Divac I, Holst M C, Nelson J, McKenzie J S
Department of Physiology, University of Melbourne, Australia.
Brain Behav Evol. 1987;30(5-6):303-20. doi: 10.1159/000118653.
Afferents of the large, electrically 'silent' frontal cortex in an egg-laying mammal, the echidna, were studied with the somatopetal axonal transport technique. This cortical area receives thalamic projections only from the anterior part of the anteromediodorsal region. The medial parts of the cortex receive afferents from more medial neurons, whereas the lateral area is innervated from the laterally placed perikarya of this thalamic region. The nonthalamic afferents to the frontal cortex are, with few exceptions, similar to the afferents of the prefrontal cortex in placental mammals. Cortical afferents originate in all layers of the contralateral symmetrical areas and in the ipsilateral paleocortex, especially the cortex in the bottom of the sulcus mu. The claustrum could not be identified, neither cytoarchitecturally nor hodologically. Pending confirmation from studies of the diencephalon, we presently conclude that the large anterior cortical area in this species corresponds to the prefrontal cortex of eutherian mammals. If this conclusion is correct, the echidna is the only species studied to date that has a proportionally larger prefrontal cortex than humans.
