Subjects were presented with two gratings with different spatial frequencies and they were asked to discriminate one from the other. Their ability to discriminate between the gratings was found to depend primarily on the ratio of their spatial frequencies over a wide range of absolute frequency. At high spatial frequencies, discrimination deteriorates when differences of subjective contrast are eliminated.
FERGUS W. CAMPBELL, JACOB NACHMIAS, and JOHN JUKES, "Spatial-Frequency Discrimination in Human Vision," J. Opt. Soc. Am. 60, 555-559 (1970)