & Lavy, 1972; Delis, Knight, & Simpson, 1983). This anomalous dominance in sinistrals may lead to a slightly increased resilience against language disorders following a lesion to the left hemisphere but an increased chance of aphasia for right side lesions(Hardyck & Petrinovich, 1977). However, Padovani, et al, believe that the term "anomalous dominance" should not be applied to such cases as F.A., where the cognitive functions are appropriately intrahemispherically organized but are simply reversed. This "mirror" dominance is instead termed "atypical cerebral dominance" (Padovani, et al, 1992). The difference between the two is that anomalous dominance refers to increased bihemispheric dominance, or cooperation and sharing of function of the two hemispheres. On the other hand, atypical dominance is defined by a strongly lateralized, mirrored dominance that entails a reversal of dominance with normal levels of bihemispheric function..

The case study by Padovani, et al (1992), provides further evidence for the possibility for the brain to show mirrored dominance and that the genetic component of left-handedness is linked to reversed cerebral laterality and atypical dominance. I will critically review the case study concerning F.A. according to a framework of different levels of analysis. I will also discuss some questions for future study in the area of atypical dominance.

Summary of the Case Study

Patient F.A., a 54-year-old non-right-handed man with a strong family history of left-handedness, displayed a typically right hemisphere syndrome following a left hemisphere stroke. Apparently, F.A., although functionally right handed, was formerly left handed as a child and could perform most functions ambidextrously. His father, one of two siblings, and his daughter were also left-handed. A Mental Status Examination was performed to determine IQ, language skills, memory, visual and spatial perception, and several other cognitive functions and mental operations.

His symptoms included severe right side hemineglect, which was to be expected. However, he also showed transcortical motor dysprosodia, spatial dysgraphia and visuo-constructive impairments, all indicative of a right hemisphere lesion. Additionally, none of the expected left hemisphere abnormalities such as aphasia, alexia, right-left disorientation, or finger agnosia were observed. On CT scan, a left fronto-temporal subcortical infarct was seen. No cerebral blood flow changes in the right hemisphere were observed in a SPECT study while the left hemisphere revealed a fronto-temporo-parietal cerebral blood flow reduction. Thus, Padovani, et al, concluded that F.A. displayed "atypical" cerebral dominance: a reversed laterality of hemispheric function. His behavioral deficits . . . were consistent with the pattern expected of a [dextral] with a right hemisphere lesion. . . . These clinical findings are, thus, consistent with a mirror "right hemisphere" syndrome following a left hemisphere damage and support the occurrence of a "true" reversed laterality of cerebral dominance with attentional and visuospatial functions inversely represented in the left hemisphere (Padovani, et al, 1992, 87).

Padovani, et al (1992), argue that such cases as F.A. in whom all functions of the right hemisphere are appropriately lateralized in the left hemisphere and vice versa are most likely not a result of delayed development of the left hemisphere or early damage to the left hemisphere. Alternatively, they suggest that atypical dominance is the result of "a specific genetic endowment" (Padovani, et al, 1992, 88).

Variations in Cerebral Asymmetry

An investigation into the causes and characteristics of reversed laterality begins with a hierarchical analysis of the brain, beginning with cells and neural systems and working up to the level of cognitive functions.

First, at the cellular and structural level, it has been found that bihemispheric representation of cognitive functions in left- and mixed-handers is correlated with an increased area of the corpus callosum, as mentioned above (Witelson, 1985). "Since callosal morphology is directly involved with interhemispheric communication, it plays an important role in hemispheric integration and possibly in hemispheric specialization" (Witelson, 1985, 665). Kimura (1973) notes that this pathway is responsible for the coordination of the activities of the hemispheres. Thus, in non-dextrals, the amount of hemispheric cooperation and sharing of function is greater than in dextrals.

Second, at the level of performance domains, it has been hypothesized that the left hemisphere, in normal right-handed subjects, inhibits speech perception in the right hemisphere, thus becoming the dominant hemisphere for speech (Kimura, 1973). There is evidence that the minor hemisphere possesses some minimal ability to express language, but it is difficult to observe because of competition from the major hemisphere for control of the motor mechanisms for the production of language. These interference effects support a rationale for the evolution of a unilateral control of language expression, namely that such lateralization was an adaptation permitting control of the unique vocal apparatus, uncomplicated by competitive antagonism between the hemispheres (Levy, 1969, 614-615).

