The fact that so many sensory systems eventually project to the cerebral cortex indicates that there are likely similarities in their processing. Evolutionarily, it’s plausible that most neural sensory systems diverged from one common system that involved early analogue of the cortex.
Within mammals, there is a large amount of variation in the number of cortical areas, with most of these subdivisions in all mammals involved in sensory-perceptual tasks. Additionally, some sensory regions of the neocortex have been well preserved throughout the mammalian lineage, including visual cortices V1 / V2, somatosensory cortices S1 / S2, and possibly the primary auditory cortex A1. In order to add additional regions from this baseline amount, evolution might have either selected for rapid duplication of one existing region, or the gradual differentiation of one region into two.
In the visual pathway, the different brain regions contain neurons with different cell morphologies that are arranged in hierarchical fashion. Elston et al gathered data from layer III pyramidal neurons of V1, V2, the medial temporal lobe, V4, the inferior temporal cortex, and finally the superior temporal polysensory of the macaque. When they arranged the data from the pyramidal cells in order they found a beautiful pattern in the basal dendritic field areas, spine densities, and somal areas. I’ve reproduced part a of their figure 2 below but check out the open-access pdf of their paper for all of the data:
This data shows that there is a hierarchical organization of pyramidal cell morphology in the visual system. If there is indeed a common form of neural processing between the various types of sensory systems, we should expect a similar hierarchical organization of cells in non-visual sensory systems as well.
Inspired by Inspired by CalTech’s Question #9 for cognitive scientists: “In mammals, somatosensory, visual, auditory, and olfactory sensory systems all project to the cerebral cortex. To what extent does this imply some common form of neural processing? Justify your answer by referring to and comparing specific details of cortical anatomy and physiology.”
Kaas JH. 1989 The evolution of complex sensory systems in mammals. Pdf.
Elston GN, Tweedale R, Rosa MG. 1999 Cortical integration in the visual system of the macaque monkey: large-scale morphological differences in the pyramidal neurons in the occipital, parietal and temporal lobes. Proc Biol Sci. 1999 July 7; 266(1426): 1367–1374. PubMed link here.