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L.L. Silveira, L. Lotman, B.P. Chalfin, D. Cheung, J. Yost, J.A. P. C. Muniz, B.L. Finlay; Scaling of the Visual Thalamus in Primates . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5683.
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Purpose:To characterize the volume and cell number (including M and P cells) of the lateral geniculate nucleus and pulvinar in primates with respect to brain and eye size, visual acuity, color vision, spatial vision and nocturnal and diurnal niche. Methods: The brains from 6 New World Primates (Saimiri ustius, Callithrix jacchus, Callicebus moloch, Aotus azarae, Cebus apella and Alouatta caraya) gathered as part of a larger study were sectioned coronally at 60 micron thickness and stained with cresyecht violet. The outlines of the lateral geniculate and pulvinar nuclei of the thalamus were traced. Using stereological techniques, the volumes and cell numbers of the nuclei were computed, including the M and P layers of the lateral geniculate nucleus. These values were compared to previously published values for tarsiers, lemurs, Old World monkeys and the great apes including humans. Results: Total cell number and volume for the lateral geniculate nucleus scaled similarly to that observed for Old World Monkeys. The numbers of cells in the M and P layers of the lateral geniculate nucleus of the owl monkey, Aotus azarae, matched the numbers of cells found in the diurnal tamarin, Callicebus moloch, very close in brain size. Over all primates, the range of variation in P cells (approximately 120,000 to 1,800,000) from small to large brain sizes well exceeds the range in variation of M cells (approximately 50,000 to 250,000) and is mismatched with P and M cell numbers in the retina, such that convergence from retina to lateral geniculate decreases for P cells and increases for M cells with brain enlargement. The scaling of the pulvinar is consistent across New and Old World Monkeys and great apes. Conclusions: The absence of a difference in M and P cell numbers in the lateral geniculate nucleus between the nocturnal owl monkey and other diurnal monkeys is consistent with the pattern of central conservation of pathways coupled with peripheral variability we have already observed for striate and extrastriate cortical pathways. The disproportionate scaling of the geniculate P cell population is a probable factor mediating producing increased cortical representation of the fovea in primates with large brains. The demonstration that the volume of the pulvinar is scales consistently over New and Old World primates and great apes suggests the time of the primate specialization of the pulvinar antedates divergence of these three groups.
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