Brodmann's ‘Localisation in the Cerebral Cortex’

The following sections are included:

• Contents

• Translator’s Introduction

When I began my work in the Neurobiological Laboratory (*1) of the University of Berlin eight years ago the task befell me to undertake a topographic analysis of the human cerebral cortex based on its cellular structure, in the context of the research programme of this institute. The practical goal of this task was essentially to provide a description of the normal histological structure of the whole human cerebral cortex, which had long been felt necessary by neuroanatomists and pathologists. In contrast to earlier research of a similar nature, the emphasis would be not only on gross divisions of the brain, such as lobes and gyral complexes, but also on the smallest gyri and parts of gyri, in order to obtain a complete picture of cortical structure and all its local modifications, and thus try to describe topographical parcellation and localisation in the cortex that would also be of value for clinicians…

The subject of the following treatise is histological localisation in the cerebral cortex, that is to say localisation which uses exclusively anatomical features as the basis for investigation, in contrast to physiological or clinical aspects. The first and most important task of such a localisational study is the parcellation of the cerebral cortex according to common anatomical features, that is the systematic grouping of structurally similar neural components and complexes and the separation of structurally dissimilar ones. In particular it is the aim of comparative localisation to identify similar or homologous parts of the cerebral cortex in different animals or animal groups on the basis of their structure. Our goal is thus to produce a comparative organic theory of the cerebral cortex based on anatomical features, as first imagined by Theodor Meynert…

In Part I we shall deal in turn with:

1. the question of the basic or “primitive” layering pattern of the mammalian cerebral cortex;

2. local differences in the cellular structure of layers within an individual brain;

3. particularities of cortical structure in different animals.

Since the first pioneering research of Meynert and Betz, a continuous stream of workers has studied the cellular lamination of the cerebral cortex and its specific modifications in man and in individual animals . It would, therefore, be reasonable to expect that there would now be a solid basis of knowledge and understanding, at least as far as the essential elements are concerned. However, while recognising the numerous valuable individual contributions that have emerged from the competition, one cannot, after critical examination, avoid the conclusion that today we are further from agreement over the basic questions than for decades, especially concerning the common features and the origins of cortical lamination. Wherever we look we see major contradictions, not only in interpretations, but also in observations. There are totally conflicting results concerning the number, organisation and nomenclature of the layers, and a complete terminological confusion reigns, making interpretation quite impossible…

The following sections are included:

• The general rules of variability

• Regional characteristics of individual layers. (Constancy and variability.)

So far we have examined the principles underlying the potential of the mammalian cerebral cortex to undergo regional modifications in cellular lamination, and have determined that the process of architectonic transformation is essentially the same in the whole mammalian class. From this emerges the important biological principle that the genesis of mammalian cortex is not only conceived according to a common plan, but it completes its further development according to standard rules…

As a result of the observations described in Part I we are now in a position to divide the cerebral cortex into structurally circumscribed regions, in other words to construct a histological topographic map of the surface of the hemispheres…

Unlike the preceding descriptions of details of individual cortical fields according to lobes or gyri, the next section will deal with the division of the surface of the hemispheres of different animals into major, structurally homogeneous zones that only partially coincide with the morphological formations of earlier nomenclatures, such as lobes, lobules and gyri, each of which may encompass several architectonic areas. The basis for this is essentially comparative anatomy and depends on the following considerations. One can indeed roughly subdivide the hemispheres of man and related gyrencephalic animals into morphologically homologous lobes. Nevertheless what corresponds in lower orders, such as small rodents and insectivores, to the frontal or temporal lobes is unfortunately impossible to determine by external inspection. However, it is quite possible to identify histological structures, and a number of such structural areas can be demonstrated in all mammals…

These are essentially expressed in the following three ways:

Similarity of overall position.

Correspondence between structural regions.

Persistence of individual areas.

Variations in areal parcellation of the cerebral cortex are, from the very nature of the subject, more numerous than the similarities between different animals. They concern both the larger structural entities, such as regions, and also the smaller areas, as is evident from the descriptions of the individual brains. Factors involved include differences in shape, position or size of these entities, that could be termed extrinsic features. Otherwise, there may be specific architectonic differences between cortical zones, such that in some species new areas are formed while, on the contrary, there may be regression, atrophy or fusion of areas, groups of areas or whole regions.

It is therefore justifiable from the outset to separate essential and non-essential differences in cortical parcellation. To the former belong variations in size, position and shape of individual areas, to the latter the appearance of new areas or regions in certain groups, whether due to new developments on the one hand or to fusion or regression on the other. Of course these different factors, that determine the particular pattern of cortical topography in a given animal, cannot be sharply segregated, as they usually compete in individual cases.

In the foregoing chapters an attempt has been made to give a coherent description of the basic features of the histological structure of the mammalian cerebral cortex based on the distribution of its neurons, that is to say the topographic, architectonic pattern of the cortex with the exclusion of its fibrous components in the neuropil. This work was intended mainly to explain the general principles of cortical structure in mammals, and form a basis for a comparative approach to localisation, the details of which still remain to be clarified, so there has necessarily been an accumulation of minute data which detracts from this general aim. Thus it was necessary to select, from the endless mass of morphological structures in the cortex of the different mammalian groups, those patterns that could be useful in answering certain questions of decisive importance for the progress of the study…

As we have seen repeatedly, until recently the concept of the morphological homogeneity of the cerebral cortex was generally popular and, most importantly, was recognised universally by anatomists and physiologists. Even neuropathology could not rid itself of this notion, in spite of ever increasing contradictory clinical observations, and to this day one finds the view from time to time, even in new textbooks, that, apart from trivial local structural modifications, the cerebral cortex is a completely homogeneously built organ containing no sharp divisions in its histological structure.

In view of this it seems justifiable to present the essential points of our observations once again, even if they are not all original and new, and despite previous necessary repetitions, and discuss briefly the validity of the principles of localisation that we have discovered for the whole mammalian class.

In his famous policy paper on the present state of pathological anatomy of the central nervous system, Franz Nissl acclaims as the greatest step forward of our times the clear recognition “that the next goal to attain in the pathological anatomy of the central nervous system does not consist in the most detailed possible identification of the affected nervous elements, nor an attempt to relate clinical signs to anatomical observations, but in the systematic investigation of individual histopathological processes underlying clinical pathological conditions, and also in the most detailed possible definition of the various histopathological states”.

It is a basic biological principle that the function of an organ is correlated with its elementary histological structure. Just as every organic structure is a product of its developmental history and, further, as development represents the sum of diverse vital processes, so in the last analysis the explanation of the emergence of organic structure and its genesis by histological differentiation is a physiological problem . Function creates organs. The recognition of this seems to me to enable those who would not otherwise be inclined, to deduce the function of an organ from its structure, and to draw conclusions about the activity of the whole organ , as well as its component parts, from its overall histological composition and the structural similarities or dissimilarities of individual components, in short from its intrinsic structural differentiation.

Although my studies of localisation are based on purely anatomical considerations and were initially conceived to resolve only anatomical problems, from the outset my ultimate goal was the advancement of a theory of function and its pathological deviations. Now the question arises as to what we can deduce from our histotopographical findings in terms of the physiology of the cerebral cortex.

The following sections are included:

• Literature

• Translator’s References

• Glossary of Species Names

• Translator’s Notes

• Index