Neuronal Representations Knowledge Stores

In the first chapter I mentioned Thomas Kuhn's proposal that discovery commences with the awareness of an anomaly or the recognition that nature has somehow violated the paradigm-induced expectations that govern normal science. Thus, the observation of an anomaly suggests that the scientific theory that attempted to explain the order in a system is inadequate and a new theory that accounts for this anomaly must be devised. A corollary of Kuhn's hypothesis would be that to be able to recognize an anomaly, one would have to have a large store of knowledge in that cognitive domain. Many of the most exciting scientific discoveries appear to happen almost by accident. An example of how important stored knowledge is to scientific discovery is the story of Alexander Fleming, who, as I mentioned previously, noticed that the penicillium mold that accidentally had blown into his bacterial cultures killed the bacteria. Prior to this accident, Alexander Fleming was performing bacteriological research and was interested in antiseptics and how the body fights infections. Thus, Fleming had a mind that was prepared for this discovery. This finding, to use Kuhn's term, was an anomaly, but it was the "prepared mind" of Fleming that discovered the importance of this phenomenon.

Because knowledge and skills are lost or degraded when the cerebral cortex is injured or degenerates, we know that knowledge is stored in the cortex. To be creative one has to have excellent stores of knowledge. In the next two sections, I discuss where knowledge is stored and how it is stored.

There have been two major theories as to how the brain stores information. Some researchers have believed that the storage of knowledge is distributed over almost the entire cerebral cortex (mass action hypothesis), whereas others believe that different forms of knowledge and different types of thinking (e.g., verbal versus visu-ospatial) are mediated by different parts of the brain (localizationist or modular hypothesis).

Franz Gall first put forth the modular or localizationist hypothesis in the latter part of the 18th and early part of the 19th centuries. He thought that intellect was mediated by the two cerebral hemispheres, united by the corpus callosum, and that the brain stem was important for controlling vital forces such as respiration. These major hypotheses were subsequently all shown to be correct. He also proposed that the brain was organized in an anatomically distributed modular fashion such that different human faculties are located in different anatomic areas of the cerebral cortex. Gall also reasoned that if certain brain functions are mediated by specific anatomic areas, then the more brain tissue a person has devoted to this function the better this person would perform this function. Because the size of the brain region influences the shape of the skull overlying this area, Gall and his students believed that one should be able to measure a person's abilities to perform different functions by measuring portions of her or his skull.

Unfortunately, Gall's modular hypothesis was temporarily discarded because it led to the pseudoscience of phrenology, with its many unfounded claims about skull shapes and mental abilities. The major problem with phrenology was that no experimental evidence supported the claims made by the practitioners. The major assumption of modularity, however, was not tested until the middle of the 19th century, when Paul Broca, a French physician and anthropologist, listened to a lecture by Auburtin, who was a student of Bouillaud, one of Gall's students. Gall noted that fluent speakers have prominent foreheads and suggested that the facility of speech is mediated by the frontal lobes of the brain. After the lecture, Broca invited Auburtin to the hospital to see a diabetic man who, as a result of a prior stroke, had weakness of his right arm and was unable to speak except for saying the word tan. Although Broca's patient was unable to speak, he was able to comprehend. Because of his diabetes he had insufficient circulation to his legs and developed gangrene. In the 19th century there were no antibiotics to treat this infection, and this man died. A postmortem examination of his brain revealed that the stroke injured his left hemisphere. The stroke was predominantly anterior, in the left frontal lobe (see Figure 3.2). Broca subsequently described eight patients who were right handed and who had lost their speech from damage to the left hemisphere. Broca's observations supported Gall's postulate of modularity or localized functions.

Figure 3.2. Diagram of the injury described by Paul Broca in a patient who lost the ability to speak fluently but could comprehend speech. The injury primarily damaged the inferior (bottom) portion of the frontal lobe.

To establish that a system is modular, however, you have to demonstrate not only that brain injury to a focal area causes a specific behavioral disturbance (as Broca demonstrated) but also that injuries to other areas of the brain cause different behavioral disorders. Further support of the modular hypothesis came about 10 years after Broca reported this nonfluent aphasic patient. Then the German neurologist Karl Wernicke reported a patient who was almost the opposite of Broca's patient in that he could speak fluently but could not comprehend speech. This patient had an injury to the posterior portion of the superior temporal lobe (see Figure 3.3). Thus, Wernicke demonstrated a double dissociation, namely, that injury to one part of the brain causes certain signs, but injury to another part of the brain induces different signs.

In addition to demonstrating modularity of the speech-language system, Wernicke initiated the development of information-processing models. His clinical observations suggested that this left posterior area of the brain contains the auditory memories of how words sound and that normally this posterior area is able to send this word-sound information to the anterior area, which Broca had found to be important in programming the movements needed to speak. From this new information-processing model one could make new predictions (see Figure 3.4). For example, if these two regions were intact but disconnected by damage to the pathway that connects these two areas (the arcuate fasciculus), a patient should be fluent and able to comprehend, but when speaking, naming, or repeating, the patient would use the wrong speech sounds because the information from the posterior area that contains the memories of word sounds could not provide the anterior

Broca's Area -

Broca's Area -

We rni eke's Area

Figure 3.3. Diagram of the injury described by Karl Wernicke of a patient who spoke in jargon and could not comprehend speech. The injury involves the superior posterior portion of the temporal lobe.

