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J Anat. Aug 2007; 211(2): 199–211.
PMCID: PMC2375766

A multiarchitectonic approach for the definition of functionally distinct areas and domains in the monkey frontal lobe


Over the last century, anatomical studies have shown that the cerebral cortex can be subdivided into structurally distinct regions, giving rise to a new branch of neuroanatomy: ‘architectonics’. Since then, architectonics has been often accused of being overly subjective, and its validity for the definition of functionally different cortical fields has been seriously questioned. Since the late 1980s, however, the problem of localization has become particularly important in functional studies of the primate motor cortex, because of evidence that (1) the primate motor cortex is made up of a mosaic of functionally specialized areas and (2) the human motor cortex shares several general organizational principles with the monkey motor cortex. Studies of the macaque agranular frontal cortex that used a multimodal cyto-, myelo- and immuno-architectonic approach have shown that architectonic borders can be reliably and consistently defined across different individuals, even at a qualitative level of analysis. The validity of this approach has been confirmed by its ability to localize functionally distinct areas precisely and to predict the existence of new functional areas. After more than a century, architectonics as a discipline goes far beyond its original aim of generating cortical maps.

Keywords: agranular frontal cortex, architectonics, functional localization, motor cortex, primate.


Between 1899 and 1902, Ramon y Cajal (1909) made a thorough study of the finer structure of several regions of the human cerebral cortex, leading to a clear definition of its basic laminar and cellular organization and to the recognition of the structural differences between cortical regions. Thanks to these studies, as well as to Meynert's work, at the beginning of the twentieth century a new branch of neuroanatomy was created, ‘architectonics’. The main goal of architectonics is not the study of the cortical structure per se, but the subdivision of the brain into regions of specific structure, the so-called ‘architectonic areas’. Strong motivation to this new discipline was soon injected by the independent works of Campbell (1905) and Brodmann (1909), which were able to subdivide the cerebral cortex into several distinct cytoarchitectonic areas.

The aim of this article is to deal with the architectonics of the monkey frontal lobe from a functional perspective. Specifically, two major and still debated issues will be addressed: the possible validity of architectonics in defining functionally distinct cortical entities, and the possibility that architectonic features provide information on the possible functions of a given area.

Given the general rule of biological sciences that different structures serve different functions, the notion that architectonic differences reflect functional differences was taken for granted since the beginning of this type of study. In particular, according to Brodmann (see Garey, 1994), each area has its own specific structure, serves a specific function and, thus, forms an ‘organ’. In subsequent years, the view that the cortex consists of a mosaic of discrete ‘organs’, a concept strongly reminiscent of the phrenological theories, was strongly questioned. The excessive enthusiasm evidenced in attributing to any minimal morphological difference the role of a functional centre along with functional data that denied any validity of architectonics in defining functionally different regions were major reasons for a loss of interest, for many years, in architectonic studies as a tool for understanding brain functions. Indeed, most of the skepticism in the functional interpretation of structural data relies on the issue of what is meant by ‘localization of functions’. In this respect, ‘function’ should not be interpreted in terms of psychological function tout-court, but as the processing performed on specific afferent information by the neural circuitry of a discrete cortical sector for the elaboration of a specific output. In recent years, neuroanatomical and functional researches in animals have significantly contributed to the study of brain circuitry and functions. In this context, non-human primates have represented a model of invaluable interest in the field of neurology. Indeed, brain imaging studies in humans have strongly supported the notion that, in primate evolution, the basic plans for the organization of the brain circuitry and functions have been conserved (see, for example, for homologies between monkey and human motor cortex, Rizzolatti & Arbib, 1998; Picard & Strick, 2001; Matelli et al. 2004).

It is largely accepted today in neuroscience that the cerebral cortex contains many functionally specialized entities, usually referred to as ‘areas’. Each area is considered to be part of largely distinct cortical circuits, subserving in parallel different brain functions. There is still no unanimous consensus, however, on what precisely constitutes a cortical area and what are the best criteria for its definition (see, for example, Van Essen, 1985). In general, three main experimental approaches, the architectural, the connectional and the functional, are considered most useful for the definition of a cortical area. Converging evidence from these three approaches is generally considered as a strong argument for a reliable identification and delineation of a cortical area (see, for example, Van Essen, 1985; Felleman & Van Essen, 1991). It is therefore widely accepted that, for understanding brain functions, a multidisciplinary approach is necessary in which structural maps of the exact location and extent of architectonic areas are continuously interfaced with functional and connectional data. In this context, the architectonic approach is the only way to obtain reliable anatomical frames of reference for the attribution of data obtained from different subjects and from different experimental approaches to a specific cortical entity.

