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Cerebral Cortex, Intellectual Functions of the Brain, Learning, and Memory
It is ironic that of all the parts of the brain, we are the least certain about the functions of the cerebral cortex, even though it is by far the largest and perhaps the most studied portion of the nervous system. However, we do know the effects of damage or stimulation of various portions of the cerebral cortex. In the first part of this chapter, the known cortical functions are discussed, and then basic theories of neuronal mechanisms involved in thought processes, memory, analysis of sensory information, and so forth are presented briefly.
Physiologic Anatomy of the Cerebral Cortex
The functional part of the cerebral cortex is a thin layer of neurons covering the surface of all the convolutions of the cerebrum. This layer is only 2 to 5 millimeters thick, with a total area of about 25% of a square meter. The total cerebral cortex has been estimated to contain over 80 billion neurons.
Figure 58-1 shows the typical histological structure of the neuronal surface of the cerebral cortex, with its successive layers of different types of neurons. Most of the neurons are of three types: (1) granular (also called stellate ); (2) fusiform; and (3) pyramidal, the latter named for their characteristic pyramidal shape.
The granular neurons generally have short axons and, therefore, function mainly as interneurons that transmit neural signals only short distances in the cortex. Some are excitatory, releasing mainly the excitatory neurotransmitter glutamate, whereas others are inhibitory and release mainly the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). The sensory areas of the cortex, as well as the association areas between sensory and motor areas, have large concentrations of these granule cells, suggesting a high degree of intracortical processing of incoming sensory signals within the sensory areas and association areas.
The pyramidal and fusiform cells give rise to almost all the output fibers from the cortex. The pyramidal cells, which are larger and more numerous than the fusiform cells, are the source of the long, large nerve fibers that go all the way to the spinal cord. The pyramidal cells also give rise to most of the large subcortical association fiber bundles that pass from one major part of the brain to another.
To the right in Figure 58-1 is shown the typical organization of nerve fibers within the different layers of the cerebral cortex. Note particularly the large number of horizontal fibers that extend between adjacent areas of the cortex, but note also the vertical fibers that extend to and from the cortex to lower areas of the brain and some all the way to the spinal cord or to distant regions of the cerebral cortex through long association bundles.
The functions of the specific layers of the cerebral cortex are discussed in Chapter 48, Chapter 52 . By way of review, let us recall that most incoming specific sensory signals from the body terminate in cortical layer IV. Most of the output signals leave the cortex through neurons located in layers V and VI, the very large fibers to the brain stem and cord arise generally in layer V, and the tremendous numbers of fibers to the thalamus arise in layer VI. Layers I, II, and III perform most of the intracortical association functions, with especially large numbers of neurons in layers II and III making short horizontal connections with adjacent cortical areas.
Anatomical and Functional Relations of the Cerebral Cortex to the Thalamus and Other Lower Centers
All areas of the cerebral cortex have extensive to-and-fro efferent and afferent connections with deeper structures of the brain. It is important to emphasize the relation between the cerebral cortex and the thalamus. When the thalamus is damaged along with the cortex, the loss of cerebral function is far greater than when the cortex alone is damaged, because thalamic excitation of the cortex is necessary for almost all cortical activity.
Figure 58-2 shows the areas of the cerebral cortex that connect with specific parts of the thalamus. These connections act in two directions, both from the thalamus to the cortex and then from the cortex back to essentially the same area of the thalamus. Furthermore, when the thalamic connections are cut, the functions of the corresponding cortical area become almost entirely lost. Therefore, the cortex operates in close association with the thalamus and can almost be considered both anatomically and functionally a unit with the thalamus; for this reason, the thalamus and the cortex together are sometimes called the thalamocortical system . Almost all pathways from the sensory receptors and sensory organs to the cortex pass through the thalamus, with the principal exception of some sensory pathways of olfaction.
Functions of Specific Cortical Areas
Figure 58-3 is a map of some of the functions of different cerebral cortical areas as determined from electrical stimulation of the cortex in awake patients or during neurological examination of patients after portions of their cortex had been removed. The electrically stimulated patients told their thoughts evoked by the stimulation, and sometimes they experienced movements. Occasionally they spontaneously emitted a sound or even a word or gave some other evidence of the stimulation.
