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The Geschwind–Galaburda hypothesis was proposed by Norman Geschwind and Albert Galaburda to explain sex differences in cognitive abilities by relating them to lateralization of brain function. The basic idea is that differences in maturation rates between the cerebral hemispheres are mediated by circulating testosterone levels, and that sexual maturation acts to fix the hemispheres at different relative stages of development after puberty.
According to the theory, male brains mature later than females, and the left hemisphere matures later than the right.
Although the Geschwind-Galaburda hypothesis has been cited in mainstream media and publication resources as a cause for left-handedness, very little research evidence (if any) has been presented to substantiate the theory. In fact, evidence has emerged suggesting that high prenatal estrogen exposure is just as likely to enhance the gene expression for left-handedness. In a study endorsed by the Centers for Disease Control (CDC), it is suggested that men who were prenatally exposed to diethylstilbestrol (a synthetic estrogen based fertility drug), are more likely to be left-handed than unexposed men.
|In cognitive abilities||Geschwind–Galaburda hypothesis|
|In eyes||Ocular dominance|
|Handedness in boxing||Southpaw stance||Orthodox stance|
|Handedness in people||Musicians|
|Handedness related to|
|Handedness measurement||Edinburgh Handedness Inventory|
|In major viscera||Situs solitus||Situs ambiguus||Situs inversus|
|Footedness in surfing||Regular foot||Goofy foot|
In human biology, handedness is the better, faster, or more precise performance or individual preference for use of a hand, known as the dominant hand. The incapable, less capable or less preferred hand is called the non-dominant hand. Right-handedness is most common; about 90% of people are right-handed. Handedness is often defined by one's writing hand, as it is fairly common for people to prefer to do some tasks with each hand. There are examples of true ambidexterity, but it is rare—most people prefer one hand for most purposes.
Colpocephaly is a cephalic disorder involving the disproportionate enlargement of the occipital horns of the lateral ventricles and is usually diagnosed early after birth due to seizures. It is a nonspecific finding and is associated with multiple neurological syndromes, including agenesis of the corpus callosum, Chiari malformation, lissencephaly, and microcephaly. Although the exact cause of colpocephaly is not known yet, it is commonly believed to occur as a result of neuronal migration disorders during early brain development, intrauterine disturbances, perinatal injuries, and other central nervous system disorders. Individuals with colpocephaly have various degrees of motor disabilities, visual defects, spasticity, and moderate to severe intellectual disability. No specific treatment for colpocephaly exists, but patients may undergo certain treatments to improve their motor function or intellectual disability.
The corpus callosum, also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mammals. It spans part of the longitudinal fissure, connecting the left and right cerebral hemispheres, enabling communication between them. It is the largest white matter structure in the human brain, about ten centimetres in length and consisting of 200–300 million axonal projections.
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter, the cerebral cortex, that is supported by an inner layer of white matter. In eutherian (placental) mammals, the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix, also join the hemispheres and these are also present in other vertebrates. These commissures transfer information between the two hemispheres to coordinate localized functions.
The term laterality refers to the preference most humans show for one side of their body over the other. Examples include left-handedness/right-handedness and left/right-footedness; it may also refer to the primary use of the left or right hemisphere in the brain. It may also apply to animals or plants. The majority of tests have been conducted on humans, specifically to determine the effects on language.
The planum temporale is the cortical area just posterior to the auditory cortex within the Sylvian fissure. It is a triangular region which forms the heart of Wernicke's area, one of the most important functional areas for language. Original studies on this area found that the planum temporale was one of the most asymmetric regions in the brain, with this area being up to ten times larger in the left cerebral hemisphere than the right.
The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The medial longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. Although the macrostructure of the two hemispheres appears to be almost identical, different composition of neuronal networks allows for specialized function that is different in each hemisphere.
Samuel Torrey Orton was an American physician who pioneered the study of learning disabilities. He examined the causes and treatment of dyslexia.
Norman Geschwind was a pioneering American behavioral neurologist, best known for his exploration of behavioral neurology through disconnection models based on lesion analysis.
In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:
The relationship between the environment and sexual orientation is a subject of research. In the study of sexual orientation, some researchers distinguish environmental influences from hormonal influences, while other researchers include biological influences such as prenatal hormones as part of environmental influences.
Researchers have suggested a link between handedness and ability with mathematics. This link has been proposed by Geschwind, Galaburda, Annett, and Kilshaw. The suggested link is that a brain without extreme bias towards locating language in the left hemisphere would have an advantage in mathematical ability.