One major argument against the validity of the classic architectonic approach is represented by the variability of the maps proposed by different investigators. Indeed, architectonics has been often accused of giving uncertain results, due to its subjective nature and to the variability of the criteria used for defining borders between areas. Over the last two decades, the combination of classic architectonic techniques with newer, mostly neurochemical, architectonic approaches has proven to be extremely useful for a more reliable and objective assessment of architectonic borders. Accordingly, the possibility of using a multimodal architectonic approach has led to a renewal of interest in architectonics as a tool for understanding brain functions and for comparative studies aimed at establishing possible homologies across different species.

In the sections below, we will review architectonic data, collected in our laboratory, focused on the structural organization of the agranular frontal cortex of the macaque monkey. Evidence will be provided that by adopting a multimodal architectonic approach the definition of different architectonic subdivisions of this cortical region becomes much more reliable and constant among different individuals, even at a qualitative level of analysis. The proposed architectonic subdivision proved to be valid for the definition of functionally distinct areas and for predicting the existence of new functional areas, contributing to the development of the present concepts on the organization of the primate cortical motor system.

The primate agranular frontal cortex

In both macaques and humans, the agranular frontal cortex (henceforth referred to also as the motor cortex) is a large cortical region, located in the caudal part of the frontal lobe and characterized by the lack of an evident layer IV. The involvement of this region in motor control was first proposed in 1863 by the British neurologist Hughlings Jackson (see Jackson, 1931), on the basis of observations on the development of somatic seizures in patients with tumours or diseases affecting the cerebral cortex. In sharp contrast with the common belief that the cerebral cortex is involved only in mentation and other high-order cognitive functions, Jackson proposed that the frontal cortex controls movement in different ways and in different combinations. He differentiated the ‘motor cortex proper’, i.e. the area that contains movement representations and sends direct and indirect projections to the spinal cord, and the ‘superimposed cortical areas’, which are connected with the spinal motor nuclei only by the intermediary of the motor cortex proper. Adapting his terminology to the principle of the hierarchy of the nervous system, Jackson ranked the motor cortex proper at the ‘middle level’ and the superimposed cortical areas among the ‘highest levels’ of the motor system.

Early cytoarchitectonic studies at the beginning of the last century suggested a possible anatomical counterpart of Jackson's functional organization of the motor system, showing that the motor cortex is structurally not homogeneous. Campbell (1905), in his cytoarchitectonic map of the cerebral cortex, proposed a subdivision of the motor cortex into a caudal and a rostral subdivision. The caudal subdivision – precentral cortex – is characterized by the presence of the giant layer V pyramidal cells described by Betz in 1874 and should mediate the executive motor functions. The rostral subdivision – intermediate precentral cortex – virtually lacks giant pyramidal cells (Betz cells) in layer V and should represent the level of the higher motor functions. Brodmann (1909) basically agreed with Campbell's view describing in both the macaque and the human brain two cytoarchitectonic areas, named area 4 and area 6, which could be distinguished for a differential distribution of Betz cells.

The possibility that architectonic differences within the motor cortex reflect functional differences was further supported by Fulton (1935) who showed that lesions within area 6 (defined as premotor cortex) in monkeys and humans produce, in addition to other symptoms, a specific deficit in the execution of skilled movements.

Evidence against Fulton's notion of a premotor cortex (area 6), functionally independent from area 4, arose in the 1940s through electrophysiological studies employing surface electrical stimulation (Woolsey et al. 1952). These studies provided detailed descriptions of the somatotopy of the primary motor area (M1) and led to the discovery of an additional somatotopically organized motor representation (the supplementary motor area, SMA) but failed to support the validity of cytoarchitectonic maps. Woolsey, in his influential study on the functional localization of the motor cortical areas, stated that ‘no single cytoarchitectural area of any worker coincides with the extent of our precentral field’.

