Mapping auditory cortex in the La Plata dolphin (Pontoporia blainvillei)

This study deals with the mapping of the primary and secondary auditory cortex. Due to their important role in echolocation they were the first areas to be examined [P.J. Morgane, M.S. Jacobs, in: R.J. Harrison (Ed.), Functional Anatomy of Marine Mammals, Comparative Anatomy of the Cetacean Nervous System, vol. 1, Academic Press, London, 1972, pp. 117-144]. We analysed the brain of a La Plata dolphin (Pontoporia blainvillei), which had been fixed in formaldehyde, embedded in paraffin, cut in sections of 20 μm thickness and stained with cresyl violet. The experimental approach being impossible, we used cytoarchitectonic variations in the neocortex. Former electrophysiological data [T.F. Ladygina, A.Y. Supin, Localization of the projectional sensory areas in the cortex of the porpoise Tursiops truncates, Zh. Evol. Biokhim. Fiziol. 13 (1978) 712-718] [Sokolov, T.F. Ladygina, A.Y. Supin, Location of sensory zones in cerebral cortex of dolphin, Dokl. Biol. Sci., Russian Original 202 (1-6) (1972)] provided the framework for the exact determination of borders between functional cortical areas. We used a stereological observer-independent procedure based on changes in volume density of cell bodies throughout the neocortex [A. Schleicher, et al., Stereological approach to human cortical architecture: Identification and delineation of cortical areas, J. Chem. Neuroanat. 20 (2000) 31-47]. Due to the computer program's high sensitivity to changes in volume density it was possible to analyse the poorly laminated dolphin cortex. The 3D-reconstruction of the auditory cortex was processed using the AMIRA® 3.0 Graphics software package comparing the main primary gyri in the histological sections with those in coronal magnetic resonance imaging scans of another intact Pontoporia brain.