Sexual orientation is an enduring pattern of romantic or sexual attraction to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender, or none of the aforementioned at all. The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However, advances in neuroscience explain and illustrate characteristics linked to sexual orientation. Studies have explored structural neural-correlates, functional and/or cognitive relationships, and developmental theories relating to sexual orientation in humans.
The history of dyslexia research spans from the late 1800s to the present.
The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult. Prenatal hormones may be seen as the primary determinant of adult sexual orientation, or a co-factor with genes, biological factors and/or environmental and social conditions.
Functional disconnection is the disintegrated function in the brain in the absence of anatomical damage, in distinction to physical disconnection of the cerebral hemispheres by surgical resection, trauma or lesion. The concept was first coined by Leisman; and Sroka, Solsi, and Bornstein Applications have included alexia without agraphia dyslexia, persistent vegetative state and minimally conscious state as well as autistic spectrum disorders.
Cerebellar cognitive affective syndrome (CCAS), also called Schmahmann's syndrome is a condition that follows from lesions (damage) to the cerebellum of the brain. It refers to a constellation of deficits in the cognitive domains of executive function, spatial cognition, language, and affect resulting from damage to the cerebellum. Impairments of executive function include problems with planning, set-shifting, abstract reasoning, verbal fluency, and working memory, and there is often perseveration, distractibility and inattention. Language problems include dysprosodia, agrammatism and mild anomia. Deficits in spatial cognition produce visual–spatial disorganization and impaired visual–spatial memory. Personality changes manifest as blunting of affect or disinhibited and inappropriate behavior. These cognitive impairments result in an overall lowering of intellectual function. CCAS challenges the traditional view of the cerebellum being responsible solely for regulation of motor functions. It is now thought that the cerebellum is responsible for monitoring both motor and nonmotor functions. The nonmotor deficits described in CCAS are believed to be caused by dysfunction in cerebellar connections to the cerebral cortex and limbic system.
Daniel H. Geschwind is the Gordon and Virginia MacDonald Distinguished Professor of Human Genetics, Neurology and Psychiatry at the David Geffen School of Medicine at the University of California, Los Angeles (UCLA). He also directs the UCLA Neurogenetics Program and the UCLA Center for Autism Research and Treatment (CART), and holds the Gordon and Virginia MacDonald Distinguished Chair of Human Genetics there. Since March 1, 2016, he has served as the Senior Associate Dean and Associate Vice Chancellor for Precision Medicine at UCLA. His brother, Michael Geschwind, is also a professor of neurology, and behavioral neurology pioneer Norman Geschwind is his father's first cousin.
Paul Satz was an American psychologist, and one of the founders of the discipline neuropsychology. His research on the relationship between the brain and human behavior spanned diverse topics including laterality, handedness, and developmental disorders. He published over 300 publications, received numerous grants and awards, and established the first neuropsychology lab. Towards the latter part of his career, Satz's research interests focused more on the cognitive deficits associated with head injury, dementia, and ageing.
Albert Mark Galaburda is a cognitive and behavioral neurologist with a special focus on the biologic bases of developmental cognitive disorders. He is the Emily Fisher Landau Professor of Neurology and Neuroscience at Harvard Medical School, the Director of the Office for Diversity, Inclusion, and Career Advancement at Beth Israel Deaconess Medical Center, Boston, and Co-director of the Harvard University Interfaculty Initiative on Mind Brain and Behavior, together with psychologist Alfonso Caramazza. He is best known for his development of the Geschwind-Galaburda hypothesis, which helps explain differences in cognitive abilities on the basis of sex hormones and immunological characteristics and their relationship to lateralization of brain function, as well as for his pioneering studies on the biological foundations of developmental dyslexia. Other work includes the anatomical organization of the auditory cortex in the brains of monkeys and humans and the neuroanatomical and neurodevelopmental bases of brain laterality and asymmetry. He attended the Six-Year Liberal Arts-Medicine Program at Boston University School of Medicine, graduating with an AB-MD degree in 1971, and completed a residency in Internal Medicine and a residency in Neurology at Boston City Hospital, now Boston Medical Center. He was trained in Medicine under Norman Levinsky and in Neurology under Norman Geschwind. He has published numerous scientific articles and books in the field of cognitive neurology, with a focus on learning disabilities and attention disorders, especially in adults.