The view of a primate motor cortex formed by two somatotopically organized areas, simply involved in executive motor functions, remained as a dogma for more than 40 years. Indeed, in the last two decades, the introduction of new and more refined connectional and functional techniques showed, first in non-human primates and, more recently, in humans (though in less detail), that the cortical motor organization is much more complex than originally thought (see, for example, Rizzolatti et al. 1998; Picard & Strick, 2001; Rizzolatti & Luppino, 2001). Among the new aspects of motor organization, some are particularly important. First, Brodmann's area 4 is functionally distinct from area 6, the classic view of a large M1 encompassing both area 4 and lateral area 6 being mistaken. Secondly, area 6 is not homogeneous, but formed by a multiplicity of functionally distinct premotor areas. Thirdly, these various premotor areas have different afferent and efferent anatomical connections and appear to play distinct functional roles in motor control.

As soon as these data started to accumulate, it became clear that a redefinition of the structural organization of the motor cortex was absolutely necessary for linking connectional and functional data to specific cortical entities and obtaining a more complete picture of the organization of the cortical motor system.

Histochemical architectonics of the monkey motor cortex

In a first attempt to find a morphological correlate of functional and connectional differences in the motor cortex, we used cytochrome oxidase histochemistry (Matelli et al. 1985). In general, the cortex, stained with this method, shows longitudinal stripes of different enzymatic activity and, in some regions, enzymatically highly active pyramids, mostly in layer V. Clear differences in cytochrome oxidase staining distinguished the motor cortex from some of the neighbouring cortical regions, such as area 3a and the frontal operculum (Fig. 1). Moreover, on the basis of regional differences in thickness and staining intensity of the stripes of higher enzymatic activity and on the presence and number of stained pyramids, the areas 4 and 6 complex of Brodmann could be parcelled into five different histochemical areas. Thus, this histochemical approach proved to be helpful in showing different subdivisions of the motor cortex. Figure 1(D) shows the histochemical map of the motor cortex obtained in this study. In analogy with von Economo & Koskinas (1925) the various identified frontal areas are referred to with the letter F and, to avoid confusion with previous nomenclature, they are labelled with Arabic numerals. This parcellation, compared with other proposed maps and with the functional scheme proposed by Woolsey, raised some new, important issues and presented some limitations. First, the rostral border of the precentral area F1 was set considerably more caudal than in previously proposed cytoarchitectonic subdivisions (Brodmann, 1909; Vogt & Vogt, 1919; von Bonin & Bailey, 1947), corresponding to only the caudal part of Woolsey's M1. Second, the identification of the mesial area F3, although representing the first demonstration of an architectonic subdivision of the mesial vs. the dorsolateral part of Brodmann area 6, corresponded to only a part of the SMA. Finally, the rostral part of the mesial and dorsal sectors of Brodmann area 6, corresponding to area FC of von Bonin & Bailey (1947), could not be defined with this approach. These questions were then addressed with the classic cytoarchitectonic approach.

Fig. 1
Low-power photomicrographs of representative fields trough the macaque motor cortex from a parasagittal (A) and a coronal (B,C) section stained via cytocrome oxidase histochemistry. Arrows mark the borders between architectonic areas. The dashed box within ...

Cytoarchitectonics of the monkey motor cortex

As mentioned above, one major argument and the source of strong skepticism against the cytoarchitectonic approach is the variability of the maps proposed by different authors. This variability appears very often to result from the different cytoarchitectonic criteria used by different investigators for defining borders between areas. On the one hand, when the main criterion used is based on single layer characteristics, the obtained maps appear to be valid only for the definition of cortical regions sharing a common functional distinguishing feature (e.g. agranular frontal cortex, koniocortex). On the other hand, when the criterion used in areal identification is based only on individual histological elements, the obtained parcellations appear to be extremely unreliable. For example, the most widely used criterion for defining the border between areas 4 and 6 is the change in number and density of giant pyramids in layer V. However, it is commonly observed that these cells do not stop abruptly, but their density decreases gradually in the rostral direction with a large degree of interindividual variability (Fig. 2). Furthermore, there is a decrease in the size of Betz cells in the medio-lateral direction (von Bonin, 1949; Zilles, 1990; Rivara et al. 2003), which appears to be related to the somatotopic organization of this area. Finally, Betz cells are not simply conventional pyramidal cells of enormous size. Indeed, since the early studies of Ramon y Cajal (see also Scheibel & Scheibel, 1978; Zilles, 2004) it has been clearly demonstrated, by using Golgi staining, that the Betz cells differ from the other pyramidal cells by their morphology: they show not only the major apical dendritic stem and typical basal dendrites, but also numerous additional dendrites originating from the whole circumference of their body. This typical morphological feature of the Betz cells is very difficult to identify in Nissl-stained material. Accordingly, if only one criterion (e.g. distribution, size and density of giant pyramids in layer V) is used alone, then the definition of the area 4/area 6 border becomes rather subjective.

