Editorial: Three‐dimensional visual space: Phenomena, theories, and applications

This special issue of Japanese Psychological Research provides a broad sampling of the various topics of research that have been conducted in the area of binocular stereopsis, which involves stereoscopic three-dimensional (S3-D) perception. Binocular stereopsis was discovered in the early 1830s, when Wheatstone invented the mirror stereoscope that was designed to present a disparate picture to each eye of a single viewer (Wade & Ono, 2012). From stereo-pairs with components that differ in the horizontal direction, a viewer can obtain a remarkable enhancement of the percept of 3-D objects and space that are depicted by the pictures. Since the discovery of binocular stereopsis from horizontal disparity, much effort in different fields of research, such as psychology, physiology, and computer sciences, has been devoted to understand the mechanism by which binocular disparities are processed (Howard & Rogers, 2002). Binocular stereopsis has attracted the attention of many researchers, not only because of the surprisingly vivid, unique, and often fantastic depth that is obtained just by fusing disparate images, each of which gives little impression of depth or even form at all, but also because one of the major functions of the visual system is to locate objects in a 3-D visual space. The role of binocular stereopsis has recently received more attention outside of the research laboratories as the marketing frenzy of S3-D visual entertainment, following the success of S3-D movies, has spread over to all sorts of visual displays and communication devices such as smart-phones, cameras, video gaming devices, and stereoscopic 3D-TV. The popularity of technologies that exploit the depth enhancement that is produced by binocular stereopsis may be traced back to the invention of Brewster's lenticular stereoscope in 1849 as well as that of Wheatstone's mirror stereoscope. The stereoscopes were very popular in the mid-19th century because they were commercialized and allowed the general public to view photographic stereo-pairs easily. Interestingly, the lenticular stereoscope and the photographic stereo-pairs were brought to Japan in 1860 (Nakazaki, 1993, p. 121) and there are records of feudal lords enjoying them during the last days of the Tokugawa shogunate (Iwashita, 1991; Nakamizo, 2003, pp. 53–54). Now, almost 150 years later, ordinary people can enjoy high-quality S3-D movies in the cinema and even at home through their own stereoscopic 3-D-TV, mainly because of major developments in digital imaging technology (Tam, Speranza, Yano, Shimono, & Ono, 2011). However, there are still many human-centered issues that need to be addressed through psychophysical research before S3-D can become truly mainstream (Tam, Speranza, & Vázquez, 2012). In this issue there are seven invited papers that cover a wide range of topics on binocular stereopsis. The first two deal with computational models of binocular stereopsis (Mitsudo, 2012; Shioiri, Matsumiya & Matsubara, 2012). Mitsudo (2012) proposes an algorithm for binocular single vision and stereopsis. He focuses attention on a process to judge whether the stereo-pairs are binocularly correlated or not. To implement such a process he suggests a framework to evaluate the reliability of horizontal disparity based on a stochastic distribution of cross-correlation coefficients of a given stereo-pair. The results obtained with his algorithm, when applied to several stereo-pairs, were consistent with those obtained with human visual perception. Shioiri, Matsumiya, and Matsubara (2012) propose a model for motion perception in depth. The model consists of two hypothesized subunits: one for processing interocular velocity differences and the other for processing changes in disparity over time. The model was based on motion-energy detection, which reflects the property of the cortical neurons that respond specifically to binocular disparity. The model was compared with psychophysical motion-perception data that were collected for stereo-pairs with manipulations of contrast and displacement, and vertical shifts between the right and left half-images. The next two papers are on experimental studies: one investigated the interaction between horizontal disparity and motion parallax (Shigemasu & Sato, 2012) and the other focused on semantic qualities of S3-D scenes (Toya & Ichikawa, 2012). Shigemasu and Sato (2012) examined how the magnitude of perceived depth is determined when horizontal disparity and motion parallax are both available as depth cues. They found that the perceived depth depends on whether there is clear contour information or not and it may also be determined by the cue that indicates a larger depth value. Toya & Ichikawa (2012) investigated the effects of horizontal disparity size and pictorial factors on semantic qualities using several stereo-pairs (from simple line stereograms with a constant disparity to picture stereograms with multiple disparities) and measured the perceived depth of the stimuli depicted by the stereo-pairs. To measure semantic qualities they employed a semantic differential method and found that semantic qualities depend on disparity size rather than pictorial factors. They also found that semantic qualities as well as perceived depth depend on the disparity distribution of the stimuli depicted by the stereo-pairs. Following the presentation of papers involving experimental studies, the next two provide a review on binocular and stereoscopic vision from antiquity to around the period of Wheatstone (Wade & Ono, 2012) and on binocular visual direction from antiquity to nowadays (Ono & Wade, 2012). Wade and Ono (2012) in their paper presented concepts, tools and observations and discussed how the invention of the stereoscopes affected subsequent research. They also claimed that what Wheatstone achieved for space perception with his stereoscope is comparable to what Newton had achieved with his prism for color vision. Ono and Wade (2012) maintained that early on there were two streams in this area of research: the optical approach and the observational approach. It is argued that initially the optical approach was widely accepted by the scientific community, but later the validity of the rules of visual direction that were developed based on the observational approach was verified experimentally and the latter “won the day.” It is highly likely that current researchers of binocular stereopsis may not pay much attention to the early history of research in this field before Wheatstone. In this sense, it is interesting for us to find out how earlier researchers had struggled to understand binocular vision and to reflect on the fact that we are their successors. The final paper describes concrete examples of how methods used for studying human visual perception have been applied to help solve human-centered problems related to stereoscopic 3-D-TV (Tam et al., 2012). In particular, it provides a review of studies that were conducted at the Communication Research Centre Canada, for example, investigating the prevalence of stereo-deficiency in a large group of people and the role of stereoscopic object motion on visual comfort. The authors concluded that studies on human visual perception are important for the development of more efficient visual communication systems. As mentioned earlier, there are ample studies on binocular stereopsis, as can be found in the Howard and Rogers (2002) book. The goal of this special issue is to bring into one convenient location a representative overview of the different types of research that have been conducted in this area. However, due to limited space, we could not include a number of areas of research in this issue. In particular, we did not include any fMRI studies, which have established that distinct cortical areas respond to different types of disparity, such as that by Parker (2007). Clinical studies were not included either. Recent clinical studies have shown that while abnormal visual experiences during a critical period may disturb the normal development of binocular stereopsis, binocular performance, including binocular stereopsis, can be recovered by means of perceptual learning (Ding & Levi, 2011), suggesting plasticity of the mechanism for binocular stereopsis. Although the topics dealt with in this issue are limited, we hope that the wide-ranging set of research areas and issues that have been covered are more than sufficient in helping readers who are interested in binocular stereopsis obtain a good taste and understanding of what have been undertaken in the past, as well as what is undergoing in the present. Importantly, we hope that the studies presented in this special issue will equip readers with a broader perspective and help stimulate them to carry research on binocular stereopsis to greater heights.

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