Handedness, Culture, Genes and Cerebral Specialization
You've probably heard about differences between "right-brained" and "left-brained" people. For instance, "right-brained" people are sometimes described as creative, while their "left-brained" counterparts are characterized as logical. These ideas sprang from the tendency of journalists to oversimplify and misinterpret research findings (Coren, 1993). Such a notion has no scientific basis, yet it has served to heighten public interest in hemispheric specialization and neuroscience in general (Hellige, 1993). In fact, despite their specialized functions, the right and left hemispheres are always in contact, forming just one unit, thanks to the corpus callosum. But research has shown that some Lateralization of the hemispheres exists; that is, each hemisphere is specialized to handle certain functions.
The left hemisphere
In 95% of right-handers and in about 62% of left-handers, the left hemisphere handles the most of language functions, including speaking, writing, reading, speech comprehension, and comprehension of the logic of written information (Hellige, 1990; Long and Bynes, 2002). But relating written information to its context involves both hemispheres. Likewise, American sign language (ASL), used by deaf persons, is processed by both hemispheres (Neville et al., 1998). The left hemisphere is also specialized for mathematical abilities, particularly calculation, and it processes information in an analitical and secuential, or step-by-step, manner. Logic is primarily a left hemisphere activity.
The right hemisphere
The right hemisphere is generally considered to be the hemisphere more adept at visual-spatial relations. And the auditory cortex in the right hemisphere appears to be far better able to process music than the left (Zatorre et al., 2002). When you arrange your bedroom furniture or notice that your favorite song is being played on the radio, you are relying primarily on your right hemisphere. The right hemisphere also augments the left hemisphere's language-processing activities.
Handedness, Culture, and Genes
Since we've been discussing right and left hemispheres, you might be wondering whether right- and left-handedness have anything to do with hemispheric specialization. Investigators have identified differences in the brains of left- and right-handers that suggest that the process of hemispheric specialization and the development of handedness may be related. On average, the corpus callosum of left-handers is 11% larger and contains up to 2.5 million more nerve fibers than that of right-handers (Witelson, 1985). In general, the two sides of the brain are less specialized in left-handers (Hellige et al., 1994). There is also evidence that new learning is more easily transferred from one side of the brain to the other in left-handlers.(Schmidt et al., 2000).
In adition, left-handlers tend to experience less language loss following an injury to either hemisphere. They are also more likely to recover, because the undamaged hemisphere can more easily take over the speech functions. On the other hand, left-handers tend to have higher rates of learning disabilities and mental disorders than right-handlers, perhaps because of differences in brain organization (Grouios et al., 1999; Hernandez et al., 1997; Tanner, 1990).
It used to be belived that cultural forces were responsible for the high incidence of right-handedness. For instance, in cultures where there is a great deal of pressure on children to adopt a right-hand preference, there are lower proportions of left-handers than there are in societies, such as the United States, where it is widely belived that forcing children to be right-handed is detrimental to their development (Wilson, 1998). Further, the proportion of left-handlers among elderly people in the United States is smaller than that for younger groups (Porak & Friesen, 2000). One possible explanation for this generational difference is that children of earlier generations who displayed a left-handled preference were more strongly pressured to switch to the right hand than were children born later. Still, right-handedness continues to predominate even in cultures that are highly tolerant of left-handedness among children.
Anthropologists belive that the biologically based predominance of right-handedness explains the pervasiveness of right-left symbolism in mythology, art, and language. Cross-cultural studies show that right-handedness is typically characterized as normal, while left-handedness is associated either with evil or with exeptional abilities (Hicks & Gwynne, 1996). For example, if you examine a deck of Tarot cards, you will see that justice is right-handed, while evil is left-handed. And we are asked to rise our right hands when we take an oath of any kind. Note, however, that the positive association of left-handedness with exeptional talent seems to have some basis in fact. Left-handlers are more numerous among artists, musicians, and political leaders (Wilson, 1998).
Consistent with the genetic hypothesis, human's handedness is evident early in life. Herpper and others (1990) found that, of the human fetuses they observed, 94.6% were sucking their right thumb and only 5.4% were sucking their left thumb. And if you carefully observe infant pointing behavior, you will see that babies almost always point with their right hand (Butterworth et al., 2002). By the age of 5, a large majority of children show a consistence preference for the right hand when manipulating objects.
Some neuroscientists belive that handedness is just one manifestation of a general "rightward bias" in the human nervous system. Even infants, for example, are more likely to attend to an object that appears in their right visual field than one that appears in the left (Butterworth et al., 2002). And more than half of left-handlers demonstrate right-side dominance with regard to motor skills that do not involve the hands, such as kicking (Bourassa et al., 1996). Moreover, only 26% of children born to two left-handed parents are left-handed themselves (McManus & Bryden, 1992). This findings suggest that a rightward bias may be a biologically based characteristic of the entire human species that has a more powerful influence on hand preference than does individual heredity.
The World of Psychology p. 57, 58, 60, 61
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