Fig. 2
Low-power photomicrographs from Nissl-stained parasagittal sections, taken from two different macaque brains, centred on the precentral gyrus. Scale bar, shown in A, applies also to B. Conventions and abbreviations as in Fig. 1.

To overcome these problems, the approach used in our studies was based on the following main principles. First, since very often architectonic features change gradually from one region to another in the range of 0.5–1 mm, borders between areas were set to indicate the intermediate points of these transitions. Second, the definition of cytoarchitectonic areas was mostly based on ‘relative’ changes (within the same case) in both single layer characteristics and individual histological elements, which could be reliably and consistently observed across different cases. Finally, different planes of section were used in order to have, for any sector of the frontal cortex, sections cut almost perpendicular to the cortical surface, resulting in optimal views of the cortical architecture.

By employing this approach, despite the interindividual variability of cytoarchitectonic features, the location and extent of the identified areas becomes much more constant among different individuals. The results of these studies (Matelli et al. 1991; see also Geyer et al. 2000; Matelli et al. 2004) fully confirmed the subdivision of the motor cortex based on histochemical criteria, but also led to the identification of two additional agranular frontal areas located in the rostral part of Brodmann area 6, a mesial one – F6 – located rostral to F3, and a dorsolateral one – F7 – located rostral to F2.

Figure 3 shows photomicrographs of representative fields through the motor cortex, illustrating the major architectonic features of some of the identified cytoarchitectonic areas and the main criteria useful for their definition.

Fig. 3
Low-power photomicrographs and higher magnification views of representative fields of cytoarchitectonic areas F1, F2, F3, F6 and F7. Scale bars, shown in the low-power and in the higher magnification view of F1, apply to all low-power and higher magnification ...

Cytoarchitectonically, F1 is characterized by low cell density, poor lamination, absent layer IV and a prominent layer V, with giant pyramidal cells arranged in multiple rows. On the mesial surface, the area next to F1 in rostral direction is F3. As for F1, F3 is basically poorly laminated. One of the most important distinguishing features of F3 is the increase in cellular density in the lower part of layer III and in layer Va. Layer Vb is pale as in F1. Some giant pyramidal cells can be observed in this layer, especially in the caudal part of this area. The agranular area rostral to F3 is F6. Unlike areas F1 and F3, this area displays an evident layer V, lacking any clear sublamination and demarcated from the less dense layers III and VI. F6 borders rostrally on a cortex in which an incipient layer IV becomes recognizable. On the lateral convexity, F2 is poorly laminated like F1, and contains more scattered giant pyramidal cells. Layer III is characterized by a narrow band of medium-sized pyramids in its lowest part. This architectonic feature represents a major criterion for setting the border between F1 and F2. Interestingly, a similar change in layer III was considered as a main criterion by Von Economo & Koskinas (1925) for setting the border between the precentral gigantopyramidalis area FA and the frontal agranular area FB. Layer V is slightly denser than in F1, while the density of layers III and V is slightly lower than in F3. F7, rostral to F2, is laminated and differs from F2 by two main characteristics: the presence of a prominent layer V and the subdivision of layer VI into two sublayers. The area rostral to F7 shows an incipient layer IV and its laminar organization is more distinct than that of the agranular frontal cortex.

The functional validity of this architectonic subdivision was fully confirmed in studies in which connectional and/or functional data were correlated with cytoarchitectonic data. Together, these data provided strong support to the development of what is, at present, the most widely accepted view of the organization of the motor cortex of the monkey. A clear example of the validity of this approach is represented by the organization of the mesial sector of Brodmann area 6, once considered to be coextensive with the SMA (Woolsey et al. 1952). Functional and connectional evidence has clearly shown that F3 and F6 correspond to two markedly distinct areas, which are also referred in the literature to as SMA proper and pre-SMA, respectively (Luppino et al. 1991, 1993; Matelli et al. 1991; Matsuzaka et al. 1992).

