prefrontal association areas have the capability of calling forth information from widespread areas of the brain and using this information to achieve deeper thought patterns for attaining goals. If these goals include motor action, so be it. If they do not, then the thought processes attain intellectual analytical goals.
Although people without prefrontal cortices can still think, they show little concerted thinking in logical sequence for longer than a few seconds or a minute or so at most. One of the results is that people without prefrontal cortices are easily distracted from their central theme of thought, whereas people with functioning prefrontal cortices can drive themselves to completion of their thought goals irrespective of distractions.
Elaboration of Thought, Prognostication, and Performance of Higher Intellectual Functions by the Prefrontal Areas—Concept of a "Working Memory." Another function that has been ascribed to the prefrontal areas by psychologists and neurologists is elaboration of thought. This means simply an increase in depth and abstractness of the different thoughts put together from multiple sources of information. Psychological tests have shown that pre-frontal lobectomized lower animals presented with successive bits of sensory information fail to keep track of these bits even in temporary memory, probably because they are distracted so easily that they cannot hold thoughts long enough for memory storage to take place.
This ability of the prefrontal areas to keep track of many bits of information simultaneously and to cause recall of this information instantaneously as it is needed for subsequent thoughts is called the brain's "working memory." This could well explain the many functions of the brain that we associate with higher intelligence. In fact, studies have shown that the pre-frontal areas are divided into separate segments for storing different types of temporary memory, such as one area for storing shape and form of an object or a part of the body and another for storing movement.
By combining all these temporary bits of working memory, we have the abilities to (1) prognosticate; (2) plan for the future; (3) delay action in response to incoming sensory signals so that the sensory information can be weighed until the best course of response is decided; (4) consider the consequences of motor actions before they are performed; (5) solve complicated mathematical, legal, or philosophical problems; (6) correlate all avenues of information in diagnosing rare diseases; and (7) control our activities in accord with moral laws.
Function of the Brain in Communication—Language Input and Language Output
One of the most important differences between human beings and lower animals is the facility with which human beings can communicate with one another. Furthermore, because neurological tests can easily assess
SPEAKING A HEARD WORD Motor cortex
SPEAKING A HEARD WORD Motor cortex
SPEAKING A WRITTEN WORD Motor cortex à /
Brain pathways for (top) perceiving a heard word and then speaking the same word, and (bottom) perceiving a written word and then speaking the same word. (Redrawn from Geschwind N: Specializations of the human brain. Sci Am 241:180,1979. ® 1979 by Scientific American, Inc. All rights reserved.)
the ability of a person to communicate with others, we know more about the sensory and motor systems related to communication than about any other segment of brain cortex function. Therefore, we will review, with the help of anatomical maps of neural pathways in Figure 57-8, function of the cortex in communication. From this, one will see immediately how the principles of sensory analysis and motor control apply to this art.
There are two aspects to communication: first, the sensory aspect (language input), involving the ears and eyes, and, second, the motor aspect (language output), involving vocalization and its control.
Sensory Aspects of Communication. We noted earlier in the chapter that destruction of portions of the auditory or visual association areas of the cortex can result in inability to understand the spoken word or the written word.
These effects are called, respectively, auditory receptive aphasia and visual receptive aphasia or, more commonly, word deafness and word blindness (also called dyslexia).
Wernicke's Aphasia and Global Aphasia. Some people are capable of understanding either the spoken word or the written word but are unable to interpret the thought that is expressed. This results most frequently when Wernicke's area in the posterior superior temporal gyrus in the dominant hemisphere is damaged or destroyed. Therefore, this type of aphasia is called Wernicke's aphasia.
When the lesion in Wernicke's area is widespread and extends (1) backward into the angular gyrus region, (2) inferiorly into the lower areas of the temporal lobe, and (3) superiorly into the superior border of the sylvian fissure, the person is likely to be almost totally demented for language understanding or communication and therefore is said to have global aphasia.
Motor Aspects of Communication. The process of speech involves two principal stages of mentation: (1) formation in the mind of thoughts to be expressed as well as choice of words to be used and then (2) motor control of vocalization and the actual act of vocalization itself.
The formation of thoughts and even most choices of words are the function of sensory association areas of the brain. Again, it is Wernicke's area in the posterior part of the superior temporal gyrus that is most important for this ability. Therefore, a person with either Wer-nicke's aphasia or global aphasia is unable to formulate the thoughts that are to be communicated. Or, if the lesion is less severe, the person may be able to formulate the thoughts but unable to put together appropriate sequences of words to express the thought. The person sometimes is even fluent with words but the words are jumbled.
Loss of Broca's Area Causes Motor Aphasia. Sometimes a person is capable of deciding what he or she wants to say but cannot make the vocal system emit words instead of noises. This effect, called motor aphasia, results from damage to Broca's speech area, which lies in the prefrontal and premotor facial region of the cerebral cortex—about 95 per cent of the time in the left hemisphere, as shown in Figures 57-5 and 57-8. Therefore, the skilled motor patterns for control of the larynx, lips, mouth, respiratory system, and other accessory muscles of speech are all initiated from this area.
