table of contents

a look at how humans think and see

Eye/Brain Physiology and Human Perception of External Reality
by David Rudd Cycleback

(c) cycleback 2003, 2005 all rights reserved


















































































































































































































































































































































































































































































































































































































































































































































































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.'


Blind Spots

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 shadows.

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 blind spot.

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 design.


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.



Binocular Vision

Humans have binocular vision, meaning that the single image we see in our mind is made from two different views-- one from each eye.

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 of it

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 colors.

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 breed.

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.


Other Senses

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. main



(c) david rudd cycleback, all rights reserved