Organization of the macaque motor cortex

The results of the adoption of a multidisciplinary, architectonic, connectional and functional approach for the parcellation of the motor cortex is represented by the map proposed by Matelli and colleagues (Fig. 4; Matelli et al. 1991; see also Rizzolatti et al. 1998; Rizzolatti & Luppino, 2001). The definition of the various identified motor areas of this map is mainly based on a combination of cytoarchitectonic, histochemical and, as shown below, immunohistochemical and neurochemical criteria, but has been also strongly corroborated by connectional and functional data. It is quite clear therefore that the organization of the monkey motor cortex appears today to be far more complex than previously thought. Basically, in this subdivision F1 roughly corresponds to area 4, whereas each of the three main sectors of Brodmann area 6, the mesial, the dorsal and the ventral, is formed by a caudal and a rostral subdivision. The basic organizational principles can be summarized as follows (for more detailed reviews on this topic, see Rizzolatti et al. 1998; Rizzolatti & Luppino, 2001):

Fig. 4
Mesial and lateral views of the macaque brain showing the architectonic parcellation of the agranular frontal cortex according to Matelli et al. (1985, 1991). The cingulate sulcus is shown unfolded. On the basis of the available data, the various body-part ...
  1. The motor cortex is a mosaic of several independent body movement representations playing a differential role in motor control. They are not simply involved in executive motor functions, but appear to play an active role in sensorimotor transformations. Some of them are also involved in cognitive functions, classically considered as an exclusive province of the associative parietal and prefrontal cortex.
  2. From rostral to F1 (primary motor area) there are six premotor areas that can be subdivided into two main classes: areas that receive their predominant cortical non-motor input from the parietal lobe (caudal premotor areas) and areas that receive their predominant cortical non-motor input from the prefrontal and the cingulate cortex (rostral premotor areas). F2, F3, F4 and F5 belong to the first class, F6 and F7 to the second. The organization of corticospinal projections is in accord with this subdivision (He et al. 1993, 1995). The caudal premotor areas have a direct access to the spinal cord, whereas the rostral premotor areas send their output only to the brain stem. Furthermore, the caudal premotor areas have direct connections with F1, whereas the rostral premotor areas lack this link.
  3. Each caudal premotor area is tightly connected with a specific set of parietal areas. Functional data, when available, indicate that the linked parietal and motor areas may share common functional properties. Thus, the parietal and premotor areas form a series of anatomical circuits partly independent one from another, which elaborate sensory information and transform it into action. This process occurs in parallel in several parieto-frontal circuits, each of which is involved in specific sensory-motor transformations. F3 and the dorsal part of F2 (around the superior precentral dimple), for this process, use somatosensory information, while F4, F5 and the rostro-ventral part of F2 use also visual information. Furthermore, these areas, by virtue of their corticospinal projections or the connections with F1, can be also directly involved in motor execution.
  4. The rostral premotor areas, by virtue of their connections with prefrontal and rostral cingulate areas, elaborate higher order cognitive information related to long-term motor plans and motivation. These areas may likely have a control function in determining when and in which circumstances the activity generated in the parieto-dependent areas becomes an actual action.

In the context of this review, what is important to note is that, when multiple architectonic criteria are considered, architectonics is an invaluable tool for bridging data from different laboratories and experimental approaches. Specifically, though with different nomenclatures, the subdivision of Brodmann area 6 into mesial, dorsal and ventral sectors, each formed by a rostral and a caudal subdivision, represents a common anatomical frame of reference for most of the students in the motor field and for atlases of the monkey brain (Paxinos et al. 2000).

Neurochemical characterization of the macaque motor cortex

In theory, differences in architecture could reflect differences in the input–output connectivity or in the organization of the intrinsic circuitry. These two possibilities are not mutually exclusive. In the first case, one should expect that different architectonic areas differ also in their connections, which is the case for most of the motor areas. In the second case, one should expect that within a given architectonic area there might coexist different patterns of connectivity, i.e. different modules that, though performing a similar kind of processing, are characterized by a differential input–output connectivity and thus are functionally distinct.

It is quite plausible to suggest that a neurochemical approach could be particularly useful for addressing these issues. For example, in V1 and early extrastriate areas, cytochrome-oxidase histochemistry was crucial for the definition of different functional modules, in spite of a cytoarchitectural homogeneity (see, for example, Merigan & Maunsell, 1993). Furthermore, as there is now clear evidence that the monkey and the human motor cortex share many common functional organizational features, a neurochemical characterization of the primate cortex could be extremely useful for establishing possible homologies between human and non-human primates.

