CEREBRAL LOCALIZATIONS. - The concept that the various somatopsychic functions have a well-determined localization in the brain has undergone continuous modifications over time and is still a problem not at all resolved; the attempt to define the question in recent decades has been based increasingly on experimental and clinical data. Historically, leaving aside fantastic, aprioristic hypotheses and interpretations devoid of a basis of secure anatomo-physiological and clinical knowledge (Hippocrates, Galen, Soemmerring, Descartes, Gall and Spurzheim; v. FRENOLOGY), the first to assume scientific value are the researches and conclusions of P. Broca (1861) on the importance of the foot of the third frontal convolution in aphasic disorders and those of Fritsch and Hitzig (1870), who, by electrically stimulating the cerebral cortex of the dog in a determined region, noticed movements of the contralateral limbs, while destruction of the same region produced a motor deficit.
From this first experiment, one has progressively arrived (thanks also to the operative verification of experimental data and clinical inductions) at the determination of true "maps" or geographical charts of the cerebral cortex, superimposed on the classic divisions into lobes, in which the various motor, sensitive and sensory localizations have a well-determined place. Modern anatomical and physiological studies have contributed to this result, with the works of von Economo, Campbell, Brodmann on the cortical areas according to the morphological appearance of the nerve cells and their arrangement in layers (cytoarchitectonics), according to the progressive covering with myelin of the different groups of nerve fibers (myeloarchitectonics, C. Flechsig and O. Vogt), or according to their different electrical activity (electroarchitectonics, Kornmüller).
With regard to the problem of cerebral localizations, one finds oneself faced with two currents: that of the localizers and that of the anti-localizers. Among the former, moreover, there are two tendencies: some go as far as the localization of psychic faculties themselves (Henschen, Wernicke, Kleist, Nielsen, Pavlov), others, more prudently, establish relationships between delimited cerebral zones and determined clinical symptomatic pictures (Foerster, Penfield). In general, the localizers tend to abandon the concept of static centers tied to particular functions, recognizing in these centers rather 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 maintain the global functioning of the cortex, while admitting the concept of a peripheral part and a central part of the cortex itself (the former would be in relation with the peripheral parts of the human body and with the periphery as a whole, while the central part, comprising the remaining areas and corresponding roughly to Flechsig's association areas, would have predominantly psychic functions: Goldstein); those who merely deny to determined cortical territories functional prerogatives in the strict sense (Monakow, Mourgue).
Thus various theories and so much divergence of conclusions are explained by the difficulty of interpreting even the simplest phenomena. In fact, when a lesion exists in a given cortical point, the possible symptomatology present will allow one to affirm at most the localization of a deficit and not of a function (Monakow), given that one observes not only the disappearance of a particular function (negative symptom), but a mixed picture of elements of this type with others provoked by the release of other centers whose activity was under the control of the lesioned center (positive symptom, according to Jackson). Moreover, the relationship between positive and negative symptoms must not be understood too schematically, varying as it does from case to case.
An observation that lends itself very well to the understanding of the coordinated activity of the various cortical areas is the comparison between what happens during an experiment and what happens instead during a voluntary act, for example, a motor one. If a determined cortical region is electrically stimulated, movements of particular muscle groups are obtained; if the same region is isolated from the surrounding areas, while stimulation obtains the identical result, the voluntary motility of those muscle groups is abolished; this is because voluntary movement cannot occur without the contribution of sensitive and sensory information that bring a whole series of complex references to the aforementioned area, from points far from the organism and from the surrounding environment. Mention should also be made of the question of hemispheric dominance: Marc Dax (1836) was the first to emphasize the importance of the left cerebral hemisphere, starting from the observation that language disorders are much more frequent in those paralyzed on the right. Currently, it is considered that the left hemispheric dominance of right-handed subjects is limited to determined functions, such as symbolic expression, the visual recognition of objects, visual orientation in space, topographical memory and knowledge of the body schema. Even in this case it would not be a matter of isolated unilateral centers, given that under anomalous conditions their functions can be vicariously taken over by the opposite hemisphere, by centers that in normal subjects probably carry out a subordinate activity. There is, however, no hemispheric dominance for the cortical functions proper to the motor, sensitive and sensory apparatus, nor for psychic phenomena in general.
With regard to the value to be given to the concept of cerebral localizations, it is worth examining the clinical results from which different forms of localization emerge. In fact, alongside zones (the so-called peripheral cortex) where anatomical, physiological and clinical data allow one to recognize a fairly precise relationship without, however, sufficiently explaining the functional disorder, there exist other zones where the lesion-symptomatology relationship presents vaguer limits, as, for example, the zones of agnosias, aphasia and apraxia. At most in these zones one can recognize some nodal point of greater importance (as for aphasia the center of Broca or the center of Wernicke); but in general here identical lesions correspond to a variable symptomatology; the clinical result of a lesion will moreover depend on its nature (infective, tumoral, degenerative, traumatic, vascular), on the terrain (age of the subject), on the manner of evolving (progressiveness, restoration, etc.).
Finally, the most important cerebral localizations are listed here with anatomo-clinical reference. The frontal syndrome includes disturbances of muscular tone, of denervation, of reflexes, of ocular movement, of coordination, of balance, of sensory-sensorial, neurovegetative activities, of language, of practical (praxic) activities, of orientation and finally psychic disorders. In the region of the fissure of Rolando are found the motor centers for the individual movements of the face, trunk and limbs. The parietal syndrome includes sensory disturbances (of the face, trunk and limbs), praxic disturbances, disturbances of the body schema, of language, of balance, of ocular movement, of trophism and sensorial disturbances (visual and gustatory). The temporal syndrome manifests itself with disturbances of the hearing of language, sensorial disturbances (gustatory, olfactory, visual), of ocular movement and of balance. The occipital syndrome includes visual disturbances (hemianopsias, cortical blindness, visual hallucinations and agnosias) and disturbances of the perception of time and of movements. The syndrome of the corpus callosum, finally, includes psychic, praxic, gnostic, language, sensory, motor and balance disturbances.
The very fact that certain functions have their seat in several parts of the cortex demonstrates the broad correlation of the various zones and gives an intuition of the complexity of the subject.
Bibliography3 works
Luisandro Canestrini