論文

We usually interpret a trapezoidal image on our retina as a slanted rectangle rather than a frontoparallel trapezoid, because we use a statistical assumption (i.e. rectangles are more common than trapezoids), called a ‘prior’, for recovering the 3D world from ambiguous 2D images. Here we report that the shape prior for recovering 3D slant can be updated differently depending on the slant axis orientation (horizontal vs. vertical). The participants were exposed to a variety of trapezoidal images surrounded by a stereoscopic reference plane. The perspective transformation of the images was interpreted as 2D shape, rather than 3D slant because the surrounding plane enhanced disparity. We found that, after continuous exposure to such images, the participants relied less on the shape information for recovering 3D slant, suggesting the update of priors via experience (i.e., rectangles are less common than trapezoids). Importantly, the learning effect was context (slant-axis) specific although partially transferred across contexts; the training with horizontal-axis slant reduced the reliance on perspective even in vertical-axis slant estimation but not vice versa. The results suggest that context-specific training is vital to update the prior for the horizontal-axis slant, whereas it is not required to update the prior for a vertical-axis slant.

A new visual illusion in which circles placed on the checkered-pattern background appear to be polygons is reported. In this article, we first demonstrated that the apparent distortion of circles in this ‘bumpy circle illusion (BCI)’ depends on the luminance difference between the circles and the components of the background. Then, with the aim of clarifying the mechanism that causes this phenomenon, the ‘low-pass filtering theory’ the ‘segmentation theory’, the ‘corner effect theory’, and the ‘completion theory’ were investigated. As a result, the low-pass filtering theory and the completion theory were rejected because they predicted the occurrence of the illusion in some modulated BCI figures that produced no illusion. The ‘segmentation theory’, which postulated that the same mechanism as in the curvature blindness illusion produces BCI, was also rejected because the same luminance assignment as for BCI image components does not produce the curvature blindness illusion. In addition, the curvature of lines appears to deform in the curvature blindness illusion, whereas the BCI does not produce an illusion of line circles, which also shows the difference between the two phenomena. The ‘corner effect theory’ is the most promising because it correctly predicts (1) how the apparent distortion of the circles appears and (2) the presence/absence of illusion with the outline circles depending on the checkerboard luminance alteration cycles inside and outside of the circles. However, the corner effect theory can only be justified if it is assumed that the strength of the effect is different depending on whether the checkered pattern is applied to the inside or outside of the circles. Whether such asymmetry does exist and the reason why the asymmetry occurs needs further investigation.

This study investigated whether the individual difference in the observer’s interpupillary distance (IPD) could cause the individual difference in perceived depth in binocular stereopsis. The horizontal retinal disparity (HRD) is one of the most potent depth cues, but the visual system could not uniquely estimate the metric depth from HRD alone. This is because the size of HRD is determined as a function not only of the size of external depth but also of the viewing distance and IPD. Thus to perceive depth veridically from HRD, observers need to adjust the scaling gain (i.e. the magnification factor when converting HRD to perceived depth), taking their IPD into account. To investigate whether the visual system performs such gain calibration, we examined the relationship between IPD size and perceived depth when observing stereograms (i.e. the visual stimulus of fixed HRD) at a fixed viewing distance with 54 observers. The results showed a significant correlation between perceived depth and IPD. This is consistent with the prediction based on the geometric relationship between depth, HRD, IPD, and viewing distance, suggesting that the visual system does calibrate the scaling gain based on the observer’s IPD. However, the measured results did not fully agree with the predictions from the geometry. The results were discussed, focusing on the errors in the estimation of scaling distance.