In this light, one approach that appears to be very promising is receptor autoradiography. Several studies by Zilles and colleagues (e.g. Zilles et al. 1991, 2004; Kotter et al. 2001) have shown that a cortical area can be characterized by measuring the regional and laminar binding patterns of tritiated ligands specifically for receptors of classic neurotransmitters. By using different ligands, a ‘neurochemical fingerprint’ can be established for each cortical area. Areas with similar neurochemical fingerprints can be grouped into ‘neurochemical families’ of areas. Functionally different areas (e.g. motor vs. somatosensory areas) are more different in terms of neurochemical fingerprints than functionally similar areas (e.g. primary motor and premotor areas). The functional similarities are reflected neurochemically by fingerprints that are similar in their laminar pattern but differ in their areal binding densities. In the macaque mesial frontal cortex, for example, a caudo-rostral increase in binding densities for [3H]AMPA, [3H]kainate and [3H]pirenzepine from the primary motor cortex (F1) to the pre-SMA (F6) in macaques (Geyer et al. 1998) has been shown. Furthermore, differences in the laminar distribution of neurotransmitter binding sites very closely match the cytoarchitectonic borders, further supporting the distinctiveness of these three motor areas. More interestingly, in the human mesial frontal cortex rostral to area 4, two areas can be distinguished, displaying cytoarchitectonic features comparable with those of F3 and F6 and showing similar changes in receptor autoradiographic patterns (Zilles et al. 1996). These two areas are today almost unanimously considered as the homologue of the monkey areas F3 and F6 and are generally referred to as SMA and pre-SMA in the literature, respectively. Therefore, the combination of cytoarchitectonic and receptor autoradiographic methods can be usefully employed in the attempt to define homologies between human and non-human primates.

Over recent years, several new microstructural techniques, based on an immunohistochemical approach, have been increasingly used to map areas in the primate cortex. Thus far, at least two of them appear quite useful for a further characterization of the monkey motor cortex.

The first technique is the immunostaining of non-phosphorylated epitopes on the neurofilament protein triplet with the monoclonal antibody SMI-32 (Hof & Morrison, 1995). This antibody labels a subset of pyramidal neurons with specific regional and laminar distribution patterns. Neurofilament proteins are involved in the maintenance and stabilization of the cytoskeleton of the axon and their immunohistochemical detection in the soma has been correlated with neuronal and axonal size and with the conduction velocity of nerve fibres. For example, the percentage of SMI-32-immunoreactive pyramidal neurons is low (typically less than 50%) in short corticocortical pathways and high (typically more than 50%) in long association pathways (Hof et al. 1995). As only a subset of pyramidal neurons is immunostained, this technique offers a simplified view of cortical architectonics, which, in many cases, allows an easier differentiation of cortical areas. Studies focused, for example, on the striate and extrastriate visual cortex (Hof & Morrison, 1995; Yoshioka & Hendry, 1995; Chaudhuri et al. 1996; Hof et al. 1996) have shown that SMI-32 immunoreactivity may be used as a powerful tool to confirm and extend previous architectonic observations based on more conventional staining techniques.

In the motor cortex, SMI-32 immunolabelling is confined mostly to layers III and V (Fig. 5). In particular, in layer V relatively large immunopositive pyramids, though much more numerous in F1, are present all over the caudal premotor areas and their distribution appears to correspond very well to the origin of corticospinal projections. In F1, there is a dense band of immunoreactivity in layer III and the strongly immunopositive giant pyramidal cells in layer V are clearly evident even in a very low-power view. Interestingly, many of these pyramids show dendrites originating from the entire circumference of the cell body, suggesting that this approach is useful for the identification of Betz cells (see above). In general, SMI-32 immunostaining in the motor cortex is maximal in its caudal part and decreases rostrally towards the prefrontal cortex (Geyer et al. 2000). This decrease, however, is not gradual. Instead, several step-like changes are obvious, which indicate not a uniform but, rather, an areal distribution pattern of SMI-32 immunoreactivity. A sharp drop in immunoreactivity in the rostral bank of the central sulcus marks the border between the primary motor and somatosensory cortex. In the rostral direction, in F2 the giant pyramidal cells in layer V almost disappear and in layer III, in spite of an overall decrease in immunolabelling, medium-sized pyramids are more evident in its lowest part. Accordingly, as with SMI-32 immunohistochemistry, the rostral border of F1 can be defined, in both non-human and human primates (Baleydier et al. 1997; Geyer et al. 2000). It should be noted, however, that the question of whether F1 is homogeneous or is constituted by different architectonic domains remains unanswered (see, for example, Geyer et al. 1996; Preuss et al. 1997; Rivara et al. 2003). Distinct decreases in layer III immunoreactivity can be found further rostrally and mark the border between F2 and F7 and between F7 and the frontal granular cortex. A similar trend, with step-like decreases in immunolabelling intensity in the caudo-rostral direction is evident also on the mesial surface at the border of F1 with F3 and of F3 with F6.

