Distribution in areas 18 and 19 of neurons projecting to the pontine nuclei: a quantitative study in the cat with retrograde transport of HRP-WGA.

Following large injections of horseradish peroxidase - wheat germ agglutinin in the pontine nuclei, corticopontine neurons in areas 18 and 19 were quantitatively mapped and flat maps showing the distribution of retrogradely labeled cells were constructed. The areal borders were defined either cyto- and myeloarchitectonically or from standard retinotopic maps presented in frontal sections (Tusa et al. 1981). Maps of the retinotopic organization in areas 18 and 19 (Tusa et al. 1979) were transferred to the present flat maps. Thus, the number and distribution of pontine projecting cells could be correlated with the retinotopic organization. The cell density (number of labeled cells per mm2 cortex) is in both areas highest in the cortex representing the lower and upper visual periphery and decreases towards the representation of the retinal central area. However, since in both areas 18 and 19 the visual field representation is twisted and portions of the visual field are magnified, the actual number of cells is higher in the cortex representing the central area and the lower medial visual field than in other parts. The cortex representing the lower hemifield contains approximately 2/3 (mean, N = 4) of the corticopontine cells in both areas. The average density of corticopontine cells increases from area 17 through 18 to 19, but the total number of cells within each of the areas is about the same (area 17: 18000 cells, area 18: 13400 cells, area 19: 17200 cells; mean, N = 4; data on area 17 from Bjaalie and Brodal, 1983). In conclusion, areas 17, 18 and 19 contribute about equally in quantitative terms to the pontine nuclei. Furthermore, assuming that the corticopontine neurons transmit spatially relevant information, there is a moderate overrepresentation of central vision and the lower medial visual field in the pontine projection from areas 18 and 19. This visual field representation is remarkably similar to that found in the corticopontine projection from area 17 (Bjaalie and Brodal 1983).