In 1894, toymaker Mr. C.E. Benham discovered that a spinning disk with a
particular pattern of black and white marks could cause people to see
colors. Mr. Benham called his disk an "Artificial Spectrum Top" and sold
it through Messrs. Newton and Co. Benham's Top (or Benham's Disk) has
puzzled scientists for over 100 years.
What's Happening? What Causes the Colors?
The
retina of the eye is composed of two types
of receptors sensitive to light: cones and rods. Cones are important for
color vision and for seeing in bright light. There are three types of
cones, each of which is most sensitive to a particular wavelength of
light. Rods are important for seeing in low light.
It is possible that the colors seen in spinning Benham disks are the
result of changes that occur in the retina and other parts of the visual
system. For example, the spinning disks may activate neighboring areas of
the retina differently. In other words, the black and white areas of the
disk stimulate different parts of the retina. This alternating response
may cause some type of interaction within the nervous system that
generates colors.
Another theory is that different types of cones take different times
to respond and that they stay activated for different amounts of
time. Therefore, when you spin the disk, the white color activates all
three types of cones, but then the black deactivates them. The
activation/deactivation sequence causes an imbalance because the different
types of cones take different times to respond and stay on for different
times. This imbalance in information going to the brain results in
colors.
Neither of these theories explains the colors of Benham's disk completely
and the reason behind the illusion remains unsolved.
ANother explanation
Benham's Disk was invented by a nineteenth-century toymaker who
noticed colours in a black-and-white pattern he had mounted on a
spinning top.
Why do we see colours? There are three types of cones. One is most
sensitive to red light, one to green light, and one to blue light. Each
type of cone has a different latency time, the time it takes to respond
to a stimulus, and a different persistence of response time, the time it
keeps responding after the stimulus has been removed. Blue cones, for
example, are the slowest to respond and keep responding the longest.
When you gaze at one place on the spinning disk, you are looking at
alternating flashes of black and white. When a white flash goes by, all
three types of cones respond. Your eyes and brain see the colour white
only when all three types of cones are responding equally. The fact that
some types of cones respond more quickly than others and that some
types of cones keep responding longer than others leads to an imbalance
that partially explains why you see colours.
The colours vary across the disk because the black arcs have
different lengths, so that the flashing rate they produce on the retina
is also different.