As discussed in previous research,
13 there are no clear reasons for the stereoscopic anisotropy. In the luminance domain, there is a different orientation anisotropy that could help us to understand the stereoscopic anisotropy, namely, humans have more sensitivity (i.e., higher visual acuities) to horizontal and vertical orientations than to oblique orientations (the oblique effect).
47–49 One of the hypotheses has suggested an ecological component where the predominance of horizontal and vertical contours and angles in natural images could explain this oblique effect.
50–53 In general, the amount of horizontal and vertical content is higher than the oblique content, however, the authors have also found different orientation bias regarding horizontal and vertical content.
50–53 Interestingly, when using broad-band oriented stimuli, the visual sensitivity was higher for oblique than for horizontal or vertical orientations.
54 Thus, although there is an overrepresentation of visual neurons tuned to cardinal orientations,
55–57 and the responses of the primary visual cortex to oriented stimuli in humans are stronger to vertical and horizontal orientations than for oblique stimuli,
58 the hypotheses that the visual system develops to match the most predominant orientation is not clear. In other words, for simple stimuli the visual system has higher sensitivity to the most predominant orientations (e.g., horizontal and vertical), but for naturalistic stimuli, it seems that the visual system may discount the most prevalent orientations to accommodate the anisotropy present in natural scenes.
53 Regarding stereopsis, recent studies have shown that neural processes reflect statistical properties of the visual scene.
59–62 For example, the distribution of natural disparities is consistent with the disparity preferences in the macaque visual cortex,
61 or that rapid binocular eye movements are adapted to the statistics of the 3D environment.
60 The matching between neuronal processes and the statistical properties poses an important question, are these neuronal processes genetically hard-wired or depend on visual experience? There are examples that are inconsistent with learning (e.g., positions of corresponding points do not change after 1 week of exposure to distorted binocular disparities),
63 but there are also examples that show the importance of visual experience.
64,65 The results from Serrano-Pedraza et al.
13 showed that the stereoscopic anisotropy strengthens during development, suggesting that this anisotropy depends on visual experience. Thus, one potential reason for this anisotropy is that it reflects the statistical properties of the visual scene. Given that the anisotropy is found mainly at low spatial frequencies, the statistics of the 3D environment should show more power at horizontal than vertical corrugations at low spatial frequencies and similar power for horizontal and vertical corrugations at high spatial frequencies. So far, no evidence about this difference has been shown. The fact that disparity thresholds for horizontal corrugations increase with aging and that disparity thresholds for vertical corrugations do not change, cannot be explained by the statistics of the 3D environment. Our new results show that the anisotropy is reduced by aging, but only because the sensitivity to horizontal depth corrugations is affected.