Putting large amounts of information together from many different sources gives a more general map, as shown in Figure 58-4 . This figure shows the major primary and secondary premotor and supplementary motor areas of the cortex, as well as the major primary and secondary sensory areas for somatic sensation, vision, and hearing, all of which are discussed in earlier chapters. The primary motor areas have direct connections with specific muscles for causing discrete muscle movements. The primary sensory areas detect specific sensations—visual, auditory, or somatic—transmitted to the brain from peripheral sensory organs.
The secondary areas make sense out of the signals in the primary areas. For example, the supplementary and premotor areas function along with the primary motor cortex and basal ganglia to provide “patterns” of motor activity. On the sensory side, the secondary sensory areas, located within a few centimeters of the primary areas, begin to analyze the meanings of the specific sensory signals, such as the following: (1) interpretation of the shape or texture of an object in one’s hand; (2) interpretation of color, light intensity, directions of lines and angles, and other aspects of vision; and (3) interpretations of the meanings of sound tones and sequence of tones in the auditory signals.
Association Areas
Figure 58-4 also shows several large areas of the cerebral cortex that do not fit into the rigid categories of primary and secondary motor and sensory areas. These areas are called association areas because they receive and analyze signals simultaneously from multiple regions of both the motor and sensory cortices, as well as from subcortical structures. Yet, even the association areas have their specializations. Important association areas include (1) the parieto-occipitotemporal association area, (2) the prefrontal association area, and (3) the limbic association area .
Parieto-Occipitotemporal Association Area
The parieto-occipitotemporal association area lies in the large parietal and occipital cortical space bounded by the somatosensory cortex anteriorly, the visual cortex posteriorly, and the auditory cortex laterally. As would be expected, it provides a high level of interpretative meaning for signals from all the surrounding sensory areas. However, even the parieto-occipitotemporal association area has its own functional subareas, which are shown in Figure 58-5 .
Analysis of the Spatial Coordinates of the Body
An area beginning in the posterior parietal cortex and extending into the superior occipital cortex provides continuous analysis of the spatial coordinates of all parts of the body, as well as of the surroundings of the body. This area receives visual sensory information from the posterior occipital cortex and simultaneous somatosensory information from the anterior parietal cortex. From all this information, it computes the coordinates of the visual, auditory, and body surroundings.
Wernicke’s Area Is Important for Language Comprehension
The major area for language comprehension, called Wernicke’s area, lies behind the primary auditory cortex in the posterior part of the superior gyrus of the temporal lobe. We discuss this area more fully later; it is one of the most important regions of the entire brain for higher intellectual function because most of these intellectual functions are language based.
The Angular Gyrus Area Is Needed for Initial Processing of Visual Language (Reading)
Posterior to the language comprehension area, lying mainly in the anterolateral region of the occipital lobe, is a visual association area that feeds visual information conveyed by words read from a book into Wernicke’s area, the language comprehension area. This angular gyrus area is needed to make meaning out of the visually perceived words. In its absence, a person can still have excellent language comprehension through hearing but not through reading; injury to the angular gyrus can result in agraphia (inability to write) with alexi a (inability to read), a condition in which a person cannot read, write, or spell words.
Area for Naming Objects
In the most lateral portions of the anterior occipital lobe and posterior temporal lobe is an area for naming objects. The names are learned mainly through auditory input, whereas the physical natures of the objects are learned mainly through visual input. In turn, the names are essential for both auditory and visual language comprehension ( functions performed in Wernicke’s area located immediately superior to the auditory “names” region and anterior to the visual word processing area).
Prefrontal Association Area
As discussed in Chapter 57 , the prefrontal association area functions in close association with the motor cortex to plan complex patterns and sequences of motor movements. To aid in this function, it receives strong input through a massive subcortical bundle of nerve fibers connecting the parieto-occipitotemporal association area with the prefrontal association area. Through this bundle, the prefrontal cortex receives much preanalyzed sensory information, especially information on the spatial coordinates of the body that is necessary for planning effective movements. Much of the output from the prefrontal area into the motor control system passes through the caudate portion of the basal ganglia–thalamic feedback circuit for motor planning, which provides many of the sequential and parallel components of movement stimulation.
