INVESTIGATION OF PREFRONTAL AREAS IN VARIOUS ANIMAL TAXA USING DIFFUSION WEIGHTED IMAGING

The Prefrontal Cortex (PFC) has attracted a particular interest since it is the latest developed cortical area which reached its apparent maximum development in the human. Although its name came from mammalian topography, being the cortex of the anterior pole of the brain, it has been shown that even other animals, such as birds, possessed an equivalent region called Nidopallium Caudolaterale (NCL) which is involved in the same functions. These multimodal associative areas have been widely studied in humans, other primates, rodents and pigeons through several techniques. Nevertheless, neuroanatomy data of other species, in particular on fiber connections, are scarce and would deserve a deeper attention, especially for those ones largely used for meat products (like sheep, cows and pigs) whose brains are wasted and poorly investigated. Even though tracing is considered the gold standard for studying fiber connections in the brain, it is mostly used in lab animals due to ethical and practical reasons. Despite this, a relatively recent MRI technique called Diffusion Weighted Imaging (DWI) has been proven to give similar results to tracing and it is applicable not only in in-vivo specimens but also in fixed ex-vivo brains. We therefore started examining the Orbitofrontal Cortex (OFC) of the sheep (Ovis aries) and compared it with that of the human and the chimpanzee. While histology and immunocytochemistry revealed a different cytoarchitectonic and neurochemical structure, DWI showed a similarity of the areas connected to the OFC, implying that although these animals do not show the same subtlety in their complex behaviours and are commonly not revered as “smart” like primates, they still possessed the basic structures for complex abstract thought. Farm animals have undoubtedly been insufficiently examined in terms of neuroanatomy, but marine mammals certainly share this neglect, mostly due to difficulty in data collection and conservation, and ethics. In these animals the cortical map is not complete, only the visual (V1/V2), auditory (A1/A2), motor (MC) and somatosensory (SSC) cortices have been localized on the telencephalon leaving the rest of the cortical surface unassigned. To overcome this problem, we applied a new DWI technique able to detect crossing fibers within a voxel even with clinical MRI parameters, the so called constrained spherical deconvolution (CSD). This new mathematical algorithm was applied to three formalin-fixed brains of adult bottlenose dolphin (Tursiops truncatus). We first performed global tractography through several steps and then qualitatively analysed few regions. Our results confirmed the feasibility, considering the long fixation time and the clinical MRI parameters, of the CSD in ex-vivo brains. Afterwards, we sought for a putative PFC in these animals, relying in the general concept that in all mammals the PFC is the major receiver of thalamic inputs from the mediodorsal thalamic nucleus (MD). Therefore, we used the CSD to detect the fiber pathways from the MD and we proved the accuracy of our results by seeding also the lateral and medial geniculate nuclei (LGN and MGN). The reverse approach was then applied, seeding first the cortices and look for their related tracts. CSD, in accordance with the previous evoked-potential and tracing studies, confirmed the location of A1/A2 and V1/V2, together with an involvement of the LGN on A1/A2 and MGN on V1/V2. The tracts related to MD detected the possible PFC location on the orbital and cranial temporal lobes. The neocortical pattern thus suggested not only a more complex thalamic integration arrangement among different areas but also a rostrolateral rotation of the areas which probably followed the so called telescoping process which occurred to the brain case of these animals during evolution.