Articulation. Finally, we have the act of articulation, which means the muscular movements of the mouth, tongue, larynx, vocal cords, and so forth that are responsible for the intonations, timing, and rapid changes in intensities of the sequential sounds. The facial and laryn-geal regions of the motor cortex activate these muscles, and the cerebellum, basal ganglia, and sensory cortex all help to control the sequences and intensities of muscle contractions, making liberal use of basal ganglial and cerebellar feedback mechanisms described in Chapters 55 and 56. Destruction of any of these regions can cause either total or partial inability to speak distinctly.
Summary. Figure 57-8 shows two principal pathways for communication. The upper half of the figure shows the pathway involved in hearing and speaking. This sequence is the following: (1) reception in the primary auditory area of the sound signals that encode the words; (2) interpretation of the words in Wernicke's area; (3) determination, also in Wernicke's area, of the thoughts and the words to be spoken; (4) transmission of signals from Wernicke's area to Broca's area by way of the arcuate fasciculus; (5) activation of the skilled motor programs in Broca's area for control of word formation; and (6) transmission of appropriate signals into the motor cortex to control the speech muscles.
The lower figure illustrates the comparable steps in reading and then speaking in response.The initial receptive area for the words is in the primary visual area rather than in the primary auditory area.Then the information passes through early stages of interpretation in the angular gyrus region and finally reaches its full level of recognition in Wernicke's area. From here, the sequence is the same as for speaking in response to the spoken word.
Function of the Corpus Callosum and Anterior Commissure to Transfer Thoughts, Memories, Training, and Other Information Between the Two Cerebral Hemispheres
Fibers in the corpus callosum provide abundant bidirectional neural connections between most of the respective cortical areas of the two cerebral hemispheres except for the anterior portions of the temporal lobes; these temporal areas, including especially the amygdala, are interconnected by fibers that pass through the anterior commissure.
Because of the tremendous number of fibers in the corpus callosum, it was assumed from the beginning that this massive structure must have some important function to correlate activities of the two cerebral hemispheres. However, when the corpus callosum was destroyed in laboratory animals, it was at first difficult to discern deficits in brain function. Therefore, for a long time, the function of the corpus callosum was a mystery.
Properly designed psychological experiments have now demonstrated extremely important functions for the corpus callosum and anterior commissure. These functions can best be explained by describing one of the experiments:A monkey is first prepared by cutting the corpus callosum and splitting the optic chiasm longitudinally, so that signals from each eye can go only to the cerebral hemisphere on the side of the eye. Then the monkey is taught to recognize different objects with its right eye while its left eye is covered. Next, the right eye is covered and the monkey is tested to determine whether its left eye can recognize the same objects. The answer to this is that the left eye cannot recognize the objects. However, on repeating the same experiment in another monkey with the optic chiasm split but the corpus callosum intact, it is found invariably that recognition in one hemisphere of the brain creates recognition in the opposite hemisphere.
Thus, one of the functions of the corpus callosum and the anterior commissure is to make information stored in the cortex of one hemisphere available to corresponding cortical areas of the opposite hemisphere. Important examples of such cooperation between the two hemispheres are the following.
1. Cutting the corpus callosum blocks transfer of information from Wernicke's area of the dominant hemisphere to the motor cortex on the opposite side of the brain. Therefore, the intellectual functions of Wernicke's area, located in the left hemisphere, lose control over the right motor cortex that initiates voluntary motor functions of the left hand and arm, even though the usual subconscious movements of the left hand and arm are normal.
2. Cutting the corpus callosum prevents transfer of somatic and visual information from the right hemisphere into Wernicke's area in the left dominant hemisphere. Therefore, somatic and visual information from the left side of the body frequently fails to reach this general interpretative area of the brain and therefore cannot be used for decision making.
3. Finally, people whose corpus callosum is completely sectioned have two entirely separate conscious portions of the brain. For example, in a teenage boy with a sectioned corpus callosum, only the left half of his brain could understand both the written word and the spoken word because the left side was the dominant hemisphere. Conversely, the right side of the brain could understand the written word but not the spoken word. Furthermore, the right cortex could elicit a motor action response to the written word without the left cortex ever knowing why the response was performed.
The effect was quite different when an emotional response was evoked in the right side of the brain: In this case, a subconscious emotional response occurred in the left side of the brain as well. This undoubtedly occurred because the areas of the two sides of the brain for emotions, the anterior temporal cortices and adjacent areas, were still communicating with each other through the anterior commissure that was not sectioned. For instance, when the command "kiss" was written for the right half of his brain to see, the boy immediately and with full emotion said, "No way!" This response required function of Wernicke's area and the motor areas for speech in the left hemisphere because these left-side areas were necessary to speak the words "No way!" But when questioned why he said this, the boy could not explain. Thus, the two halves of the brain have independent capabilities for consciousness, memory storage, communication, and control of motor activities. The corpus callosum is required for the two sides to operate cooperatively at the superficial subconscious level, and the anterior commissure plays an important additional role in unifying the emotional responses of the two sides of the brain.
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