Binocular Depth Inversion
1981; Springer Nature; Volume: 245; Issue: 1 Linguagem: Inglês
10.1038/scientificamerican0781-148
ISSN1946-7087
Autores Tópico(s)Historical Geography and Cartography
ResumoBinocular 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
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