Distribution of corticopontine neurons in visual areas of the middle suprasylvian sulcus: Quantitative studies in the cat

Abstract

This study deals with the distribution of corticopontine neurons in four of the lateral suprasylvian visual areas, the anteromedial, anterolateral, posteromedial and posterolateral areas. Following large injections of horseradish peroxidase-wheat germ agglutinin in the pontine nuclei, labelled cells were quantitatively mapped. The borders of the four areas were determined cyto- and myeloarchitectonically or from standard retinotopic maps presented in frontal sections (Tusa et al., 1981). 50 Flat maps were constructed of each area showing the distribution of retrogradely labelled neurons. Maps of the retinotopic organization in the lateral suprasylvian visual areas (Palmer et al., 1978; Tusa et al., 1981) 29,50 were transferred to the flat maps. Thus, the density and number of labelled cells within each area and within smaller zones representing different visual field blocks could be determined. The average cell density (average number of labelled cells per mm 2 of flattened cortex) is highest in the anteromedial area. The average density in this area is higher than in areas 17,18 and 19. The total number of labelled cells is highest in the posteromedial area. The lateral syprasylvian visual areas together have a stronger pontine projection in terms of actual number of cells than each of areas 17, 18 and 19. [Data on areas 17, 18 and 19 from Bjaalie and Brodal (1983) and Bjaalie (1985).] 6,8 The four areas investigated have different internal distribution of corticopontine cells. The posteromedial and posterolateral areas have lower densities of labelled cells in the representations of the retinal central area than in the representations of the visual periphery. However, due to the enlarged cortical representations of central vision, the actual number of corticopontine cells per visual field block is higher in the cortex representing the central area; i.e. the central visual field region is moderately over-represented in the corticopontine projection from these areas. In the anteromedial and anterolateral areas, unlike in all other areas so far investigated, the cortical representations of the central visual field contain higher densities of corticopontine cells than the representations of the lower visual periphery. However, since the central area is only very weakly over-represented compared to the lower visual periphery in terms of cortical volume in these areas, the over-representation in the corticopontine projection of central vision compared to the lower visual periphery is fairly moderate. Thus, the over-representation of central vision in terms of number of corticopontine neurons seems to be of a fairly constant magnitude in all visual areas so far investigated (including areas 17, 18 and 19), even if the over-representation in terms of cortical volume varies dramatically between the various cortical visual areas.