Cerebral Localization

The theory that different parts of the brain subserve different functions is known as localization theory. It is consistent with the basic biological principle that structures that do not look alike should have different functions. Localization of function is the theoretical backbone of modern neuropsychology, neurology, and related disciplines, all of which attempt to correlate specific behaviors with specific brain parts.

In the history of the brain sciences, it is possible to conceive of the theory of localization as being applied to the whole and then to increasingly smaller parts (Finger, 1994). At first the question seemed to be "Why is the brain special and how is it different from other organs, such as the heart?" This was an important question in classical and Hellenistic Greece, where opinions were divided. Aristotle (384-322 b.c.), the greatest of the Greek naturalist philosophers, believed that the heart was the seat of sensory and cognitive functions and that the brain simply tempered "the heat and seething" of the heart. In this regard, he was consistent with the archaic Greeks and the Egyptians. In contrast, Plato (c. 429-348 b.c.), in agreement with the thoughts of the Hippocratic physicians, believed that intellect belonged not in the heart but in the head.

During the Roman period, Galen (130-200 a.d.) reasoned that the seat of the highest soul, and hence the seat of intellect, had to be the brain itself. He listed imagination, cognition, and memory as basic components of intellect, but in his surviving writings he did not localize these functions in different parts of the brain (Finger, 2000). What he did write was that the soft front of the brain is likely to be sensory (better able to receive impressions), whereas the harder back (cerebellar) region is likely to be motor.

The church fathers of the fourth and fifth centuries a.d. went one step further when they localized imagination, intellect, and memory in the different hollow ventricles of the brain. One of the earliest advocates of ventricular localization was Nemesius (fl. 390 a.d.), a bishop in Syria. He local ized perception in the two lateral ventricles, cognition in the middle ventricle, and memory in the posterior ventricle. This early localization theory was also embraced by St. Augustine (354-430), and it was broadly accepted in its general form for more than 1,000 years (i.e., some writers varied the ventricles that were associated with these particular functions).

During the Renaissance, as scientists returned to dissection and experimentation, observation began to replace conjecture. Leonardo Da Vinci (1472-1519) made molds of the ventricles to reveal their shape, and Andreas Vesalius (1514-1564) showed that the ventricles vary little across mammals. Consequently, the idea of ventricular localization slowly gave way, and increased attention began to be given to differences in the size and makeup of the brain itself.

The freer thinking and questioning of the Renaissance set the stage for Thomas Willis (1621-1675), an Oxford physician, who published his Cerebri Anatome in 1664 (Finger, 2000). Willis proposed that the corpus striatum, which he defined as all white matter between the basal ganglia and the cortex, plays a role in sensation and muscle movement, and that the cerebral cortex controls memory and the will. The cerebellum (which was broadly defined to include some pons and midbrain) was thought to regulate involuntary, smooth motor functions, such as breathing. This division of the brain into working parts, based partly on comparative anatomy, partly on clinical material, and partly on speculative theories, helped to change thinking about the functional organization of the brain.

The opening decades of the 1800s proved to be an especially important time in the history of localization theory. First, Julien Jean César Legallois (1770-1840) provided the first accepted localization within a region of the brain. In 1812, he pinpointed the area responsible for respiration within the medulla. In addition, the seeds were planted for modern cortical localization theory when Franz Gall (17571828) presented his theory of organology (his assistant Spurzheim preferred the word phrenology) at about the same time (Finger, 1994, 2000; Young, 1970). Gall maintained that different areas of the cerebral cortex govern different mental faculties and that cranial features reflect the development of these different organs of mind. Among other things, Gall was convinced that humanity's highest functions (e.g., speech) belong in the front of the cerebrum.

The theories of Gall and his followers stimulated great debate. Some scientists thought they had merit, others found them absurd, and still others believed that cortical localization made sense but that cranioscopy was a dead end that must be replaced by careful neurological examinations. The latter position was advocated by Jean-Baptiste Bouillaud (1794-1881). This Frenchman began to localize speech in the anterior lobes in 1825, and he then spent decades collecting clinical and autopsy material supportive of speech localization.

The debates over cortical localization continued to heat up until 1861, when Paul Broca (1824-1880) presented his celebrated case of M. Leborgne ("Tan") in Paris (Finger, 2000). His sickly, hospitalized patient had lost his capacity for articulate language (among other things), and an autopsy revealed a lesion involving the third frontal convolution of the left hemisphere. Broca's localization of a center for articulate language toward the back of the frontal lobes went further than Bouillaud's broader anterior lobe idea, and it became the first cortical localization to receive broad acceptance.

In 1865, Broca published another landmark paper. As his collection of cases continued to grow, he had recognized that the left hemisphere must be special for speech. Left hemispheric dominance for speech was something that Marc Dax (1770-1837) had written about in 1836. Unfortunately, Dax failed to make his findings public in his lifetime. Thanks to his son Gustave, however, his report on more than 40 cases also appeared in print in 1865, the same year as Broca's own paper on the subject (Finger & Roe, 1996; Joynt & Benton, 1964).

In 1870, Gustav Fritsch (1838-1927) and Eduard Hitzig (1838-1907) discovered the motor cortex of the dog, first by stimulation and then by ablation (Finger, 2000). This was a significant accomplishment, because it showed that cortical localization could be studied under controlled conditions in laboratory animals. In the wake of this paper, lesion studies were conducted to localize sensory functions, such as vision and hearing, as well as higher intellectual functions, such as attention and memory. The leader of the new localization movement was David Ferrier (1843-1928), a Scottish physiologist-physician who moved to London. His most influential book, The Functions of the Brain, was first published in 1876 (Finger, 2000). Three other early contributors to the cortical localization movement were Hermann Munk (1839-1912), a German, and Victor Hors-ley (1857-1916) and Edward Schafer (1850-1935), two Englishmen.

The success of localization theory was not, however, based solely on pathological (brain lesion) material—it also received good support from other sources. In 1875, Richard Caton (1842-1926) of Liverpool reported that cortical electrical activity varied in accord with Ferrier's maps when animals chewed, looked at flashing lights, and so forth. In addition, Paul Flechsig (1847-1929) showed that different cortical areas become myelinated at different times, a finding he correlated with the gradual attainment of different functions. Cytoarchitectonic studies of the cerebral cortex, such as those of Korbinian Brodmann (1868-1918), represented another significant source of support for localization theory.

Today, efforts continue to divide the brain into smaller functional units, albeit with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) scans and other sophisticated tools that could not even have been imagined when Broca was living. In this regard, localization theory is alive and well. However, so is the mod erate holistic notion, which holds that the brain is not a collection of independent functional parts but a remarkably unified organ made up of interacting specialized parts that are laid down by the genes and shaped by experience.

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