Colour as Input to Stereopsis

Neural structures carrying information concerning stereopsis belong to the magnocellular sub system of the visual pathway. Regarding the magnocellular system as insensitive to colour, one would expect a high threshold for chromatic stimuli to evoke stereoscopic depth perception. In the present study, a stereogram together with an equiluminant background was generated on a colour monitor. The monitor was calibrated by a tristimulus colorimeter before each experimental session. The chromaticity coordinates of both stereogram and background could be varied in small steps. The author and two naive subjects participated in the experiments. Two patterns of three vertical bars formed the stereogram which was viewed through a stereoscope. With binocular fusion, the spatial arrangement of the bars gave rise to a horizontal, crossed or uncrossed, disparity of 10 min arc for one of the bars. Accordingly, this bar appeared to float either in front of, or behind, the other two, provided the colour difference between the stereo pair and the background was large enough. Under equiluminant conditions, the minimal colour difference necessary to evoke a depth perception exceeded the just noticeable colour difference at the chromaticity coordinates chosen (x = 0.265; y = 0.499) by a factor of only 3 to 4. In control experiments, the use of an optical system to compensate ocular chromatic aberrations did not measurably alter the results. These findings indicate that colour has an input to stereopsis which is stronger than a purely magnocellular system could provide.