We measured binocular summation for the detection of red-green chromatic, blue-yellow chromatic, and luminance contrast. The results indicated a higher degree of binocular summation for luminance contrast (reaching the level of neural summation) than for chromaticity contrast (probability summation), showing that the former is more effective than the latter for the binocular visual system, in agreement with studies on stereopsis and binocular summation for visual reaction time. (C) 2003 Wiley Periodicals, Inc.