The brain of vertebrates is divided, from front to back, into two hemispheres which are largely symmetric in their form. The animation shows the division into hemispheres for the human brain.
| Human brain |
|---|
|
|
Right cerebral hemisphere
Left cerebral hemisphere
https://
en.wikipedia.org/wiki/Cerebral_hemisphere |
Lateralization
While the hemispheres are largely symmetric in their form, they do not share equally in all the functions controlled by the brain. The cells of the cortex are partly different in their composition and function. Some of these differences characterize an entire species, including, e.g., dogs and humans. In frogs, e.g., the left hemisphere is responsible for their vocalizations (Lieberman 2002: 50). Other such differences between the hemispheres characterize an individual. This kind of functional and microscopic asymmetry is the lateralization of the brain.
In mammals, each hemisphere controls the limbs of the opposite side. One side is commonly dominant; this is certain for cats, rats and humans, among others. The hemisphere in control of the dominant hand is the dominant hemisphere. The following table summarizes some of the differences between the two hemispheres as they relate to language. These differences obtain at a statistical level and are valid for right-handers. For 30% of the left-handers, the distribution of the properties is inverted.
| left hemisphere | right hemisphere |
|---|---|
| Lobus temporalis larger than opposite one | Lobus frontalis larger than opposite one |
| analytic reasoning | holistic reasoning |
| language processing: subconscious processing of grammar, lexical access |
non-verbal, incl. spatial, reasoning; prosody |
| exact calculation | social interaction, fierce emotions |
On balance, various areas on both sides of the brain are involved in the processing of language. However, the dominant hemisphere is mainly responsible for the properly semiotic operations, matching expression and meaning and calculating the structure of complex messages both in production and in perception. The non-dominant hemisphere is mainly responsible for the holistic handling of messages, their emotional aspects, their social appropriateness and consequences, in short, for their integration with everything else. Both in humans and in apes, the planum temporale (including Wernicke's area) and Broca's area are asymmetric in having special functions in the dominant hemisphere.
At birth, the brain has a high degree of plasticity, which produces considerable differences in brain functions between individuals. Neural plasticity decreases with age. However, if a region in one hemisphere is damaged, the corresponding region of the opposite hemisphere can take on its functions even in adults, although to a decreasing extent. This includes, specifically, damages in Broca's and Wernicke's areas.