Fig. 5
Photomicrographs of representative fields of areas F1, F2, F3, F6 and F7 showing the laminar distribution pattern of SMI-32 immunoreactivity. Scale bar, shown in F1 (upper left), applies to all photomicrographs, except the higher magnification view of ...

Interestingly, in addition to different areal distribution patterns, SMI-32 immunoreactivity also shows regional common immunoarchitectonic features that distinguish different cortical domains. For example, the border of Brodmann area 19 with the parietal cortex is quite easy to define (Luppino et al. 2005a), and the temporal cortex is characterized by the presence of immunopositive pyramids in the upper part of layer III, which represent a distinguishing feature of this cortical domain (Hof & Morrison, 1995). In the frontal lobe, a very sharp decrease in layer III immunoreactivity clearly distinguishes the motor cortex from neighbouring cortical regions, for example from the opercular frontal cortex. It is interesting to note, however, that rostral to the agranular frontal cortex, a cortical strip including the dorsal prearcuate cortex and a more dorsal region rostral to F7 still presents a relatively high immunoreactivity. This chemoarchitectonic feature distinguishes it from more rostral prefrontal areas 46 and 9 (Fig. 6). This observation is not surprising if it is considered that this prefrontal sector, corresponding to area 8 of Walker (1940), is involved in the control of eye movements (e.g. Bruce et al. 1985; Schall et al. 1995) and therefore may be considered, in a broad sense, as a part of the motor functional domain. Similarly, in the cingulate sulcus, the cingulate motor area 24d, located ventral to F3 (Luppino et al. 1991; Matelli et al. 1991), displays a higher immunoreactivity with respect to areas 24b and 24a of the cingulate gyrus.

Fig. 6
Low-power photomicrographs of SMI-32-immunostained parasagittal sections through the transition between the agranular with the granular frontal cortex (A) and the superior arcuate sulcus (B). Dashed box within the section drawings indicates the location ...

While SMI-32 immunoreactivity, by labelling a subset of pyramidal neurons, may give information on specific aspects of the output components of the cortex, other immunoarchitectonic approaches appear useful in giving information on the organization of the intrinsic cortical circuitry. This is the case, for example, of the immunolabelling for calcium-binding proteins, in particular parvalbumin (PV) and calbindin (CB). Other studies have shown that CB and PV immunostaining labels different cell populations, represented mostly by inhibitory interneurons (Conde et al. 1994; see also Hof et al. 1999), with specific physiological and morphological features (Zaitsev et al. 2005). Both PV- and CB-immunopositive cells appear also to show differential areal distributions which have been used for distinguishing different prefrontal (Dombrowski et al. 2001; Elston & González-Albo, 2003), temporal (Kondo et al. 1994), and sensory and polisensory areas (Kondo et al. 1999) and for distinguishing the primary motor area from the premotor cortex (Elston & González-Albo, 2003)

Preliminary evidence obtained in our laboratory indicates that CB immunoreactivity shows a differential regional distribution in the agranular frontal cortex (Fig. 7). In particular, although transitions between different immunostaining patterns are not as sharp as transitions in cytoarchitectonic features, CB immunostaining appears quite useful for a further characterization of the different classes of motor areas. In F1, there is dense population of CB-immunopositive cells, confined to layer II and the uppermost layer III, and sparse immunopositive cells in the deeper layers. Rostral to F1, in both F2 and F3, CB-immunopositive cells have a similar distribution, although they appear to be more numerous in the deeper layers. A faint band of immunopositive neuropil, however, is evident in these two areas, in correspondence with layer V. In contrast, in the two rostral premotor areas F7 and F6, CB immunoreactivity is considerably denser. Immunopositive cells, although densest in layer II, clearly extend more deeply in layer III. Furthermore, the band of neuropil in layer V is much more evident than in the caudal premotor motor areas. Further rostrally, a sharp increase in the number of immunopositive neurons and in the neuropil staining intensity, as well as the presence of two bands of neuropil in the deeper layers, clearly distinguishes the granular prefrontal cortex from F7 and F6. These observations suggest that functional specializations of motor areas rely not only on their specific patterns of cortical and subcortical connectivity, but also on the organization of their intrinsic cortical circuitry.

