We live in a 3D world, and we perceive a 3D world. However, how we actually achieve 3D vision is an intriguing psychological question. After all, we only have a 2D retina, so where does the extra dimension come from?

The most common answer to this question is to do with the fact that we have two eyes. Having two eyes means we get two slightly different views of the world. This can be seen by simply closing one eye after the other and switching views between them. You can see that objects in the world appear in different positions relative to each other. This is called “binocular disparity” and is the basis of depth perception. If you notice, objects that are nearer to you seem to shift position in your visual more than those in the background do. The brain takes the information from both eyes and combines it to interpret these disparities and give us a sense of depth. This method of perceiving 3-dimensions is known as “stereopsis” or “stereoscopic vision”. Charles Wheatstone demonstrated stereoscopic vision experimentally in the 1800s, with his invention of the stereoscope, now often used at fun fairs and similar places. Wheatstone took two photographs of the same scene using cameras whose lenses were a few centimetres apart. This produced two pictures corresponding to the images formed on the retinas of each eye. Wheatstone’s stereoscope consists of a peephole for each eye through which the pictures can be seen – the “left picture” through the left peephole, and the “right picture” through the right peephole. An observer will see a 3D image when looking through the stereoscope. This demonstrates that disparity produces a sense of depth.

However, people with only one functioning eye are still able to perceive a 3D world. Such people are obviously not using stereopsis. Also, not everyone develops stereoscopic vision. Children with a squint - or strabismus – (a slight misalignment of the eyes) do not develop stereopsis unless their condition is corrected very early in life, yet they can perceive depth too. There must be other ways to perceive in 3D than just stereopsis.

Hermann von Helmholtz proposed that we use a series of “cues and clues” to perceive the 3D world. The cues and clues he identified were:

Occlusion: If an object is occluded, or covered, by another object in our visual field, then we infer that that object is behind the other. We need to discern that the two objects are separate. Ideas of how this is done is related to Gestalt psychology.

Shading/shadow: The shadows objects cast on each other gives us an idea of their relative positions, given that we know where the light source is. This light source has been the Sun throughout our evolution, so there are effects where we find that the brain automatically makes the assuption that the light source is coming from above. For instance, if we look at pictures of craters on the moon, we see them as either craters or mounds depending on which way up we hold the picture. The confusion arises due to the fact that the pattern of shadow on a crater being lit from above is the same as mounds being lit from below. Since the brain assumes light is coming from above, we see mounds lit from below as craters lit from above, and craters lit from below as mounds lit from above.

Geometric perspective: This relies on the simple principle that parallel lines appear to converge as they go into the distance. This is a good method of perceiving depth so long as we live in a “carpentered” world, with its straight lines and right angles. There are some African tribes, such as the Zulu tribe, that live in a world based around circles rather than right angles, and they do not have the ability to use perspective as a method of seeing depth. The natural, or maybe developed, inclination to use perspective in this way can be demonstrated when watching sporting events on television. Advertisers sometimes paint trapezoidal motifs on the field of play in such a way that when viewed from the camera the lines appear to from a right angled shape, which then appears to “jump out” from the ground, as if it were stood up. The Ames Room is also a good demonstration of this effect.

There are other cues and clues, including analysing the relative sizes of objects. If an object’s distance doubles, its image size on the retina halves. This can help us to gain some idea of depth, but is limited in its use as it only works with objects that we know the size of. Other humans are particularly useful yardsticks for this, as we are all a fairly uniform size, and we can relate this directly to our own size too.

It has been postulated that we use the angle between our eyes to read the distance an object is from us. When we fixate on an object, our eyes converge on it, and this angle of vergence can give a distance in accordance with trigonometric principles. It has been found that people can fairly accurately judge distance in this way for objects up to three metres away. At around this point, the angle of vergence of the eyes becomes such that they are almost looking parallel to each other.

One final method of seeing a 3D world is that of “optic flow”. This method, formulated by James Gibson, says that movements in the world provide “optic flow patterns”, which the brain reads to give us a perception of three dimensions. If we move around, we see the objects of the world moving across our visual field relative to each other. If we move side to side, perpendicular to our field of view, we see that the more distant an object is, the slower it moves across our visual field. If we move forwards or backwards, parallel to our field of view, we see that objects nearer to use move faster across our visual field. Gibson’s proposition that we use these optic flow patterns to perceive the 3D world has had much supporting evidence. Work by David Lee, using his “swinging room”, has shown that optic flow patterns affect our balance.

No one of these methods of seeing the 3D world is used alone. They all have their strengths and weaknesses. The most effective, however, is stereopsis. After all, we evolved to have two eyes for a reason. None of the methods are really able to directly represent the 3D world. All the information we use is detected on the 2D retina, therefore any sense of three dimensions we have is constructed by the brain. The brain can reconstruct the 3D world incorrectly at times (for instance with the pseudo-3D advertisements painted on sports fields, and with Wheatstone’s stereoscope). In reality, we do not really perceive the 3D world at all - we infer it.

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