Increment thresholds with various low background intensities at different locations in the peripheral retina.

A set of increment threshold data as a function of test-flash diameter, background luminance, and retinal eccentricity is presented. It is shown that for low background intensities the results can readily be described by simple transformations of flash diameter and background luminance: The threshold is independent of eccentricity if the quotient of diameter and eccentricity is constant and if the flash is presented on a background for which the product of background luminance and the square of eccentricity is constant. At an eccentricity of 50 deg, Ricco's law is violated: A small stimulus has a threshold 10 times as high as a large stimulus. On the basis of results found by other investigators for smaller eccentricities, it is concluded that the receptive field size at 50 deg of eccentricity is more than 10 deg (for low background luminances). For eccentricities smaller than 50 deg, a data analysis is given in order to derive an appropriate measure of the size of the sample units. This analysis shows that with increasing background luminance the decrease in the size of the sample unit is steplike rather than gradual.