Stereopsis is the process in visual perception leading to the sensation of depth from the two slightly different projections of the world onto the retinas of the two eyes. The differences in the two retinal images are called horizontal disparity, retinal disparity, or binocular disparity. The differences arise from the eyes’ different positions in the head.

The effect appears to be processed in the visual cortex in binocular cells having receptive fields in different horizontal positions in the two eyes. Such a cell is active only when its preferred stimulus is in the correct position in the left eye and in the correct position in the right eye, making it a disparity detector.

Because each eye is in a different horizontal position, each has a slightly different perspective on a scene yielding different retinal images. Normally two images are not observed, but rather a single view of the scene, a phenomenon known as singleness of vision. Nevertheless, stereopsis is possible with double vision. 

Stereopsis was first described by Charles Wheatstone in 1838. He recognized that because each eye views the visual world from slightly different horizontal positions, each eye’s image differs from the other. Objects at different distances from the eyes project images in the two eyes that differ in their horizontal positions, giving the depth cue of horizontal disparity, also known as retinal disparity and as binocular disparity. Wheatstone showed that this was an effective depth cue by creating the illusion of depth from flat pictures that differed only in horizontal disparity. To display his pictures separately to the two eyes, Wheatstone invented the stereoscope.

Leonardo da Vinci had also realized that objects at different distances from the eyes project images in the two eyes that differ in their horizontal positions, but had concluded only that this made it impossible for a painter to portray a realistic depiction of the depth in a scene from a single canvas.

Stereopsis became popular during Victorian times with the invention of the prism stereoscope by David Brewster. This, combined with photography, meant that tens of thousands of stereograms were produced.

In the 1960s, Horace Barlow, Colin Blakemore, and Jack Pettigrew found neurons in the cat visual cortex that had their receptive fields in different horizontal positions in the two eyes. This established the neural basis for stereopsis. Their findings were disputed by David Hubel and Torsten Wiesel, although they eventually conceded when they found similar neurons in the monkey visual cortex. In the 1980s, Gian Poggio and others found neurons in the monkey brain that responded to the depth of random dot stereograms.

In the 1970s, Christopher Tyler invented autostereograms, random-dot stereograms that can be viewed without a stereoscope. This led to the popular Magic Eye pictures.

The Muller-Lyer illusion is an optical illusion consisting of nothing more than an arrow. It consists of two arrow like figures, one with both ends pointing in, and the other with both ends pointing out. When asked to judge the lengths of the two lines, which are equal, viewers will typically claim that the line with inward pointing arrows is longer.

One possible explanation is that one sees the lines as three dimensional, such as the outgoing and ingoing corners of a room. Another possible explanation is that the line with arrows pointing inwards may simply appear longer because the arrows themselves extend past the line.

The illusion is not cross cultural. Non western subjects, and particularly subjects whose day to day surroundings are usually not rectangular, as with few buildings, doors, walls, are much less likely to be affected by it. Researchers discovered that the Zulu people, whose typical dwellings are circular thatched huts with no angular walls, were rarely susceptible to the illusion.

One possible explanation states that the Muller-Lyer illusion occurs because the visual system processes that judge depth and distance assume in general that the angles in configuration corresponds to an object which is closer, and the angles out configuration corresponds to an object which is far away. Basically, there seems to be a simple heuristic that takes those configurations as 90 degree angles. This heuristic speeds up the interpretation process, but gives rise to many optical illusions in unusual scenes.

Neural nets in the visual system of human beings learn how to make a very efficient interpretation of 3D scenes. That is why, when somebody goes away from us, we do not see him getting shorter. And when we stretch one arm and look at the two hands we do not see one hand smaller than the other. We should not forget that, as visual illusions show us quite clearly, what we see is an image created in our brain. Our brain projects the image of the smaller hand to its correct distance in our internal 3D model. This is what is called the size constancy mechanism.

In the Muller-Lyer illusion, the visual system detects the depth cues, which are usually associated with 3D scenes, and incorrectly decides it is a 3D drawing. Then the size constancy mechanism makes us see an erroneous length of the object which, for a true perspective drawing, would be further away.

In athletics, the four minute mile is the running of a mile, or 5280 feet, in less than four minutes. It was first achieved in 1954 by Roger Bannister. The four minute barrier has since been broken by many male athletes, and is now the standard of all professional middle distance runners. In the last 50 years the mile record has been lowered by almost 17 seconds.

When Bannister crossed the finish line of Oxford’s Iffley Road track on May 6, 1954, he could hardly see straight. Completing the mile in 3 minutes, 59.4 seconds, he had not only trimmed two seconds off the world record, but also run the world’s first sub four minute mile.

“People thought it was like bouncing off a brick wall,” explains close rival John Landy, who had come within three seconds of the four minute mark six times.

“It was a sense of relief,” said Bannister, recalling the momentous event more than 50 years later. “There was a mystique, a belief that it couldn’t be done, but I think it was more of a psychological barrier than a physical barrier.”

Landy, who broke Bannister’s record with a 3 minute 58 second finish only six weeks later, argues otherwise. “It has nothing to do with psychology,” he says. “It was just a matter of having the right runners at the right level of training and the right set of circumstances.”

John Walker, a distance runner from New Zealand, managed to run 129 sub four minute miles during his career, during which he was the first person to run over 100 sub four minute miles, and American Steve Scott has run the most sub four minute miles, with 136. Currently, the mile record is held by the Moroccan Hicham El Guerrouj, who set a time of 3 minutes 43.13 seconds in Rome in 1999.

Another illustration of the progression of performance in the men’s mile is that in 1994, forty years after Bannister’s breaking of the barrier, the Irish runner Eamonn Coghlan became the first man over age 40 to run a sub four minute mile.

No woman has yet run a four minute mile. The current women’s record holder is retired Russian Svetlana Masterkova, with a time of 4 minutes 12.56 seconds.

In 1997, Daniel Komen of Kenya ran two miles in less than eight minutes, doubling up on Bannister’s accomplishment.