We rni eke's Area

Figure 3.3. Diagram of the injury described by Karl Wernicke of a patient who spoke in jargon and could not comprehend speech. The injury involves the superior posterior portion of the temporal lobe.

Figure 3.4. Diagram of Karl Wernicke's model of how the left hemisphere mediates speech. A1 = primary auditory cortex, which performs auditory analyses of incoming stimuli; W = Wernicke's area, which stores the memories or representations of the speech sounds (phonemes) that comprise words; AF = arcuate fasciculus, which is a bundle of nerve fibers that carry information from Wernicke's area to Broca's area; BA = Broca's area, which contains the knowledge of how to move the articulatory apparatus to make speech sounds; and M = motor cortex, which sends this information to the brain stem that contains the motor neurons that move the muscles in the tongue, lips, and mouth.

area with this information. Subsequently, as predicted by Wernicke's information-processing model, this type of aphasic disorder was described and was called conduction aphasia.

Broca's and Wernicke's seminal studies led to the golden age of the study of brain-behavior relationships called neuropsychology. This golden age lasted until the First World War, and then there was a shift in position to the mass action or nonlocalization hypothesis. The

Figure 3.4. Diagram of Karl Wernicke's model of how the left hemisphere mediates speech. A1 = primary auditory cortex, which performs auditory analyses of incoming stimuli; W = Wernicke's area, which stores the memories or representations of the speech sounds (phonemes) that comprise words; AF = arcuate fasciculus, which is a bundle of nerve fibers that carry information from Wernicke's area to Broca's area; BA = Broca's area, which contains the knowledge of how to move the articulatory apparatus to make speech sounds; and M = motor cortex, which sends this information to the brain stem that contains the motor neurons that move the muscles in the tongue, lips, and mouth.

reason for the decline of the localizationist approach is not fully known, but there were probably two major factors. The first was a change in the political-philosophical Zeitgeist. Most of the early localizationist work was done on the European continent, primarily in France and Germany. After the First World War these continental European powers lost much of their power and their influence on Western thought, but the English-speaking countries such as the United States and the United Kingdom flourished. The Anglo-American social and political systems were strongly influenced by the philosophic writings of John Locke, who proposed that the brain was like a "tabula rasa" or a blank wax tablet. Unlike the modularity hypothesis that suggests anatomic specialization, the tabula rasa is uniform and featureless until it receives impressions gained by experience.

The shift toward this antilocalizationist view of brain organization was strongly propelled by the Harvard psychologist Karl Lashley, who removed different parts of rodents' brains to see if there were any specific areas of the brain that, when removed, caused a specific behavioral deficit. Because he found no localized regions where knowledge was stored but rather knowledge seemed to be diffusely represented, he formulated the theory of "mass action." A corollary of this hypothesis is that whatever the location of a brain injury, the more tissue that is damaged, the poorer the animal performs on any task. Unfortunately, during this antilocalizationist period (1920s to 1962), clinical neurologists who presumably should have been interested in localization of function did little to advance knowledge in this field, and many of the British neurologists had a strong negative attitude toward localizationist thinking. Sir Henry Head, one of the leaders of British neurology, wrote inflammatory and derogatory comments about Karl Wernicke's reports. Head (1926) wrote, "No better example could be chosen of the matter in which writers of this period were compelled to lop and twist their cases to fit the Procrustean bed of their hypothetical conceptions." Fortunately, a localizationist and connectistic renaissance began in 1962, when Norman Geschwind, Edith Kaplan, Harold Goodglass, and their students clearly demonstrated brain modularity in patients with brain lesions. The advent of new neuroimaging techniques, such as computer tomography (CT) and magnetic resonance imaging (MRI), began to allow investigators to localize lesions in living patients rather than having to rely on postmortem examination. This technological advance allowed the resurrected science to flourish.

Information-processing models help us understand how the brain works and enable us to ask further questions about brain functions. They presume that the brain stores and processes information in highly interconnected modular systems. These modular systems process information both serially and in parallel. In the past 20 years, we have been able to visualize brain functions by using radioisotopes (e.g., positron-emission tomography or PET scanning) or strong magnets (i.e., functional magnetic resonance imaging or fMRI). When an area of the brain becomes active it uses more energy, and to provide that energy, the active part of the brain receives an increased amount of blood. The PET and fMRI methods can detect the areas of the brain that are active by locating the areas that are receiving increased amounts of blood. These functional imaging studies provided further support to Gall's modularity hypothesis and support the work of local-izationists such as Broca and Wernicke. Strong magnets also can be used to detect small brain currents (magneto-encephalography). This technique permits neuroscientists to measure the time between stimulus presentation and cortical processing. As predicted by information-processing models, these studies demonstrate that the brain processes stimuli using both serial and parallel processing.

Diabetes 2

Diabetes 2

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