The prefrontal association area is also essential to carrying out “thought” processes . This characteristic presumably results from some of the same capabilities of the prefrontal cortex that allow it to plan motor activities. It seems to be capable of processing nonmotor and motor information from widespread areas of the brain and therefore to achieve nonmotor types of thinking, as well as motor types. In fact, the prefrontal association area is frequently described simply as important for elaboration of thoughts, and it is said to store on a short-term basis “working memories” that are used to combine new thoughts while they are entering the brain.
Broca’s Area Provides the Neural Circuitry for Word Formation
Broca’s area, shown in Figure 58-5 , is located partly in the posterior lateral prefrontal cortex and partly in the premotor area. It is here that plans and motor patterns for expressing individual words or even short phrases are initiated and executed. This area also works in close association with the Wernicke language comprehension center in the temporal association cortex, as we discuss more fully later in the chapter.
An especially interesting discovery is the following: When a person has already learned one language and then learns a new language, the area in the brain where the new language is stored is slightly removed from the storage area for the first language. If both languages are learned simultaneously, they are stored together in the same area of the brain.
Limbic Association Area
Figures 58-4 and 58-5 show still another association area called the limbic association area. This area is found in the anterior pole of the temporal lobe, in the ventral portion of the frontal lobe, and in the cingulate gyrus lying deep in the longitudinal fissure on the midsurface of each cerebral hemisphere. It is concerned primarily with behavior, emotions, and motivation. We discuss in Chapter 59 that the limbic cortex is part of a much more extensive system, the limbic system, that includes a complex set of neuronal structures in the midbasal regions of the brain. This limbic system provides most of the emotional drives for activating other areas of the brain and even provides motivational drive for the process of learning itself.
Area for Recognition of Faces
An interesting type of brain abnormality called prosopagnosia is the inability to recognize faces. This condition occurs in people who have extensive damage on the medial undersides of both occipital lobes and along the medioventral surfaces of the temporal lobes, as shown in Figure 58-6 . Loss of these face recognition areas, strangely enough, results in little other abnormality of brain function.
One may wonder why so much of the cerebral cortex should be reserved for the simple task of face recognition. However, most of our daily tasks involve associations with other people, and thus one can see the importance of this intellectual function.
The occipital portion of this facial recognition area is contiguous with the visual cortex, and the temporal portion is closely associated with the limbic system that has to do with emotions, brain activation, and control of one’s behavioral response to the environment, as we see in Chapter 59 .
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 58-7 , where the temporal, parietal, and occipital lobes all come together. This area of confluence of the different sensory interpretative areas is especially highly developed in the dominant side of the brain—the left side in almost all right-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 call intelligence. Therefore, this region has been called by different names suggestive of an area that has almost global importance: the general interpretative area, the gnostic area, the knowing area, the tertiary association area, and so forth. It is best known as Wernicke’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 of Wernicke’s area in a conscious person occasionally causes a highly complex thought, particularly 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 experienced include complicated visual scenes that one might remember from childhood, auditory hallucinations such as a specific musical piece, or even a statement 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 different patterns of sensory experiences.
Angular Gyrus—Interpretation of Visual Information
The angular gyrus is the most inferior portion of the posterior parietal lobe, lying immediately behind Wernicke’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 experiences 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 may not be able to interpret their meanings. This condition is called alexia, or word blindness. The term “dyslexia” is used to describe difficulty in learning about written language, not complete word blindness.
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 hemisphere is called the dominant hemisphere . In about 95% of all people, the left hemisphere is the dominant one.
Even at birth, the area of the cortex that will eventually become Wernicke’s area is as much as 50% 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.
The following theory can explain the capability of one hemisphere to dominate the other hemisphere. 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 lobe, the left side normally begins to be used to a greater extent than is the right side. 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 less well developed. Therefore, the left side normally becomes dominant over the right side.
In about 95% of all people, the left temporal lobe and angular gyrus become dominant, and in the remaining 5%, either both sides develop simultaneously to have dual function or, more rarely, the right side alone becomes highly developed, with full dominance.
As discussed later in this chapter, the premotor speech area (Broca’s area), located far laterally in the intermediate frontal lobe, is also almost always dominant on the left side of the brain. This speech area is responsible for formation of words by exciting simultaneously 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 90% of 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 also capable of controlling motor activities in both hemispheres. For this purpose, they use mainly fiber pathways in the corpus callosum for communication between the two hemispheres. This unitary, cross-feeding organization prevents interference between the two sides of the brain; such interference could create havoc with both mental thoughts and motor responses.
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