Retinal sensitivity and the critical area of the spatial summation in the foveal and parafoveal regions were investigated by using a fundus-controlled perimeter to monitor accurately a subject's fundus picture and target position on the retina. The first experiment showed that retinal sensitivity and the critical area change continuously from fovea to periphery and that the diameter of the critical area is a linear function of retinal eccentricity. In the second experiment, this linear relationship was verified by another method.