Outline:•  What is Stereopsis?•  Kinds of Stereograms•  Brief History- da Vinci, Wheatstone, Brewster,

etc•  Stimuli used•  some fundamental data

What is Stereopsis?Stereopsis- n. The perception of depth based on

horizontal, relative, retinal disparity.Retinal Disparity- n.The difference in position of an image on

the left and right retinas, measured in visual angle units of degrees, minutes, or seconds of arc.

Stereoscopic Stimuli in Vision ScienceThree common kinds of stimuli used for measuring stereopsis: 1) dots & lines (�local� targets) �

First experiments used these, still used for basic stereo testing2) random dot stereograms �

Tests �global stereopsis.� �Used in some clinical tests �and for making �Magic Eye� �pictures.

3) sine wave gratings �

Used for testing stereopsis with fine, medium, �and course scale patterns

Retinal disparity is a difference of angles

Perception of Depth•  The perception of depth depends on the

combination of several factors:– Retinal Disparity– Motion Parallax– Shading cues– Interposition cues (overlap of objects)– Size / Perspective – Blur

Under the best circumstances, the smallest detectable disparity is about 3 arc seconds.�

�How much depth is 3 arc sec? It depends on

viewing distance!

•  Converged at 6 meters, 3 arc sec = 1 cm –  Thickness of a finger

•  Converged at 0.6 meters (arms length)3 arc sec = 0.1 millimeters–  Ridges in your fingerprint

The 3-D �fovea� for stereoacuity

From Tyler chapter in Vergence Eye Movements Schor & Ciuffreda eds. (1983)

Stereoacuity values are lowest at the point of fixation. ��Targets that are in the periphery show higher thresholds.��Targets that are closer or farther from the plane of fixation�show higher thresholds.

History•  Galen (AD 200) Alhazen (AD 1000), Leonardo da Vinci

(1500), Aguilonius (1600), all wrote about binocular parallax and double vision. Leonardo knew that binocular parallax contributed to depth, and Aguilonius worked out the geometric horopter. But none of these thinkers understood depth from disparity.

•  Wheatstone (1802-1875) invented the mirror and the lenticular stereoscopes in 1832. His device allowed him to experiment systematically with image differences and to firmly establish disparity as a basis for depth perception. We credit Wheatstone with �discovering� stereopsis.

•  Brewster (1781-1868) re-invented lenticular stereoscope in 1849, claimed credit, and fought with Wheatstone for years.

•  1860�s to 1890�s 3-D photography craze takes over world!

Early Stereoscopes

LeftWheatstone Mirror Stereoscope (1838) �or �mirror haploscope.� Images are placed on left and right sides and fused through mirrors.RightBrewster Stereoscope. Images are placed side by side and fused through combined lenses/prisms. Millions of people owned these in the late 1800s.

Stereo photos

3D viewing

Early 3D movies used red/green (anaglyph) glasses.

Modern 3D movies use polarizing glasses. Don’t tilt your head!

3D television uses special shutter glasses that flicker to show frames alternately to left and right eyes.

The future is Virtual Reality, with a separate image for each eye.

How Stereograms are made and viewed•  1) photograph or construct two pictures from different viewpoints•  2) present them separately to the two eyes

Stereoscope – uses mirrors or lenses to aim eyes at two different pictures.

anaglyph (red/green glasses) – good for demospolaroid glasses – used in 3D moviesfree fusion – Just cross your eyes to blend two side by side images.phase haploscope – used in 3D television. Left and right eye images

alternate on screen. Fast shutters block left and right eyes alternately in sync with display.

Virtual Reality goggles – head mounted stereoscopeAuto stereograms – used for Magic Eye pictures, requires you to cross

or uncross the eyes a little bit.

Development of stereo vision

Stereoscopic vision and the control of eye alignment develop rapidly in the first few months of life. (top left) If the eyes are not properly aligned during this critical period, binocular vision does not develop properly. The bar plot shows results for kids at about 5 years of age. (top right)Surgery or injections to straighten the eyes does not usually help. (Bottom left)

Training adult amblyopes to improve stereoacuity

Vidamurthy et al. (2016)

Subjects were all adult amblyopes or strabimics, people with limited stereo vision..They were tested for stereo vision before and after playing a virtual reality game 35 times over 8 to 11 weeks. The task was to squash a bug with a beer can.Contrast was reduced in the dominant eye to provide a better binocular balance.Stereo vision was measured with a clinical test called “randot circles”. Stereo was measured before, after, and 2 months after training.Some, but not all showed dramatic improvement in their stereopsis, and this effect was still present 2 months after training.