However, development of language and writing skills has been shown to be influential in the level of bihemispheric cooperation. Illiterate subjects showed a marked increase in right-ear speech perception over control groups, leading to the assumption that speech was encoded both spatially as well as literally in illiterate subjects (Tzavaras, Kaprinis, & Gatzoyas, 1981). Kimura also found that, in speech, the hand contralateral to the "speaking" hemisphere makes more free movements, demonstrating that "the left hemisphere is particularly well adapted not for the symbolic function in itself but for the execution of some categories of motor activity that happen to lend themselves readily to communication" (1973, 78).

Third, the study of mental operations involved in variations of cerebral asymmetry deal mostly with the perception of speech and auditory stimuli. For example, Kimura and Folb (1968) found that auditory information is processed in the left hemisphere (i.e. speech) if it includes sounds that can be articulated, even if they are perceived as nonsense. This is indicative of a specialization of the hemispheres for specific mental operations. In the case of F.A., no deficits in speech perception were observed, pointing to a lesion in the speech-producing hemisphere. Additionally, Hörster & Ettlinger (1985) mention that language disorders are more likely to occur in left-side lesions in right handers than in left handers.

Finally, in the area of cognitive functions, an individual's right hemispheric linguistic abilities are more often under the continuous inhibitory influence of the left hemisphere. However, the linguistic abilities of the non-speaking hemisphere can be measured by appealing to that hemisphere's specific cognitive strategies (Tzavaras, Kaprinis, & Gatzoyas, 1981). An example of such a strategy would be a dichotic listening task involving tones instead of words or digits (Kimura, 1973).

The various sources of cerebral asymmetry are most easily correlated with hand preference. Handedness, seen in many animals other than humans (Kolb & Whishaw, 1996), has been correlated to variations in learning, speech, and spatial perception (Hörster & Ettlinger, 1985; Hardyck & Petrinovich, 1977). The development of a preference for either the right or left hand has been found to be affected by familial or cultural practices, education, environment, genetic factors, or even brain damage (Hardyck & Petrinovich, 1977). Kimura (1973) suggests that the right hand, being dominant in most individuals for manual behavior, is preferred as a result of the dominance of the left hemisphere, which has a special control over some aspects of many skilled acts.

Several theories, including environmental, anatomical, and genetic theories, have been proposed for the evolution of hand preference and may illuminate the role that cerebral asymmetry plays in handedness.

Levy and Nagylaki (1972) described an environmental model for the development of handedness in a child that proposed that, should the dominant hemisphere for speech be damaged in a young child, the brain would be able to utilize either hemisphere for speech, resulting in possibly reversed laterality or anomalous dominance. Unfortunately, this theory hinges on the presence of an early unilateral insult to produce anomalous or atypical hemispheric dominance.

Two theories for the anatomical basis of cerebral asymmetry and, ultimately, handedness, hinge on a differential flow of blood to the brain and size of the corpus callosum. Carmon, Harishanu, Lowinger, and Lavy (1972) examined the asymmetries in the circulatory system of the brain and found that the blood volume in the nondominant hemisphere appeared to be larger than that of the dominant hemisphere. Perhaps an anatomical asymmetry in the blood supply to the brain results in differential amounts of blood flowing into or being contained in the two hemispheres. These asymmetries lead to different processes of maturation and functional development in the human hemispheres, which in turn result in functional asymmetry of the brain. (Carmon, Harishanu, Lowinger, & Lavy, 1972)

Unfortunately, no information involving F.A.'s circulatory system was available. Additionally, the relationship between hand preference and callosal morphology may be part of the anatomical substrate of functional asymmetry, further defining the possible anatomical basis of handedness (Witelson & Goldsmith, 1991).

The most convincing theory for both the degree and variation of functional asymmetry is a genetic theory. Hardyck and Petrinovich (1977) discuss the influence of a family history of left handedness on cerebral organization. The more genetically "left-handed" an individual is, the more reversed the cognitive functions of the hemispheres. Those exhibiting anomalous dominance were also found to recover function following brain damage more quickly than do those that were more lateralized, whether right- or left-handed (Luria, 1970).