Fig. 7
Upper part: low-power photomicrograph of a CB-immunostained parasagittal section through the whole extent of the motor cortex, taken at a mediolateral level similar to that of Fig. 1(A). Lower part: photomicrographs of representative fields of areas F1, ...

A multiarchitectonic approach for the definition of a new ventral premotor area

The data reviewed above indicate that neurochemical architectonic features can be informative not only of the location and extent of a given architectonic area, but also of its possible assignment to a given functional domain. In a recent study (Luppino et al. 2005b) we adopted a combined cytoarchitectonic and neurochemical approach to re-examine the organization of the ventral premotor area F5, in the light of evidence showing that this area is functionally not homogeneous (see, for example, Rizzolatti & Luppino, 2001). Figure 8 shows a low-power photomicrograph of a parasagittal section, stained for SMI-32 immunoreactivity, through the whole extent of the inferior limb of the arcuate sulcus. Two areas can be distinguished in this sector, a more caudal and a more rostral one, referred to as the ‘posterior’ (F5p) and the ‘anterior’ (F5a) subdivision of F5. In Nissl-stained material F5p shows a columnar arrangement and a clear subdivision of layer V into a dense layer Va, populated by small pyramids and a layer Vb in which relatively large pyramids are evident. In contrast, F5a is characterized by the presence of a dense and homogeneous layer V. F5p displays a relatively high content of SMI-32-immunopositive pyramids in lower layer III and scattered relatively large pyramids in layer V. In contrast, SMI immunoreactivity in F5a is much lower, with immunopositive medium-sized pyramids confined to the lowest part of layer III. Interestingly, a sharp change in the distribution of Cb immunoreactivity also distinguishes these two areas. In F5p, CB-immunopositive cells are mostly confined to layer II and the uppermost part of layer III, while in F5a these cells are very numerous in the entire extent of layer III. Accordingly, F5a shows a general pattern of SMI-32 and CB immunoreactivity which closely resembles that observed in F6 and F7, suggesting an assignment of this area to the class of the rostral premotor areas. This hypothesis found strong support in a functional magnetic resonance study (Nelissen et al. 2005) in which the above mentioned architectonic features were considered for the definition of regions of interest for the analysis of regional changes in blood flow. Indeed, the results of this study indicate that F5a appears to play a higher order role in action coding with respect to F5p, suggesting that F5a can be considered as a newly defined rostral premotor area. Furthermore, considering the suggested homologies of the monkey F5 with at least part of the human Broca's region (Brodmann's areas 44 and 45; see, for example, Rizzolatti & Arbib, 1998), these data suggest that F5a is a candidate for the homologue of the human area 44.

Fig. 8
Upper part: low-power photomicrograph of an SMI-32-immunostained parasagittal section centred on the precentral gyrus and the whole extent of the inferior arcuate sulcus. The level at which the section was taken is shown in the drawing of a dorsal view ...

Concluding remarks

The data reviewed above indicate that, a century after the foundation of this discipline, architectonics remains an actual and successful approach, whose power for understanding brain functions has for a long time been underestimated and now goes far beyond the original aim of subdividing the brain into regions of specific structure. Indeed, a multimodal architectonic approach, based on a combination of cytoarchitectonic criteria and on other complementary techniques, is quite successful for a reliable definition of the structural organization of the cortex. These structural maps must, however, be continuously interfaced with functional and connectional data and accordingly updated. In this way, architectonics appears of crucial value for comparison of data from different individuals and different disciplines, and for the identification of new cortical areas and for comparative studies.


The authors are supported by grants from the Ministero dell’Universita’ e della Ricerca (PRIN 2006, no. 2006052343-002). A.B. is supported by a fellowship from the EU (Marie Curie, Early stage training programme ‘Sensoprim’ MEST-CT-2004-007825).


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