Three experiments were performed (a) to derive indices of spatial summation for flickering white and three part-spectrum targets, on the assumption, verified in a previous experiment, that the index was the ratio of the slopes of the functions for CFF/log area and CFF/log luminance and (b) to investigate the effect of target/surround contrast on these functions.
It was found that, for part-spectrum targets at least, spectral composition is not a determinant of CFF and luminance is the effective variable. Further, the slope of the CFF/log luminance function increases with target area, depending upon the contrast between target and surround. It is suggested that these effects can be accounted for by postulating a family of sigmoid curves, representing the slope of the CFF/log luminance function as a function of log target size, with contrast as the parameter. These curves would have common upper and lower asymptotes, the points of inflection depending on the contrast.
P. J. FOLEY and J. KAZDAN, "Area-Intensity Relations within the Fovea for Flickering White and Part-Spectrum Targets," J. Opt. Soc. Am. 54, 547-550 (1964)