Barbara Gillam, Phillip Marlow; Stereoscopic adaptation to relative perceived slant. Journal of Vision 2011;11(11):339. doi: 10.1167/11.11.339.
Studies of stereo slant adaptation have generally concluded that the slant after-effect (SAE) results from adaptation to perceived slant rather than to disparity gradients, with greater adaptation to greater perceived slant. Here we report a contrary example. We compared the SAE for a single slanted surface, as conventionally used, with adaptation to a surface with relative as well as absolute slant information. Despite measurably increasing perceived slant, the addition of relative slant much reduced the SAE. In Experiment 1, a central random dot surface (5.2°H × 7.8°W) was magnified horizontally (4%) in one eye consistent with slant around the V-axis. Frontal plane surfaces (2.6°H × 7.8°W), when present, flanked the central surface at a separation of 0.5 deg vertically. Observers fixated within a central area of the slanted central surface with or without the flanking surfaces for one minute. Then the SAE was measured by nulling the apparent slant of the central surface using a staircase procedure. The flanking surfaces were present/absent at test. The SAE was found to be significantly lower for the relative slant condition. Additionally, the presence of relative slant for the test stimulus much reduced the SAE even following adaptation to a single slanted surface. In Experiment 2 relative slant at adaptation or test likewise reduced the SAE when the central surface was slanted around the H-axis and was flanked horizontally by frontal plane surfaces. The results indicate that the SAE found for single slanted surfaces is not mainly due to adaptation of perceived slant, but in large measure due to slant normalisation, which is much reduced under conditions of relative slant at adaptation or test. The results are also inconsistent with the view that the SAE is principally due to the adaptation of disparity gradient detectors, which would still operate under conditions of relative slant.