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  • Title: The corticopontine projection from area 20 and surrounding areas in the cat: terminal fields and distribution of cells of origin as compared to other visual cortical areas.
    Author: Bjaalie JG.
    Journal: Neuroscience; 1989; 29(1):81-93. PubMed ID: 2469039.
    Abstract:
    Visual area 20 in the cat projects to the pontine nuclei and thereby gives input to the cerebellum. The termination of the fibres and the distribution of the cells of origin were studied with anterograde and retrograde transport of horseradish peroxidase-wheat germ agglutinin. Fibres from area 20 terminate ipsilaterally in the medial half of the rostral three-quarters of the pontine nuclei. The labelled fibres occur in multiple well-restricted patches usually some distance away from the peduncle. The retrogradely labelled cells in areas 20a, 20b, and the adjacent posterior suprasylvian visual area were quantitatively mapped. Areal borders were placed according to the maps of Tusa and Palmer (J. comp. Neurol. 193, 147-164) and Updyke (J. comp. Neurol. 246, 265-280). In terms of cortical densities (cells per mm2 of cortex) area 20a is among the visual areas with the highest densities of corticopontine neurons (data from other visual areas from the author's previous works). Densities are 30-60% lower in area 20b than in 20a. The posterior suprasylvian visual area was found to be among the visual areas with the lowest densities of corticopontine neurons. Due to the small size of the areas investigated, the total number of corticopontine cells within them is small compared to many other areas. Within areas 20a and 20b, cortical densities are higher in the representation of visual space below than above the horizontal meridian. Furthermore, cortical densities are generally somewhat higher in the region devoted to central vision compared to regions devoted to the visual periphery. Since the part of cortex in areas 20a and 20b devoted to central vision is weakly over-represented also in terms of cortical volume, it follows that the "visual field density" (cells per degree of visual field) of corticopontine cells is highest in the central visual field representation. The finding of an over-representation of central vision compared to peripheral vision in the corticopontine projection from areas 20a and 20b is particularly interesting in conjunction with previous findings in other visual areas. In the cortical representations of central vision, areas with high magnification factors (i.e. areas that greatly emphasize central vision in terms of cortical volume, such as areas 17, 18, and 19) have relatively low cortical densities of pontine projecting cells, whereas areas with low magnification factors (such as areas 20a, 20b, and some of the lateral suprasylvian visual areas) have relatively high cortical densities. The "visual field density" of corticopontine neurons appears therefore to be fairly constant from area to area as concerns central vision.
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