A quantitative study of the projection area of the central and the paracentral visual field in area 17 of the cat

Summary1.In the course of long oblique penetrations through the postlateral gyrus a variation in the position of the receptive fields (RF-scatter) of single cells recorded extracellularly is observed. This is superimposed on the continuous topological representation of the retina. Spezifying the RF-positions by the azimuthal and elevation coordinates of their geometrical centers, the standard deviation (SD) of the mean RF-positions of cells recorded in 200 μm long horizontal sections of cortex is calculated and the total radial scatter of RF-positions (Sanderson, 1971) as defined: $$\sqrt {\left( {{\text{4SD azimuth}}} \right)^2 + \left( {4SD{\text{ elevation}}} \right)^2 } $$ is determined. The radial scatter is found to have its smallest value (1 degree visual angle (v.a.)) in the projection area of the functional center of the area centralis increasing to 3–4 degrees v.a. at 10 degrees eccentricity.2.The mean RF-diameter as defined: $$\sqrt {{\text{mean RF - area}}} $$ is centrally 0.7 degrees v.a. increasing to 2.6 degrees v.a. at 10 degrees eccentricity. The ratio of the largest RF-diameter to the smallest RF-diameter is between 7–9 and remains almost constant over the central 10 degrees of the projection area. The magnification factor (M) as defined: mm Cortex/degree v.a. is centrally 2.3, decreasing paracentrally to 0.6.3.The cells in area 17 whose RFs have the same direction in the visual field constitute the spatial subunit of the retinocortical projection. The diameter of the spatial subunit is calculated as: $$\sqrt {\left( {{\text{total radical scatter}}} \right)^2 + \left( {{\text{average RF - diameter}}} \right)^2 } $$ . The spatial subunit functionally represents, therefore, that part of the visual field whose location and area is calculated by averaging over the RFs of the individuals of its cell population. It is found that the cells belonging to a spatial subunit are distributed within a cortical cylinder of 2.6–2.8 mm in diameter, the peak of the distribution coinciding with the central axis of the cylinder.4.Within the projection area of the central 10 degrees of the retina in area 17 the spatial subunits have the same diameter. This suggests that each retinal ganglion cell is functionally connected with an equal number of cells in area 17 irrespective of its position within the retina and that, therefore, the retinocortical projection is organized on the basis of a stereotyped schema if a basic spatial relationship is concerned.