Comprehensive Interpretative Function of the Posterior Superior Temporal Lobe-“Wernicke’s Area” (a General Interpretative Area)
The somatic, visual, and auditory association areas all meet one another in the posterior part of the superior temporal lobe, shown in Figure 57–7, where the tem-poral, parietal, and occipital lobes all come together. This area of confluence of the different sensory inter-pretative areas is especially highly developed in the dominant side of the brain—the left side in almost allright-handed people—and it plays the greatest single role of any part of the cerebral cortex for the higher comprehension levels of brain function that we callintelligence. Therefore, this region has been called bydifferent names suggestive of an area that has almost global importance: thegeneral interpretative area, the gnostic area, the knowing area, the tertiary association area, and so forth. It is best known asWernicke’s area in honor of the neurologist who first described its special significance in intellectual processes.
After severe damage in Wernicke’s area, a person might hear perfectly well and even recognize different words but still be unable to arrange these words into a coherent thought. Likewise, the person may be able to read words from the printed page but be unable to recognize the thought that is conveyed.
Electrical stimulation in Wernicke’s area of a con-scious person occasionally causes a highly complex thought. This is particularly true when the stimulation electrode is passed deep enough into the brain to approach the corresponding connecting areas of the thalamus. The types of thoughts that might be experi-enced include complicated visual scenes that one might remember from childhood, auditory hallucina-tions such as a specific musical piece, or even a state-ment made by a specific person. For this reason, it is believed that activation of Wernicke’s area can call forth complicated memory patterns that involve more than one sensory modality even though most of the individual memories may be stored elsewhere. This belief is in accord with the importance of Wernicke’s area in interpreting the complicated meanings of dif-ferent patterns of sensory experiences.
Angular Gyrus—Interpretation of Visual Information. Theangular gyrus is the most inferior portion of the pos-terior parietal lobe, lying immediately behind Wer-nicke’s area and fusing posteriorly into the visual areas of the occipital lobe as well. If this region is destroyed while Wernicke’s area in the temporal lobe is still intact, the person can still interpret auditory experi-ences as usual, but the stream of visual experiences passing into Wernicke’s area from the visual cortex is mainly blocked. Therefore, the person may be able to see words and even know that they are words but not be able to interpret their meanings. This is the condi-tion called dyslexia, orword blindness.
Let us again emphasize the global importance of Wernicke’s area for processing most intellectual functions of the brain. Loss of this area in an adult usually leads thereafter to a lifetime of almost demented existence.
Concept of the Dominant Hemisphere
The general interpretative functions of Wernicke’s area and the angular gyrus, as well as the functions of the speech and motor control areas, are usually much more highly developed in one cerebral hemisphere than in the other. Therefore, this hemi-sphere is called thedominant hemisphere. In about 95 per cent of all people, the left hemisphere is the dom-inant one.
Even at birth, the area of the cortex that will even-tually become Wernicke’s area is as much as 50 per cent larger in the left hemisphere than in the right in more than one half of neonates. Therefore, it is easy to understand why the left side of the brain might become dominant over the right side. However, if for some reason this left side area is damaged or removed in very early childhood, the opposite side of the brain will usually develop dominant characteristics.
A theory that can explain the capability of one hemisphere to dominate the other hemisphere is the following.
The attention of the “mind” seems to be directed to one principal thought at a time. Presumably, because the left posterior temporal lobe at birth is usually slightly larger than the right, the left side normally begins to be used to a greater extent than the right. Thereafter, because of the tendency to direct one’s attention to the better developed region, the rate of learning in the cerebral hemisphere that gains the first start increases rapidly, whereas in the opposite, less-used side, learning remains slight. Therefore, the left side normally becomes dominant over the right.
In about 95 per cent of all people, the left temporal lobe and angular gyrus become dominant, and in the remaining 5 per cent, either both sides develop simul-taneously to have dual function, or, more rarely, the right side alone becomes highly developed, with full dominance.
As discussed later, the premotor speech area (Broca’s area), located far laterally in the intermediate frontal lobe, is also almost always domi-nant on the left side of the brain. This speech area is responsible for formation of words by exciting simul-taneously the laryngeal muscles, respiratory muscles, and muscles of the mouth.
The motor areas for controlling hands are also dominant in the left side of the brain in about 9 of 10 persons, thus causing right-handedness in most people.
Although the interpretative areas of the temporal lobe and angular gyrus, as well as many of the motor areas, are usually highly developed in only the left hemisphere, these areas receive sensory information from both hemispheres and are capable also of con-trolling motor activities in both hemispheres. For this purpose, they use mainly fiber pathways in the corpuscallosum for communication between the two hemi-spheres. This unitary, cross-feeding organization pre-vents interference between the two sides of the brain; such interference could create havoc with both mental thoughts and motor responses.
Role of Language in the Function of Wernicke’s Area and in Intellectual Functions
A major share of our sensory experience is converted into its language equivalent before being stored in the memory areas of the brain and before being processed for other intellectual purposes. For instance, when we read a book, we do not store the visual images of the printed words but instead store the words them-selves or their conveyed thoughts often in language form.
The sensory area of the dominant hemisphere for interpretation of language is Wernicke’s area, and this is closely associated with both the primary and sec-ondary hearing areas of the temporal lobe. This close relation probably results from the fact that the first introduction to language is by way of hearing. Later in life, when visual perception of language through the medium of reading develops, the visual informa-tion conveyed by written words is then presumably channeled through the angular gyrus, a visual associa-tion area, into the already developed Wernicke’s lan-guage interpretative area of the dominant temporal lobe.
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