What we see is dependent on more than the actual color & brightness!


Color Contrasts  our perception of colors is affected by the presence of adjacent colors


Brightness Adaptation  our perception of brightness is affected by more than can be

accounted for through the changing diameter of the iris and the saturation effects due to

diminished rhodopsin & iodopsin at higher light levels.


            There is something in the wiring……


Simultaneous Contrast


Lateral Brightness Adaptation


Our estimate of the brightness of a region is affected by that of the adjacent/surrounding



The presence of a white background

reduces the sensitivity into the gray region!

Mach Bands - Although constant within a band, the brightness appears to change at the edges.


(It also seems to depend on whether the region is at the fovea…)




The strength of the stimulus determines the rate of neural pulses being sent by

photoreceptors to ganglion cells. (the size/strength is not affected). The response is

essentially logarithmic:



The "magnitude" system of star brightness that was developed by astronomers over 2

thousand years ago was discovered to obey the relationship (approximately):




Neural synapses contain both excitatory and inhibitory connections. With enough

inhibitory pulses arriving from an adjacent regions, the net transmitted signal is

diminished  Lateral Inhibition.


Induced Color


The inhibitory effect apparently also works for each color receptor. A bright color can

induce inhibition of that same color in an adjacent region  Goethe's shadow experiment.


Here is an example from a web page by NASA.




Successive Contrast

The above effects to not go away instantaneously with the change in stimulus. They can

affect later perceptions!




Stare here first

Then here




The human eye's response dies out about 0.1 second after a brief flash of light. To fuse a

series of images to appear continuous requires frames be shown at about 40 frames/sec.

Run too slowly, we see individual "snapshots" as in strobe flashes.


Often, film is shot at a lower frame rate, and to compensate each frame is projected twice.

CRT displays (TV screens) will scan every other line during the first pass, then

"interleave" the rest on a second pass, covering the whole screen in 1/60 sec. (Frame

rates can differ in different countries, depending on the frequency of the current input).


However, this can lead to some interesting effects, such as where (say) in a western

movie, wagon wheels appear to turn "backwards"!


Physiology & Psychology


Physiology  many animals seem to have special "shape detectors" hard-wired into their



Psychology  form perception can "shift"



Which of the 3 faces did you see first?


Color Blindness


Most (~97%) of people are trichromats.


Some are dichromats (more males than females)  usually "red-green" blindness caused

by lack of red-sensitive comes.


True monochromats are rare


Take a color-blindness test here but beware perception shifts (or is it just me?)


And then there are other complex effects…..




Say the COLOR, NOT the WORD





A nice series of illusion of motion can also be found here.