The insular region: Part of the prefrontal cortex?

Prefrontal and insular regions in different mammalian species are described on the basis of Brodmann's cytoarchitectonic findings. It is shown that the prefrontal cortex, defined cytoarchitectonically, and defined as the projection area of the mediodorsal thalamic nucleus, is often a considerably discordant area. An inverse relationship is assumed between the extent of insular cortex, which on the basis of mediodorsal thalamic afferents has to be termed prefrontal, and frontal cortex, which on the basis of the existence of a granular cell layer four has to be termed prefrontal. It is hypothesized that, because of a lower concordance of additional criteria, on which cortex may be termed prefrontal, and because of evidence, which makes the role of the mediodorsal nucleus for a definition of the prefrontal cortex questionable, nonprimates in comparison to primates in general will possess only “weaker copies” of the primate prefrontal cortex. Tentative support for this view is indicated.