Specializations Of Limbic Cortex

The limbic cortex, located along the margin (limbus) of the cortical mantle, is a functionally heterogeneous region that includes structures involved in emotion and memory. Limbic cortex forms a continuous band that extends from the cingulate region, which lies immediately anterior and superior to the corpus callosum, through the retrosplenial region, which is wrapped around the posterior end (splenium) of the corpus callosum, and into the parahip-pocampal region of the medial temporal lobe. Much of the cortex of the lim-bic region has a simplified laminar structure, and is considered allocortex or periallocortex rather than isocortex, but some portions have a well-developed laminar organization and are classified as proisocortex or isocortex. The allo-cortex (hippocampus and primary olfactory cortex) bears some resemblance to the three-layered cortex of reptiles, which makes it tempting to view the limbic region as being frozen in the evolutionary past. This is not the case: even the early cortical cartographers noted important variations among mammals in the areal subdivisions of the limbic region (Brodmann, 1909). There have been few modern comparative studies of the organization of limbic cortex, but there are reasons to think that stem primates evolved specializations within this territory.

One region of interest is cingulate cortex. In primates, cingulate cortex is usually divided into an anterior region (comprised of Brodmann's areas 24, 25, and 32) and a posterior region (areas 23 and 31) (Brodmann, 1909; Vogt et al., 1987). The cingulate cortex is adjoined posteriorly by retrosplenial cortex. Homologues of the anterior and retrosplenial territories are recognizable in most eutherians that have been examined, and older studies identified a posterior cingulate region in a variety of taxa. Modern studies of rats and rabbits, however, depict anterior cingulate area 24 as adjoining the retrosplenial cortex directly, without an intervening posterior cingulate territory (Vogt, 1985; Vogt et al., 1986). This raises the possibility that primate posterior cingulate cortex is neomorphic. The posterior cingulate cortex underwent further modification later in primate evolution, in stem haplorhines or anthropoids (Armstrong et al., 1986; Preuss and Goldman-Rakic, 1991b; Zilles et al., 1986). The posterior cingulate region is connected with cortical areas known to have visuospatial functions (including PP and DLPFC) and functional studies place it in a network of structures involved in spatial attention and memory, eye movements, and mental navigation (e.g., Maguire, 1997; Vogt et al., 1992).

Another possible locus of evolutionary change is the posterior parahip-pocampal cortex. The parahippocampal region comprises a set of areas present in a wide variety of eutherians; these include the entorhinal, perirhinal, and prorhinal areas, which are located in the temporal lobe adjacent to the hippocampus. Primates possess an additional pair of areas, termed TH and TF, which lie posterior to the entorhinal-perirhinal region. These areas have been identified in both strepsirhine and anthropoid primates (Preuss and Goldman-Rakic, 1991b) but are not usually attributed to nonprimate mammals. It has been suggested that a territory in rats termed the postrhinal area is homologous to primate posterior parahippocampal cortex (Burwell et al., 1995). It is possible that postrhinal cortex is homologous to area TH—both share a fairly simple laminar organization—but it seems unlikely that postrhinal cortex also includes a homologue of the thick, well-laminated area TF. The parahippocampal areas are important way stations in pathways that link isocortical areas to the hippocampus, a structure involved in the formation of long-term memories (Squire and Zola, 1996), and the likelihood that primates possess one or more new divisions of parahippocampal cortex suggests they may have evolved new memory-related systems.

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