BRAIN LOCALIZATION. — The concept that various somatopsychic functions have a well-defined localization in the brain has undergone continuous modifications over time and remains an unresolved problem; in recent decades, attempts to define the issue have increasingly relied on experimental and clinical data. Historically, setting aside fantastic, a priori hypotheses and interpretations lacking empirical foundation,
BRAIN LOCALIZATION
the presence of a symptom at a given cortical point will at most allow one to affirm the localization of a deficit rather than a function (Monakow), since not only the disappearance of a particular function (negative symptom) is observed, but also a mixed picture of such elements alongside others produced by the release of other centers whose activity was under the control of the injured center (positive symptom, according to Jackson). Furthermore, the relationship between positive and negative symptoms should not be interpreted with excessive schematism, as it varies from case to case.
An observation particularly useful for understanding the coordinated activity of various cortical areas is the comparison between what occurs during an experiment and what happens during a voluntary act, for example, a motor act. If a specific cortical region is electrically stimulated, movements of particular muscle groups are obtained; if the same region is isolated from surrounding areas, while stimulation produces the identical result, voluntary motility of those muscle groups is abolished. This occurs because voluntary movement cannot take place without the contribution of sensory and sensory information that conveys a series of complex references to the aforementioned area from distant points within the organism and the surrounding environment. The issue of hemispheric dominance must also be mentioned: Marc Dax (1836) was the first to emphasize the importance of the left cerebral hemisphere, based on the observation that language disorders are far more frequent in right-sided paralysis. Currently, it is considered that the left hemispheric dominance in certain individuals is limited to specific functions, such as symbolic expression, visual recognition of objects, visual orientation in space, topographic memory, and knowledge of the bodily schema. Even in this case, it would not involve isolated unilateral centers, since under abnormal conditions their functions can be assumed by the opposite hemisphere or by centers that, in normal subjects, likely perform subordinate activities. There is, however, no hemispheric dominance for the cortical functions proper to the motor, sensory, and sensory apparatus, nor for psychic phenomena in general.
Regarding the value to be attributed to the concept of brain localization, it is advisable to examine clinical findings that reveal different forms of localization. Indeed, alongside areas (the so-called peripheral cortex) where anatomical, physiological, and clinical data allow for the recognition of a fairly precise relationship without, however, sufficiently explaining the functional disturbance, there are other zones where the lesion-symptomatology relationship presents vaguer boundaries, such as, for example, the zones of anxiety, aphasia, and apraxia. In these areas, one can identify certain nodal points of greater importance (such as Broca’s area or Wernicke’s area for aphasia), but in general, identical lesions correspond to variable symptomatology; the clinical outcome of a lesion will also depend on its nature (infectious, tumorous, degenerative, traumatic, vascular), the subject’s constitution (age), and the manner of progression (progressive, restorative, etc.).
The following are the most important localizations with reference to anatomo-clinical data. The frontal syndrome includes disturbances of muscle tone, denervation, reflexes, oculomotor function, coordination, balance, sensory-sensory activities, neurovegetative functions, language, practical (praxic) activities, orientation, and finally psychic disorders. In the region of the Rolandic fissure, motor centers for individual movements of the face, trunk, and limbs are found. The parietal syndrome comprises sensory disturbances (of the face, trunk, and limbs), praxic disturbances, bodily schema, language, balance, oculomotor function, trunk, and sensory disturbances (visual and gustatory). The temporal syndrome manifests with disturbances of language hearing, sensory disturbances (gustatory, olfactory, visual), oculomotor function, and balance. The occipital syndrome includes visual disturbances (hemianopsia, cortical blindness, visual hallucinations, and visual anxiety) and disturbances of time perception and movement. Finally, the syndrome of the corpus callosum involves psychic, praxic, gnostic, and language disturbances, as well as sensory, motor, and balance disorders.
From this initial approach, progress has been made—also thanks to the surgical verification of experimental data and clinical inductions—toward the determination of true “maps” or geographical charts of the cerebral cortex, superimposed on the classic lobar divisions, in which various motor, sensory, and sensory localizations have been precisely established. This result has been achieved through modern anatomical and physiological studies, including the work of von Economo, Campbell, and Brodmann on cortical areas based on the morphological aspect of nerve cells and their layering (cytoarchitectonics), the progressive myelination of different groups of nerve fibers (myeloarchitectonics, C. Flechsig and O. Vogt), or their differing electrical activity (electroarchitectonics, Kornmüller).
Regarding the problem of brain localization, two main currents emerge: that of the localizers and that of the anti-localizers. Among the former, there are two tendencies: some go so far as to localize even the psychic faculties themselves (Henschen, Wernicke, Kleist, Nielsen, Pavlov), while others, more cautiously, establish relationships between delimited cerebral zones and specific clinical symptomatic patterns (Foerster, Penfield). In general, localizers tend to move away from the concept of static centers tied to particular functions, recognizing instead in these centers dynamic organizers of functions, replacing the term “localization” with the more appropriate one of “correlation.” The anti-localizers are also divided into two groups: those who advocate the global functioning of the cortex, while admitting the concept of a peripheral and a central part of the cortex itself (the former would be related to the peripheral parts of the human body and the entire periphery, while the central part, comprising the remaining areas and roughly corresponding to Flechsig’s association areas, would have predominantly psychic functions: Goldstein); and those who simply deny specific cortical territories any strict functional prerogatives (Monakow, Morgue).
Such varied theories and the divergence of conclusions are explained by the difficulty of interpreting even the simplest phenomena. Indeed, when a lesion exists in
(from G. Spatuzzi, *Alcuni oggetti della cartella materna*, in *Nuovi progressi in Med.*, vol. III, no. 2, Rome 1957, p. 38, fig. 1) Brain Localizations — Cytoarchitectonic map of the human cortex according to Brodmann. The numbering of the areas is conventional and follows the most commonly used scheme.