Reverse perspective

Abstract: Binocular Depth Inversion Sometimes a solid object seen with both eyes can seem to reverse perspective. A study of this geometrically irrational experience suggests that ordinary depth perception is somewhat precarious by John A Visitor to the Haunted Mansion at Disneyland in California sees among other things a pair of hu­ man faces that appear to rotate in a mys­ terious and sinister way as he walks by. They are in fact inside-out relief masks, but because of the lighting and the way they are mounted the visitor unwittingly reverses their depth, perceiving them in­ correctly as normal faces. This uncon­ scious reversal of perspective gives rise to the apparent rotation. Besides mystifying visitors to the Haunted Mansion this illusion pre­ sents a problem for theories on the per­ ception of visual form. The problem is not the apparent rotation of the fac­ es; psychologists have known for some time that whenever a three-dimensional object is perceived in reverse perspec­ tive, it will seem to rotate as the observ­ er's head moves. What is puzzling is the perspective reversal itself. Ordinarily people see the three-dimensional forms of things correctly, and reversals of per­ spective occur only in special circum­ stances that deny the brain its normal visual cues to depth. One such circum­ stance is viewing the three-dimensional form with one eye closed, so that the depth cues provided by binocular vision are eliminated. The masks at Disney­ land, however, show that sometimes ob­ jects are routinely perceived inside out in spite of the availability of all the nor­ mal depth cues, including those due to binocular vision. How can such a ma­ jor perceptual mistake occur? And giv­ en that it does happen sometimes, why is it so rare? In this article I draw a distinction be­ tween the kind of perspective reversals produced by ambiguous pictures such as the famous Necker cube and those experienced in viewing actual three­ dimensional objects, such as the masks in the Haunted Mansion. I shall refer to the latter type of illusion as depth inversion. Reversible-perspective pic­ tures and their perceptual consequences are quite well known; drawings such as the Necker cube have illustrated countc less psychology textbooks since the 19th r. Yellott,Jr. century, and artists have explored the same theme for much longer. Depth inversion of solid objects also has a long scientific history. References to the phenomenon date from the 18th century, and in the 19th century it was studied by such notable figures as Her­ mann von Helmholtz and Ernst Mach. In this century, however, it seems to have been neglected until 1970, when the British psychologist Richard L. Gregory drew attention to it again in his book The Intelligent Eye. Gregory's dis­ cussion stimulated my interest and led me to devise several experiments to de­ termine whether objects can be seen in reverse perspective when the brain is truly in full possession of all the depth information available in normal vision, including in particular the information provided by binocular vision, which according to classical accounts should make the illusion impossible. The re­ sults of these experiments indicate that under appropriate conditions the brain is prepared to override all its sensory cues to depth and create an inside-out visual world that defies geometrical analysis but nonetheless seems just as realistic as normal visual experience. T hese experiments on binocular depth inversion are the subject of this arti­ cle, but to put them in context it will be helpful to first consider monocular in­ version, which is much easier to explain. Depth inversion of a three-dimensional object viewed with one eye can be un­ derstood if one thinks of visual experi­ ence as the outcome of a process in which the brain tests hypotheses about the three-dimensional shapes of objects against the evidence provided by their retinal images. With one eye alone the only potential source of unequivocal in- formation about depth is accommoda­ tion, or change of focus, and the brain normally gives this cue little or no weight in its judgment of distance. Ac­ commodation therefore presents no bar­ rier to the acceptance of an inside-out shape as being real. All the other mon­ ocular cues to depth are intrinsically ambiguous. The evidence they provide cannot exclude inside-out hypotheses, although they can render such hypothe­ ses statistically unlikely in the sense that, say, a tree rotating in synchrony with movements of the head is an improbable object. Apparently this is normally enough to enable the brain to guess correcrtly about the shapes of things seen monocularly. If sensory evidence becomes sufficiently impoverished, however, the brain may accept an inside-out hypothesis that is compatible with the retinal image. In such a case visual experience is totally transformed to agree with the hypothe­ sis, intellectual knowledge of the correct form notwithstanding. Yet the inverted object now seen still incorporates all the information available on the retina, just as in normal vision. The only difference is that now every depth cue is visually reinterpreted in order to agree with a false premise. Now consider the situation in binoc­ ular vision. The key to my explanation of monocular inversion (an explanation borrowed from Helmholtz, Mach and Gregory) is the fact that all the monocu­ lar cues to depth can be consistently rec­ onciled with an inverted-object hypoth­ esis. When both eyes view an object, however, no such reconciliation is possi­ ble. Binocular vision provides depth in­ formation that is geometrically incom­ patible with depth inversion, in other words information that should enable INSIDE-OUT FACE, made as the mold of a bust, is showu iu side and front views on the oppo­ site page. Looked at from the front it is more easily seen as a normal face because the brain overrides th'e depth cues that suggest an object as improbable as an inside-out face. (The rever­ sal is made easier when, as in the front view here, the lighting eliminates shadows that might aid the brain in making the correct interpretation.) A three-dimensional inside-out face seen in reversed perspective seems to rotate and to follow an observer who is moving laterally past it. © 1981 SCIENTIFIC AMERICAN, INC

Abstract: Trompe l'oeil pictures have been produced for hundreds of years. They attempt to create the impression of a surface that has different three-dimensional structure to the work; successful examples of trompe l'oeil typically constrain the observer's viewpoint and require use of a single eye. The works of Patrick Hughes are in relief but are painted to appear like conventional flat pictures; those parts that protrude from the picture plane are pictorially distant, or in reverse perspective. Movements of the observer result in fluid distortions of the pictorial image. These distortions occur with binocular observation and over a wide range of viewing distances.

Abstract: Contents: Preface Introduction The role of time in the pictorial art On reverse perspective a " a critical reading Registering presence in the icon 'Seeing the world with the eyes of God': an alternative explanation of 'reverse perspective' Conclusion Sample analysis: an analysis of Rublev's Trinity icon in the terms proposed in this book Glossary of terms Bibliography Index.

Abstract: The English artist Patrick Hughes has created an extraordinary class of painted artpieces, most commonly referred to as 'reverspectives'. They consist of truncated pyramids and prisms with their smaller faces closer to the viewer, in such a way as to allow a realistic scene to be painted on them. The works of art contain rich perspective and other painted cues that conspire to elicit an illusory depth percept that is the reverse of the physical depth arrangement. This reverse depth is obtained under a wide range of viewing conditions, and competes with the veridical depth percept in a classical bistable paradigm that was found to exhibit a high degree of hysteresis. Under the illusory depth percept, reverspectives appear to move vividly as the viewer moves in front of them. This paper reports two experiments that were designed to assess the effectiveness of the painted cues in eliciting the illusory depth percept by using three different measures for the strength of the illusion. As expected, the illusion was favored by monocular viewing and large viewing distances. The results from these two experiments are in close agreement with each other, and they indicate that the painted cues are powerful in influencing the ultimate percept.