Despite popular misconception, humans do not see a direct representation
of external reality, but a translation formed by their eyes and
mind. This is not some coffee house philosophical argument, but
physiological fact. Human eyes do a good, but far from perfect
job at detecting light.
This page is an introuduction to the physiology of seeing
and offers several interesting examples of optical distortion.
A Very Brief Overview of the Phsysiology of Seeing
When a human looks at an object, light from the object enters
the eyes. The light goes through the cornea, which is a clear
covering, then through the pupil which is a clear circle in the
colored part of the eye called the iris. The pupil gets larger
(dilates) when there is little light and smaller when there is
little light. The lens focuses the light through the aqueous
humor, a clear liquid, and onto the retina. The retina, in the
back of the eye, contains millions of tiny photo sensors that
detect the light. There are two main kinds of photo sensors:
rods and cones. Shaped like rods, rods detect shades and forms
and are needed for night and peripheral ('out of the corner of
the eye') vision. Rods are not good at detecting color. Shaped
like cones, cones are needed for seeing details, seeing in the
bright daylight and seeing colors. Cones do not work well in
low light, such as at night. Rods and cones cover the entire
retina except for a spot where the optic nerve connects to the
brain. The optic nerve carries the information received from
the retina to the brain, where the brain translates it into the
single image we perceive, or 'see.'
All humans have blind spots, which are spots where the eye cannot
see. The blind spot in an eye corresponds to the spot on the
retina where the optical nerve connects the retina to the brain.
At this spot there are no light detecting cells and, thus, this
spot cannot detect light. A small object can disappear from view
at the spot.
In everyday life the blind spot goes unnoticed. This is in
part as the eye is constantly looking around, getting a wide
and varied range of views. It is also in part as the brain uses
the information from both eyes to create the single mental vision.
What one eye misses, the other often picks up.
As its optical nerve connects differently, the octopus has
no blind spot.
Detecting your blind spot
To detect your blind spot using the above red dot/green dot
picture on the next page, close your right eye and look at the
GREEN dot. Slowly move your head towards the picture. At one
point the RED dot will disappear. Notice that the missing spot
is filled in white by your mind, so it appears as if nothing
is missing from your view. This illustrates how your blind spot
goes unnoticed during daily living. Many people live their entire
life not knowing they have a blind spot.
Humans have more glaring blind spots. Due to the placement
of our eyes in our head, we can't naturally see behind us, under
our feet, from the top of our head, behind our elbows. A common
saying to explain why we didn't notice something is, "I
don't have eyes in the back of my head." And it's common
knowledge that if you want to sneak up on a person you approach
from behind. We compensate for these blinds spots by turning
around, moving our heads, using a mirror or other reflection,
saying "Who's that behind me?," listening, noticing
Other animals have different eye placement and fields of view.
As a robin has its eyes on the side of its head, it has better
side view but worse directly ahead view. The robin's life depends
on its being able to detecting predators from the side and back.
When hunting for worms in the grass, robins turn their heads.
Some think they are turning their ear to listen for worms, when
they are turning their heads to see in front of them. A wolf,
which is a hunter stalking prey, has eyes placement best suited
for seeing ahead. The wolf sees better straight ahead, but its
side to side vision is worse than a robin's. A crocodile has
eyes that rise above the rest of its head. Not only does this
create a different field of view, but allows the crocodile to
see above water while the most of its head and body are hidden
below water. The eyes serve as periscopes.
Afterimages: Seeing What Isn't There
Afterimages are when, after staring at an object, you look away
and still see an image of the object. An example is when you
still see the nighttime headlights of a car, even when your eyes
have closed and the car has turned away. Another is when, after
looking away from a light bulb or candle in the dark, you still
see light in the shape of the bulb or candle.
Afterimages happen after the retina's photosensors (the rods
and cones in your eyes) become oversaturated, or burned out,
from staring at a particular color. This burning out is comparable
to lifting weights in the weight room. After doing enough bench
presses you lose your bench press strength and will be able to
lift only lighter weights. Your muscles are burnt out, if only
temporarily, from lifting big weight. Similarly, after staring
at a large area of a single color, the eye's photosensors lose
their strength for that color. If, afterwards, the eyes look
at a blank piece of paper, the photosensors will be weak towards
the previously stared at color but fresh and strong for detecting
the other colors. This imbalance causes the mind to perceive
the image (the afterimage), but in the color opposite to the
original color. To the mind, the weakness towards one color means
the presence of the opposite primary color is stronger. Quirky
perhaps, but this is the way the brain works. If you are staring
at a green image, the afterimage should be red. After staring
at a yellow image, the afterimage should be blue. The mind sees
afterimages in primary colors, so any non-primary color will
be seen as the primary opposite.
Though they occur almost constantly, afterimages usually go
unnoticed. Afterimages are best observed when focusing on a single
color or object for a lengthy of time. However, in normal viewing,
we view a wide range of objects and colors at once and our eyes
are always moving around, the view constantly shifting. In these
cases, the afterimages are minor and get lost in the visual shuffle.
We barely if at all notice them.
Natural delay in processing light
If in the dark you pass a lit match in front of your face
you will see a trail of light following the match. If you pass
you hand quickly in front of your eyes in daylight, your hand
will be a blur. Related to afterimages, this effect happens in
part because your eyes and brain don't process light instantaneously.
It takes a small fraction of a second for the eyes and mind to
translate the light that enters the eyes into the mental image
we see in our minds.
This effect, along with the afterimage and binocular vision,
aids in making our blind spot disappearing. As our eyes naturally
move around, there is a lingering of image that helps cover the
The following shows examples of afterimages, and a few also
involving the process delay.
