Effect of the Tilted Vertical Horopter on Visual Recognition.

The horopter, which refers to the plane in visual space where targets are seen as single and in a common visual direction, is integral to stereoscopic depth perception. Although the horizontal horopter has been analyzed extensively, the vertical horopter has virtually gone unexamined. Recent research indicates that the vertical horopter is tilted away from the observer and that such a tilt differentially alters the perceptibility of targets located above and below the horizontal plane of fixation. The present experiments explore the consequences of horopter tilt for the processing of information from stereoscopic displays. The following results were obtained 1 For targets located 10 deg above and below fixation, in front depth or with crossed disparity, forced-choice recognition performance is 10-15 lower for the below fixation targets, when the display is physically vertical 2 The asymmetry for above- and below-targets is reversed when the display is physically tilted rearward and restored when the display is tilted forward 3 The same reversals in asymmetry occur when the display is physically upright and the fixation plane is placed in back depth and in front depth relative to the plane of the targets 4 These relationships are not substantially altered by changes in head and eye position of the observer and 5 The tilted horopter does not alter the apparent size of targets above and below fixation. Overall, the results indicate that the vetical horopter is tilted and influences recognition performance under stereoscopic conditions.