Although the two cerebral hemispheres appear to be nearly symmetrical, the left hemisphere has a dominance over the right in 90% of people with regard to language comprehension and motor production (Wernicke's and Broca's areas respectively), with each having anatomical, chemical, and functional specializations. Since the left cerebral hemisphere is highly specialized in language understanding, it is the side of the brain where the values or moral codes of a given culture is deeply internalized; thus, there is also a dominance of the left with regard to social adaptation; so, it has the cultural logic. We must remember that the left hemisphere deals with the somatosensory and motor functions of the right side of the body, and vice versa, due to the pyramid nerve crossing in the medulla oblongata.
Damage to the temporal region known as Wernicke’s area generally results in aphasias that are more closely related to comprehension—the individuals have difficulty understanding spoken or written language even though their hearing and vision are unimpaired. Although they may have fluent speech, their speech is incomprehensible. In contrast, damage to Broca’s area, the language area in the frontal cortex responsible for the articulation of speech, can cause expressive aphasias—the individuals have difficulty carrying out the coordinated respiratory and oral movements necessary for language even though they can move their lips and tongue. They understand spoken language and know what they want to say but have trouble forming words and putting them into grammatical order.
The potential for the development of language specific mechanisms in the left hemisphere is present at birth, but the assignment of language functions to specific brain areas is fairly flexible in the early years of life. Thus, for example, damage to the perisylvian area of the left hemisphere during infancy or early childhood causes temporary, minor language impairment, but similar damage acquired during adulthood typically causes permanent, devastating language deficits. By puberty, the transfer of language functions to the right hemisphere is less successful, and often language skills on this side are lost permanently.