Consider what happens when you fixate on an image, such as a letter—the X on this page—with both eyes. Images of the letter project to the central part of each retina (the fovea), and the brain fuses them into one. You see one X, not two. English physiologist Charles Sherrington suggested in the early 20th century that this blending was a mysterious psychological process occurring in the mind, requiring no actual confluence of messages into a single brain area. We must not confuse mental fusion with physiological fusion, he urged.
We now know he was wrong: binocular fusion is a physiological process. The X, or any point on which you fixate, falls on what is, functionally and geometrically, termed corresponding retinal points. In fact, any point from an entire plane (or, strictly speaking, from a slightly curved surface), centered on the fixation point, would stimulate corresponding retinal points and be seen as a single object (any letter on this page, not just the one on which you fixate, appears singly). As neuroscientists David Hubel and Torsten Wiesel of Harvard University discovered in a series of groundbreaking experiments in the 1960s, individual cells in the visual cortex, so-called binocular cells, receive input from both eyes, specifically from corresponding retinal locations, thus providing a mechanism for perceptual fusion.
Yet if binocular neurons were only excited when identical input arrived from both eyes, you would have trouble perceiving real 3-D objects. John “Jack” Pettigrew, then a young medical student in Canberra, Australia, noted this fact in the mid 1960s, reasoning that the neural mechanism for stereopsis must entail another set of binocular neurons, ones that signal retinal disparity by processing noncorresponding retinal points.
What Pettigrew (along with his colleagues Horace B. Barlow, Colin Blakemore and Peter Bishop) found was that Hubel and Wiesel’s description was only partially correct. Sure enough, corresponding points from the retinas send signals that converge on single neurons in the visual cortex. It is as if there is a map of each eye’s image in the brain, and these maps are in registration (speaking anatomically); that arrangement makes sense overall. But many noncorresponding points also converge on and activate binocular cells. It is these neurons that signal stereo depth because they are, in effect, measuring the horizontal scatter between the left and right eye images. As a consequence, what you have even at this early stage is not a flat 2-D map of the world on the cortex but a 3-D map. This fact was probably the most important discovery about binocular vision since Wheatstone’s insight.
Of course, we have progressed much since Wheatstone’s days. Instead of drawings, we can mimic the two eyes’ views using a camera. Look at any 3-D scene and take one picture from the left eye’s vantage point. Then shift the camera to the right eye’s location and take a second picture. Print the two photographs, place a vertical partition so that each eye gets only its own image and, lo and behold, the image transforms into a 3-D scene. (See the example at a.) Such stereograms were highly popular in Victorian drawing rooms (they were carefully stashed away if they were pornographic, proudly passed around at family gatherings if they were travel series).
The best way to view them is through a stereoscope, which incorporates lenses and prisms or mirrors for more natural accommodation and convergence. But you can try the rudimentary partition method just discussed. With some practice, you can get the eyes aligned to fuse the images and see stereo depth. It is well worth the effort.
Another stereo illusion you can construct and experience is the Pulfrich effect, described, ironically, by the famous one-eyed scientist Carl Pulfrich in 1922 (experimenting on others, of course). Hang a weight on the end of an 18-inch string and set it in motion like a pendulum, moving back and forth horizontally in a single plane (its speed gradually accelerates as it approaches the center and decelerates again as it reaches the top at the other end). Now put a filter (sunglasses will do) in front of one eye alone. Astonishingly, you will see the pendulum making an elliptical 3-D excursion toward and away from you! With a left eye filter, motion will be clockwise, as seen from above; counterclockwise with a right eye filter. And the darker your glasses, the greater the depth of the ellipse you will see. Remove the filter, and it goes back to the 2-D swing of a regular pendulum.