The perceived relative direction of a pair of features can be intermediate between their relative direction in the two eyes. It is known that the perceived relative direction of the features can be altered by making their disparity gradient steep. Features at the edge between two surfaces at different depths form steep disparity gradients. These features are typically next to features on the same surface with the same disparity. These additional features were found to mitigate the effect of steep disparity gradients on perceived direction. The effect of the additional features was found to vary with their contrast, their contrast polarity and their separation relative to the features that define a steep disparity gradient. Hence, these stimulus properties determine whether disparity gradients affect the perceived direction of features at surface edges. That the perceived relative direction of a pair of features can be an average of the angles that they subtend at each eye supports the theory that perceived visual directions are referred to a point midway between the eyes (cyclopean eye). However, from this reference point, an occluding surface would hide features on the background that are visible to the left eye next to its left edge or visible to the right eye next to its right edge. Ono et. al. (2002) proposed that the perceived direction of these features is shifted laterally from their true direction when the occluder is fixated and the occluder is shifted in the opposite direction when the background is fixated. Erkelens et. al. (1996) proposed that the perceived direction of features is referred to the eye that views the monocular region instead of a cyclopean eye so that both surfaces may be perceived in their true direction. In the present study, two occluders were arranged so that a left side edge was physically aligned with a right side edge. The monocularly visible background texture next to these edges was also physically aligned. Consistent with the first proposal, the monocular regions or the edges of the occluders were found to appear laterally offset from one another indicating that their perceived direction was incorrect. Erkelens et. al. (1996) found that the perceived direction of the left and right edges of an occluding surface relative to a line on the background was found to be more strongly influenced by their position in the eye that views the monocular region of the background next to the edge than by their position in the other eye. In the present study, this Erkelens-bias was found with fixation on the background surface, but was not found with fixation on the occluder, which is also consistent with the first proposal. The Erkelens-bias decreased with the lateral separation of a target line on the occluder from its edge. The Erkelens-bias was found even when all texture on the surfaces was eliminated except for the outline of a foreground surface and a single line on the background surface. This indicates that the Erkelens-bias does not require the presence of either steep disparity gradients or monocular texture. The Erkelens-bias was eliminated by presenting the outline as the farthest feature in the stimulus, which is incompatible with it specifying the edges of a foreground surface.
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