Explorations to Accompany Exhibit Set 3 Perception

by Paul Doherty

The great reference books for perception are:

Seeing the Light, Brill et.al. This gives an accurate picture of human vision with examples.

Eye and Brain, Richard Gregory A classic.

Perception  Irvin Rock


A great website for visual illusions has been created by Prof. Ken Brecher from Boston University. http://lite.bu.edu/

Prof. Brecher is a physicist who publishes research in color in astronomy.

He has created interactive applications using Flash10. http://lite.bu.edu/vision-flash10/applets/lite/lite/lite.html

I will point to appropriate versions of his applications in connection with the Exploratorium exhibits below.


After Image

After Image Snack A bright light flashes a pattern on the retina. This results in an afterimage which changes over time as the dark afterimage becomes a bright afterimage known as a positive afterimage. The apparent size of the afterimage appears to change for some people as they view the afterimages on a nearby paper and on a distant wall. The image is fixed in size on the retina and has a constant angular size in the field of view, but the eye-brain adjusts the perception of size based on its perception of distance.

After Image Exploration, Explore the afterimages created by a moving minimaglite. The positive afterimages change color with time.

Color Afterimages project LITE, examine the afterimage color of red green and blue.

A careful explanation of color afterimages.

Locating Rays look at a bright point of light, notice the rays, where are they?

Seeing Your Retina  Use a dim point of light in motion to see the blood supply to your retina.

Benham's disk

Benham's Disk Snack The exact reason why alternating flashes of black and white when surrounded by a white rim produce a sensation of color is not understood.

Benham's Disk  Project Lite with variable speed direction and patterns to explore.

See Slow Blue below. The response times of the three cones in the eye are different.

Bird in a Cage

Bird in a Cage Snack

Lori's Bird in a cage mathematics

Color Blindness test

Color Blindness Cards these precisely printed cards are expensive but they are good tools for studying perception. Inexpensive student colorblindness tests are available here www.colorblind.to  for about $10. 8% of men are red-green colorblind.

Ishihara Colorblindness test RG online Project Lite

Color Contrast Our perception of color depends on the surrounding colors.

Gray Step The retina enhances edge perception. One part of the retina is connected to adjacent parts and can affect the response of adjacent parts. This  is called lateral inhibition. This Gray step from project Lite is adjustable.

Lateral Inhibition A model of how the retina might do lateral inhibition, this is called the DOG model difference of gaussians. The images show how you see the illusion as well as the results of applying the difference of gaussian model to the image.

Background color affects perception  Project Lite, Explore the perception of color as you change the background color.

The Land Effect compare banana colors using a gray mask try 50% red.

Color Reversal A bright backlit green tree appears against a red background. The tree may be observed through a hole in a rotating disk. When the disk rotates one way the image of the tree is followed by a white region. When it rotates the other way the image is followed by a black region. When a part of the retina is exposed to bright green light it adapts so that if it is followed by a white region the green cone responds less than the blue and red cones and a magenta afterimage is seen. If the white region persists longer than the hole-allowed view of the green tree then the overall color of the tree will switch from green to magenta. This does not happen when the tree image is followed by a black region.

Color reversal  from Project Lite, on a computer screen.

Green Lips The red green and blue cones in our eyes combine their signals to produce the perception of color. A full color image can be created by projecting three different images one in shades of red, one in green and one in blue. Interesting changes in the image occurs when the colors are switched so that green light illuminates the red pattern.

Colored Shadows Red, green and blue lights are separated and cast colorful shadows.

Additive Color Mixing  an Exploratorium online exhibit mix R G B to match a random color.


Macula Exhibit Exploration Draw the shape of the image you see at the macula exhibit. Some people do not see an image at all. The key scientific idea here is to be honest and report what you see. Just because a large number of people see something does not mean that you do. Honesty is the soul of science.

Macula Project Lite 

Long Path Diffraction used to create laser speckle. This exhibit produces a bright laser speckle pattern. When the observer moves the pattern moves. How the pattern moves is a function of the eye of the observer so different observers see different things. Even a right eye and a left eye of the same person may give a different answer.

Peripheral Vision

Peripheral Vision Snack  You only see sharply enough to read within a small angle just a few degrees of your center of view. In the center, the macula, you have only cones which require bright light. That's why you need to look to the side of a dim star to see it. This is known as averted vision. At large angles away from the macula you have fewer and fewer cones so you lose color perception at large angles. You can begin to detect objects in motion beynd 90 derees.

Rainbow Edges A simple lens like that in the eye has different focal lengths for different colors of light. This is called chromatic aberration. It is particularly apparent when looking at a white disk surrounded by a black background.

Slow Blue The response time of the three cones to light is different. The blue cone responds to a light stimulus more slowly than the green or red cones. This can lead to a misperception in the location of a moving colored dot.

Slow Blue Project Lite, shake the three colors back and forth and notice if you perceive a lag in the motion.

The differing response times of the cones also play a role in the colors perceived in Benham's disk.


Scientific Explorations with Paul Doherty

© 2010

13 Sep 2010