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.
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