Discussion

At last, we come to the discussion of the cerebral organization of left-handed individuals, a topic most relevant to the case of F.A., a sinistral who exhibited a most extremely atypical cerebral organization. Since left-handers, as a group, show greater bihemispheric representation of cognitive functions, it has been found that sinistrals also have greater anatomical connectivity between the hemispheres, leading to an increased sharing of cognitive function (Witelson, 1985). Additionally, it has been found that strongly familial left-handers have right-hemispheric speech along with left-hemispheric prosodic and tonal perception (Kimura, 1973; Delis, Knight, & Simpson, 1983; Dronkers & Knight, 1988).

Padovani, et al, in exploring the cognitive deficits of F.A., examined at various levels the degree of lateralization, the localization of cognitive function, and the cerebral organization of F.A. By using several measures and inventories in the Mental Status Examination, they were able to assess the neuropsychological state of F.A.'s cognitive functioning. The Mental Status Examination was carried out 2 weeks after F.A. suffered the stroke and was repeated until discharge. This test was conducted to measure F.A.'s abilities at the cognitive level.

F.A.'s speech, according to Padovani, et al, was "fluent, monotonic, mildly tangential, without paraphrasias, agrammatism or word-finding problems. Comprehension was excellent even for complex material" (83). These results were quite uncharacteristic for a right-handed patient with a left-hemisphere lesion. As Levy and Nagylaki (1972) observed, aphasia in left-handed individuals is often temporary or non-existent following lesions to the left or right hemisphere, as was the case with F.A. However, in a writing task, it was evident that a severe spatial dysgraphia and right-side hemineglect was present as a result of damage to the non-dominant hemisphere.

F.A. showed no deficits in motor functioning, but showed sever deficits in visuo-spatial functioning, affective characteristics, and visuo-spatial memory, providing further evidence that the insult had occurred in the non-dominant hemisphere. When F.A. performed CT and SPECT scans, it became clear that the infarct had occurred in the left hemisphere, pointing to the fact that the right hemisphere retained speech function and other normally-left-hemisphere abilities. Thus, through analysis that examined F.A. abilities at the cognitive, mental operation, and neural systems levels, Padovani and colleagues were able to fully explore F.A.'s cognitive deficits and examine his atypical cerebral organization through lesion analysis.

To fully examine the causes of F.A.'s reversed hemispheric dominance, it would be necessary to map and measure the asymmetries of the circulatory system supplying his brain. According to Carmon and colleagues (1972), his atypical dominance would be a result of asymmetrical blood volume. Perhaps we would find that F.A. also had reversed laterality in his circulatory system as well, with the aortic arch bending toward the right side instead of the left, and the normally enlarged left ventricle being located on the right. Furthermore, it would be necessary to examine the circulatory and cognitive asymmetries of F.A.'s surviving relatives to determine the genetic heritage contributing to the atypical dominance in his right hemisphere. If F.A. was indeed strongly left-handed and his brain strongly lateralized, then, for the diagnosis of "atypical" dominance would show normal mass of the corpus callosum, which could be examined through further neuroimaging using MRI.

Many questions remain to be answered in regards to atypical dominance. What is the evolutionary significance of cerebral asymmetry? Does hemispheric dominance develop from infancy or is it genetically preprogrammed? How does strength of lateralization affect survivability and recovery rates following unilateral insult? Through further analysis of case studies similar to that of F.A., we may be begin to answer these questions.

Throughout this paper, I have analyzed the case study by Padovani, et al, in relation to the hierarchical framework of different levels of analysis. The causes and characteristics of cerebral laterality and anomalous dominance were extensively reviewed to provide a background for the analysis of F.A.'s case. The case study by Padovani and colleagues found that a patient exhibited reversed laterality. F.A., a left-handed neurological patient that suffered a left hemisphere stroke, showed no signs of aphasia, or deficit in speech. Although it has been shown in numerous studies that the left hemisphere of the brain is responsible for speech function, we see a case where the dominance of the "speaking" hemisphere is laterally reversed. Many questions remain to be answered in regards to the causes and characteristics of atypical dominance, but through analysis of cases such as that of F.A., we may find some answers into the localization of function in the human brain.

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