Relationship between prefrontal and limbic cortex: a comparative anatomical review

Certain cortical areas of the frontal lobe which are included in the limbic system on functional grounds and by virtue of their hypothalamic and amygdaloid connections must also be considered part of the prefrontal cortex if the latter is defined as the projection field of the mediodorsal thalamic nucleus (MD). This ambiguity has resulted in general confusion regarding the anatomical organization of these areas. The present review attempts to clarify these issues by briefly discussing the historical development of the concepts of limbic and prefrontal cortex, then reviewing comparative data on cytoarchitectural structure and afferent connections among several orders of mammals. It is shown that in all cases the entire cerebral cortex can be divided into concentric rings of allocortex, mesocortex and isocortex. The cortical projections of MD and the amygdala overlap primarily in the mesocortical regions which constitute limbic cortex, and the MD projection field extends further to include the granular isocortex of the frontal lobe. This close correspondence between cytoarchitectonic structure and afferent connections in different groups of mammals suggests that these anatomical features are fundamental aspects of cortical organization and that they be used to re-orient terminology such as limbic cortex and prefrontal cortex, as well as guide our understanding of the functional roles played by these cortical areas.