Area 4 is found at the other end of the spectrum than the anterior cingulate cortex, a limbic region that lies at the foot of laminar differentiation of the motor cortical system (Barbas Pandya, 1987; Morecraft et al., 2012).NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptConfusing areas of different types: apples and oranges in one basketThe idea of systematic variation in cortical architecture is significant because it applies to all cortical systems–sensory and association areas alike. The fundamental idea is that the phylogenetically ancient limbic areas lie at the foot of every cortical system, and the gradual changes in architecture are observed in every system, including the cortical premotor-motor system (Sanides, 1964; Sanides Krishnamurti, 1967; Sanides, 1970; Barbas Pandya,Eur J Neurosci. Author manuscript; available in PMC 2015 June 01.Garc -Cabezas and BarbasPage1987; Morecraft et al., 2012). The cortical motor system can be traced to the cingulate gyrus Carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone site through areas now known as cingulate motor areas, and then through successive steps to medial and then lateral premotor areas, culminating in the primary motor cortex, or area 4. The systematic variation in cortical lamination makes it possible to identify types of cortices rather than rely only on their specific features. Agranular limbic areas have three layers because they lack a granular layer IV and their superficial (II II) and deep (V I) layers are not distinct. These areas lie near the primary olfactory areas and the hippocampus, as the classical students of architecture observed. Near the agranular areas, there is another type of limbic cortex characterized by a poorly developed layer IV–the dysgranular cortices. Nearby areas that have six layers belong to the general type of eulaminate cortex. But the latter too, show differences in laminar structure, with some areas having a specialized layer IV with subdivisions, as Brodmann and others observed for the primary visual cortex. Brodmann made a critical error in stating that area 4 is agranular along with the anterior cingulate. He grouped together areas of vastly different types, i.e. areas that are at the opposite spectrum with respect to cortical structure. In concluding that some areas differ in the number of layers they have, with some having more layers (e.g. the visual) or fewer in others (e.g. the anterior insula), Brodmann saw variation locally but missed the global principle, namely that the changes in cortical structure are systematic. The concept of cortical type has provided the basis to predict the pattern in the laminar origin and termination of corticocortical connections, as first postulated for primates (Barbas, 1986; Barbas Rempel-Clower, 1997; Rempel-Clower Barbas, 2000; Barbas et al., 2005), and later shown that it applies to other species (Hilgetag Grant, 2010). By analogy with the visual system, corticocortical connections have been called `feedforward’ when they originate in the upper layers (mostly layer III) of one area and terminate in the middle layers (including layer IV) of another area [reviewed in (Felleman Van Essen, 1991)]. Viewed in the context of cortical type and the systematic variation in cortical architecture, `feedforward’ connections BIM-22493 site simply describe those that originate in an area with more elaborate laminar structure than the area of termination (Barbas Rempel-Clower, 1997). Corticocortical connections have been called `feedback’.Area 4 is found at the other end of the spectrum than the anterior cingulate cortex, a limbic region that lies at the foot of laminar differentiation of the motor cortical system (Barbas Pandya, 1987; Morecraft et al., 2012).NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptConfusing areas of different types: apples and oranges in one basketThe idea of systematic variation in cortical architecture is significant because it applies to all cortical systems–sensory and association areas alike. The fundamental idea is that the phylogenetically ancient limbic areas lie at the foot of every cortical system, and the gradual changes in architecture are observed in every system, including the cortical premotor-motor system (Sanides, 1964; Sanides Krishnamurti, 1967; Sanides, 1970; Barbas Pandya,Eur J Neurosci. Author manuscript; available in PMC 2015 June 01.Garc -Cabezas and BarbasPage1987; Morecraft et al., 2012). The cortical motor system can be traced to the cingulate gyrus through areas now known as cingulate motor areas, and then through successive steps to medial and then lateral premotor areas, culminating in the primary motor cortex, or area 4. The systematic variation in cortical lamination makes it possible to identify types of cortices rather than rely only on their specific features. Agranular limbic areas have three layers because they lack a granular layer IV and their superficial (II II) and deep (V I) layers are not distinct. These areas lie near the primary olfactory areas and the hippocampus, as the classical students of architecture observed. Near the agranular areas, there is another type of limbic cortex characterized by a poorly developed layer IV–the dysgranular cortices. Nearby areas that have six layers belong to the general type of eulaminate cortex. But the latter too, show differences in laminar structure, with some areas having a specialized layer IV with subdivisions, as Brodmann and others observed for the primary visual cortex. Brodmann made a critical error in stating that area 4 is agranular along with the anterior cingulate. He grouped together areas of vastly different types, i.e. areas that are at the opposite spectrum with respect to cortical structure. In concluding that some areas differ in the number of layers they have, with some having more layers (e.g. the visual) or fewer in others (e.g. the anterior insula), Brodmann saw variation locally but missed the global principle, namely that the changes in cortical structure are systematic. The concept of cortical type has provided the basis to predict the pattern in the laminar origin and termination of corticocortical connections, as first postulated for primates (Barbas, 1986; Barbas Rempel-Clower, 1997; Rempel-Clower Barbas, 2000; Barbas et al., 2005), and later shown that it applies to other species (Hilgetag Grant, 2010). By analogy with the visual system, corticocortical connections have been called `feedforward’ when they originate in the upper layers (mostly layer III) of one area and terminate in the middle layers (including layer IV) of another area [reviewed in (Felleman Van Essen, 1991)]. Viewed in the context of cortical type and the systematic variation in cortical architecture, `feedforward’ connections simply describe those that originate in an area with more elaborate laminar structure than the area of termination (Barbas Rempel-Clower, 1997). Corticocortical connections have been called `feedback’.