Orientational versus horizontal disparity in the stereoscopic appreciation of slant

Twenty stereograms with needles either plunging in depth or untilted were constructed. When the geometry of the needles was unbiased, the tilt of the needles was correctly and rapidly appreciated. When the needles were biased so as to remove either their orientational disparity, or the difference in horizontal disparities at the tips, they could be seen, depending on the subject and the nature of the bias, either with or without slant. Orientational disparity proved to be, with two different testing methods, clearly more effective than horizontal disparity in conveying the information of slant. Biased needles at -45 degrees were more often rejected as untilted than biased needles at +45 degrees. The orientational disparity information was ineffective with crosses that combined +45 degrees and -45 degrees needles. The reaction time and the nature of the percept were correlated, the tilted percept taking longer to mature than the untilted one in biased stereograms. Of the seventy tested subjects, one appeared to make no use at all of horizontal disparity in the stereoscopic appreciation of slant.