7.8 Now you see it, now you don't

3D-illusion is a fun subject. It is enjoyed by people with binocular vision, i.e., the two eyes must work together and each with a good central vision. The foundation is stereopsis which can be exploited for 3D illusion. And, based on preliminary functional MRI, stereopsis seems to be coded in the cortex adjacent to the intraparietal sulcus. Not surprisingly, in patients with amblyopia, there is a decreased activation in V1, the primary visual cortex.The above image shows a stereopsis testing kit, known as the Stereo Fly - found in all eye doctors' offices. Children are asked to put on the polarizer spectacles, look at the huge scary fly on the right, and pinch its wings. On the left panel are 9 sets of buttons and 3 rows of cartoons, both of which with decreasing depths. The children are asked to push the buttons/cartoons from the most to the least obvious. Stereopsis can thus be quantified. This test is based on retinal disparity. In real life, the two eyes look at a 3D object, each from a different angle (as part of convergence), and the images are then reconstructed in the brain and perceived as 3D. The Stereo Fly is in 2D, the 3D effect is really a simulation of the retinal disparity.

There are many other ways of appreciating pseudo-3D images, with (e.g., the red-green 3D glasses at IMAX theaters, or the polarizers shown above), and without equipment (by converging or diverging the eyes to fuse the images). The basic principle is the same: one eye sees (or is allowed to see) one image of an object taken from one angle and the fellow eye, another image taken from a slightly different angle. The two images then fuse together to produce 3D. This angle is determined by the distance between the two eyes, known as the interpupillary distance or PD - typically around 65mm in adults.

In addition to superimposing two images as in the Stereo Fly above, there are different ways of generating stereograms. The example below is a computer-generated single-image random-dot stereogram, in which, a semi-sphere can be seen by simply diverging your eyes (just stare at it, the eyes/brain will take care of the rest):

(From wwwchem.uwimona.edu.jm:1104/gifs/caff3d.gif)

A more decorative, copy-righted version is known as the Magic Eye. Actually, a very simple way of making stereograms is to photograph the same object twice each at a slightly different angle to simulate the viewing position of the two eyes. Then lined up the two pictures horizontally, and by converging the eyes or using a stereoscope, the 3D effect becomes apparent. 3D cameras also have been in existence since the 1960s.

In humans, both stereopsis and motion parallax co-exist. The advantage of stereopsis is depth perception especially at close range if monocular cues are ambiguous. At a distance, parallax dominates, e.g., a house behind a tree moves in the same direction of your motion and the tree in the opposite direction. To the brain, this clue indicates the house is farther away than the tree from you. So by relying on parallax, the distance judgment of an amblyope or a one-eyed person is not impaired at all.

Optical illusion, on the other hand, can be appreciated monocularly. And based on fMRI, Area MT (medial temporal) in the brain seems responsible. An example of optical illusion is shown in the image below, squares A and B actually have the same shade. You can print it out, cut out A and B and compare them side by side.

(From: http://wapedia.mobi/en/Image:Grey_square_optical_illusion.PNG)

If you don't want to bother printing anything, look at the above diagram, the cords all have the same length, yet the one on the left appears shorter. There are simply too many varieties of optical illusions to mention here. Luckily, most are now available on the Net.

Optical illusion is based on the brain's interpretation drawing upon past visual experiences. Indeed Area MT is associated with recognition of the spatial localization of one's self. You might say that MT imposes its will on the eyes - the true origin of optical illusion.

The above are two more unique aspects of higher-order vision that make our visual world vibrant and exciting. Of course, you should also remember: "Seeing is believing" is not without limits.