Ocular Equivocation: The Rivalry Between Wheatstone and Brewster

Binocular portraiture

Pictorial portraits are viewed with two eyes despite the fact that they are mostly monocular: they have been produced from a single viewpoint (either by painters or photographers). The differences between the images on each eye are a consequence of the separation between them rather than differences in two pictorial images. Viewing with two eyes detracts from the monocular cues to depth within the singular portrait because of information for the flatness of the pictorial surface. Binocular portraits, on the other hand, incorporate differences between two pictorial images producing perceptual effects that cannot be seen by a single eye alone. The differences can consist of small disparities that yield stereoscopic depth or large ones that produce binocular rivalry. Binocular portraits require viewing with a stereoscope, many varieties of which exist. Those shown here are anaglyphs which can be observed through red/cyan filters. They are not conventional stereoscopic portraits where the sitter is imaged from two slightly different locations. Rather, the binocular processes of cooperation (stereoscopic depth perception) and competition (binocular rivalry) are manipulated in the binocular portraits. The subjects shown in the anaglyphic portraits have been involved in the science and art of binocular vision.

History of Virtual Reality and Augmented Reality in Neurosurgical Training

Virtual reality (VR) and augmented reality (AR) are rapidly growing technologies. Both have been applied within neurosurgery for presurgical planning and intraoperative navigation, but VR and AR technology is particularly promising for the education of neurosurgical trainees. With the increasing demand for high impact yet efficient educational strategies, VR- and AR-based simulators allow neurosurgical residents to practice technical skills in a low-risk setting. Initial studies have confirmed that such simulators increase trainees' confidence, improve their understanding of operative anatomy, and enhance surgical techniques. Knowledge of the history and conceptual underpinnings of these technologies is useful to understand their current and future applications towards neurosurgical training. The technological precursors for VR and AR were introduced as early as the 1800s, and draw from the fields of entertainment, flight simulation, and education. However, computer software and processing speeds are needed to develop widespread VR- and AR-based surgical simulators, which have only been developed within the last 15 years. During that time, several devices had become rapidly adopted by neurosurgeons, and some programs had begun to incorporate them into the residency curriculum. With ever-improving technology, VR and AR are promising additions to a multi-modal training program, enabling neurosurgical residents to maximize their efforts in preparation for the operating room. In this review, we outline the historical development of the VR and AR systems that are used in neurosurgical training and discuss representative examples of the current technology.

Alfred Wilhelm Volkmann on stereoscopic vision

Wheatstone's demonstration of binocular single vision and depth with stimulation of non-corresponding points reverberated throughout mid-nineteenth century German visual science. It challenged the received view that single vision was a consequence of retinal correspondence otherwise objects were seen double. Wheatstone also argued that stimulation of corresponding points could yield double vision. He interpreted his experimental observations in psychological rather than physiological terms, as did Helmholtz later. Volkmann addressed both of these challenges in a long article on stereoscopic vision published in 1859. While he accepted the first of the questions Wheatstone posed Volkmann was more cautious with regard to the second. Volkmann was an experimentalist who applied psychophysical methods to determine thresholds for stereoscopic depth perception. In line with many of his colleagues in Germany, he took issue implicitly with Wheatstone's approach: how can the detailed quantitative experiments supporting Vieth and Müller's interpretation of binocular single vision be derailed by simple observations with a stereoscope? Unlike most of his colleagues, Volkmann was swayed in Wheatstone's favor through his own experiments: both physiological and psychological processes are involved in stereoscopic depth perception.

On the Art of Binocular Rivalry

Binocular rivalry has a longer descriptive history than stereoscopic depth perception both of which were transformed by Wheatstone's invention of the stereoscope. Thereafter, artistic interest in binocular vision has been largely confined to stereopsis. A brief survey of research on binocular contour rivalry is followed by anaglyphic examples of its expression as art. Rivalling patterns can be photographs, graphics, and combinations of them. In addition, illustrations of binocular lustre and interactions between rivalry and stereopsis are presented, as are rivalling portraits of some pioneers of the science and art of binocular vision. The question of why a dynamic process like binocular rivalry has been neglected in visual art is addressed.

Helmholtz at 200

Hermann von Helmholtz was born 200 years ago, but his influence on vision research is enduring. His legacy in vision is celebrated visually with anaglyphs that combine portraits of him with illustrations from his publications. Emphasis is directed principally to his Treatise on Physiological Optics. Among the optical instruments Helmholtz invented were the ophthalmoscope, ophthalmometer, and telestereoscope. Mention is made of his investigations into accommodation, colour vision, eye movements, stereoscopic vision, binocular rivalry, and lustre. Helmholtz also presented his analyses of vision and art in several of his Popular Lectures.

On the Origins of Terms in Binocular Vision

Vision with two eyes has been commented upon for many centuries, and the principal concern has been with binocular single vision. The terminology we apply to binocular vision developed rapidly after the invention of the stereoscope in the early 19th century. The origins of terms such as anaglyph, binocular lustre, chromatic stereoscope, cyclopean eye, dichoptic, horopter, pseudoscope, rivalry, stereoscope, stereograph, and stereopsis are described together with portraits of those who introduced them.