Discussed here is the more elementary information concerning brain symmetries and asymmetries. It is hoped that this information will be informative for those who wish to have a basic understanding of current research articles.

Asymmetries are commonly found in the human organism. The more prominent and noticeable areas concern handedness differences and eye, ear and foot differences. While these display preference differences there are also many physical asymmetries such as size and shape differences within the paired hemispheres of the brain and within corresponding areas of the left and right hemispheres.

Specialization occurs in each hemisphere of the brain although as to any rule there are generally many exceptions. For simplicity sake, I will initially generalize. The left hemisphere specializes in the language functions and the right hemisphere in the non verbal functions. At birth the left hemisphere is generally larger than the right which leads some researchers to hypothesize that there is a genetic factor which gives preference to language acquiring abilities. They believe that the early predisposition for acquisition of language is what sets the left side of the brain's initial dominance. You can factor into this the life span tendency in cultural factors of preference for analytical thinking verses holistic thinking and you have a nature verses nurture debate.

In western cultures we praise those attributes which are generally specialized in the left brain hemispheres, whereas, in eastern cultures emphasis is placed on a whole rather than part, or gestalt, type of thinking which is more specialized by right brain hemisphere. What I am saying here is that despite genetic factors, societies and cultures can, to a certain extent, mold the tendency for greater exercise of right or left brain dominance. Another consideration for exercise of left or right brain functioning would be levels of development. Early childhood is, in fact, a time where there is considerable focus on language acquisition which would indicate a more prevalent use of left brain functioning. Within the western culture we tend, in our education system, to continue with those activities which encourage the further use of this dominance rather than equalizing it with activities which would also stimulate more right brain functioning. I think we are selling ourselves short.

Recent research since the early 1960's is indicating more cross over of brain functioning than was earlier believed. There research had been developed through the study of the functioning of patients with brain lesions or split brain patients following cerebral cortex surgery. Two of the early investigators identified two area of the left hemisphere of the brain which caused speech interference. Broca identified an area in the posterior inferior portion of the left frontal lobe that, when damaged, caused an inability to adequately verbalize a thought. Speech was stunted in quantity and consisted of repetition of one or two words. Further study indicated that these patients could understand both the written and verbal word but could not express their understanding. Damage to this area was a result of "motor output instead of a receptive comprehension) disorder." (pp 53)

Another investigator, Karl Wernicke, also studying similar disorders, claimed to have discovered a second speech center within the left hemisphere. He identified an area in the left temporal lobe just posterior to the primary auditory cortex. Wernicke's patients who suffered damage to this area of the left hemisphere endured deficits in the comprehension of words spoken. These patients would speak rapidly, often mispronounce words, and incorporate meaningless terms within their attempts at speech. In the case of Wernicke's disorder the patients were able to recover partially as the right hemisphere gradually took over the function of speech, although the competency did not return to previous levels.

Damage to the right hemisphere of the brain is not generally as dramatic as that with the left. The functioning of the right hemisphere is more gestalt in nature. It perceives patterns globally so it can compensate easier to isolated damage to one specific area of it's right hemisphere. It should also be remembered that in most western cultures the functions held by the right hemisphere of the brain are given secondary status to those of the left hemisphere. Language is given a primary God status in the western cultures. Early research centered around disorders of speech. Spatial disorders such as the affective aphasia, which were the result of right brain damage, were not researched until much later.

Increased right hemispheric activity has been identified in such affective disorders as unipolar depression and schizophrenia and shows heightened left hemispheric activity. Kinsbourne posits that in normal subjects, left and right brain activity are generally balanced. With verbal cues, dominance shifts to left brain dominance and with visiospatial cues it is shifted to the right hemisphere. When the left brain hemisphere is activated than the subject's right side is dominant including right ear and vision dominance. Right brain activation conversely causes left vision and ear advantage.

There appears to be differences in brain lateralization between the genders. Females tend toward bilateralization and males toward more specialized lateralization. Females are more adept at verbal left hemisphere dominance with a crossover to the right hemisphere for verbal abilities. Males are dominant for spatial abilities held by the right hemisphere. One theory states that the specialization may have occurred during early evolution with females who required more communication abilities to rear their young. The males made use of more visiospatial abilities in their skills required for the survival of their families, such as hunting and migration.

Brain structure differences also appear within the genders. Studies involving hemispheric weights indicate that the female brain has less hemispheric difference in weight, with more symmetrical sylvian fissures and temporal planums than in male brains. This fact, according to researchers, tends to support the female bilateralization theory. Other research involves the maturation rate and the effects of hormones on brain circuitry between males and females. The author does not discount the influence of environment but also supports a genetic factor for the differences of hemispheric specialization between the genders. In studies of rats, brain development was not significantly different until the fetal developmental stage where androgens are introduced into the system. Where testosterone levels increased the brain asymmetries occurred and when researchers castrated the males at birth the normal cortel asymmetries were not evidenced, "rather, thicker left (than right) corti were observed by 90 days of age. Newborn females who had their ovaries removed showed the typical male asymmetries of significantly thicker right corti."(p 143) This indicated that testosterone effects postnatal development of the hemispheres. There is conflicting research concerning techniques involved in experimental research concerning the gender differences of brain lateralization. One such study involved the measurement of blood flow within the subjects while performing various tasks geared toward right hemispheric involvement. Results indicated that both subjects had increased cerebral blood flow in the left hemisphere while performing tasks involving verbal analogies. For tasks involving spatial problem solving, females had a significantly higher level of cerebral blood flow than their male counterparts. These results were replicated by other researchers. Initial speculation was that these results negated previous theories of female bilateralization. Another opinion posits that the differences in the increase of cerebral blood flow was more a result of the females increased efforts toward performance of the task than was the hemispheric laterality. The author states "Females presumably were having more difficulties processing the spatial inputs because their bilateral verbal blueprint was in direct conflict with that nonverbal function." (p 152)