If you stare up close for about a minute at the below color squares,
then stare at the corresponding white space below, you may perceive
the colors in reverse.
If you stare at the below circular design, you should see
movement of some sort, such as pulsating, shifting and/or rotating.
This is is caused by how the eyes and mind detect and interep
the information. As your eyes naturally move, even if slightly,
an afterimage follows with your eyes causing the appearance of
movement that does not exist. The rotating black and white design
was intentionally designed to play on the afterimage and other
visual conceits. To the human mind, if any printed picture is
going to move on the page, it will be this circular, rotating
The below is another design that often produces the appearance
of movement when stared at-such as rotating, pulsating and/or
shifting. Even though the image is stationary, it's difficult
to not visually perceive it as stationary.
Humans have binocular vision, meaning that the single image we
see in our mind is made from two different views-- one from each
Our binocular vision gives at least two notable advantages.
First, we have a wider field of view than if we had only one
eye. The right can see further to the right and the left further
to the left. The single vision in our mind shows more than either
single eye can see.
A second advantage is the two views give us good, if not perfect,
depth perception. People who are blind in one eye and animals
with only one eye have worse depth perception than the average
human. The mythical Cyclops might appear an unbeatable warrior,
but a wily human opponent could take advantage of the monster's
poor depth perception.
Triangularism and Calculating Depth
Binocular vision produces the perception of depth in a way
similar to how triangularism measures length in applied mathematics.
When looking at a distant point using only one view it is hard
to impossible to measure the distance accurately. In applied
mathematics, triangularism can accurately calculate this distance
from point a to point b by creating an imaginary triangle. Trianglularism
has long been used in the real world to measure distant objects,
like islands and boats at sea and when surveying land.
Triangularism: From point a alone, it can be impossible to accurately
calculate distance to point b. In the real world, point a could
be you standing on land and point b an anchored boat out at sea.
However, by taking measurements from point a, then taking a measurement
from nearby point c (perhaps a walking distance away), then measuring
the distance from point a and c, one can create an imaginary
triangle that calculates the distance from point a to point b.
It's just a matter calculating angles and doing the math.
Two eyes give the mind a similar two point view of an apple
or house, and the mind uses these two views to help guestimate
distance. This is mostly done subconsciously. You simply reach
out and grab that pencil or penny or door knob or hanging ceiling
fan string or stairway railing, no problem. When you wear an
eye patch, you may discover it's more difficult to grab things
on the first try.
The Hole In The Hand Illusion
This simple trick plays with your binocular vision to make
it appear as if you have a hole in your hand. Roll a normal piece
of 8x11" paper into a tube and place it next to your hand
as shown in the following picture. With one eye look through
the tube and with the other at your hand. With a little bit of
shifting you should perceive what appears to be a large hole
through your hand. Your mind takes the two distinct views to
create one odd bizarre view.
The viewer would look through the tube
with his left eye and at his right hand with his right eye
As you can see, you don't see physical reality but a translation
When you are look at a living room or bowl of apples or painting
or mountain range, the image you see is not a direct representation
of the objects. The image you see is a translation made by your
eyes and mind. As demonstrated, binocularism (changing two views
into one), afterimages (images created by the eyes/mind), unnoticed
blind spots, inability to see colors in low light and countless
other purely physiological occurrences ensure that our mental
image is always different than the objects viewed.
Everything we perceive involves visual illusion.
What color is a red ball when the lights are turned off? Remember
that red is part of the visible light spectrum.
If you believe that there is a God who purposely created animals,
why do you think He gave humans such limited eyesight? Why do
you believe He gave some animals better eyesight than humans?
Infrared viewers, such as night vision goggles, do not allow
humans to see infrared light, but translate infrared light into
visible light. We will never see infrared light, and can only
guess how an infrared viewing animal perceives the light.
Humans categorize and label objects in part by visible colors.
Many animals, flowers, gems and even humans are defined by their
As defined by the American Kennel Club, a cairn terrier can
come in all colors except white. If a cairn terrier is born white,
it's not a cairn terrier. It's a West Highland Terrier, a different
If we could see infrared and ultraviolet light our categorizations
and names of objects, including terriers, would be different.
A mirror mirrors what is in front of it. If you place an apple
two feet in front of the mirror, an identical looking apple will
look as if it's the same distance behind, or into, the mirror.
Curiously, if you use triangulation to measure the distance to
the apple in the mirror, the apple will measure as being two
feet behind the mirror. Both our eyes and scientific measurement
say there is an apple two feet behind the mirror's surface.
If a human perceives a person in a magazine picture and a
dog does not, which animal has the better perception? Humans
often use as evidence of a dog's dimwittedness that the dog 'doesn't
see' the human being on the television screen, when, of course,
there isn't really a person on the screen. The dog is faulted
for not seeing what isn't there.
Smell, taste, sound and touch effect your visual perception.
For example, your visual perception of a pie shaped object may
be confirmed, corrected or confused by the smell. You judge distance
by sound-something is usually softer the further away it gets.
In the dark, people typically feel about for walls, doors and
tables. Echoes can fool you into misjudging location.
While humans depend mostly on sight, other animals depend
more on other senses. The blood hound has worse than human eyesight,
but uses its advanced sense of smell to find lost people that
even trained police detectives cannot find. In these instances,
the blood hound's non-seeing perception is more accurate than
all of the detectives senses combined. This explains why many
police departments have blood hounds on the staff.
(c) david rudd cycleback, cycleback.com
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