**Document ID**: 08_18_00_1

**Date Received**: 2000-08-18 **Date Revised**: 2000-11-09 **Date
Accepted**: 2000-11-09

**Curriculum Topic Benchmarks**: S6.4.8, S15.4.1, S15.4.5, S15.4.6

**Grade Level**: \[9-12\] High School

**Subject Keywords**: aviation, eyes, vision, anatomy, physiology,
psychology, optic disc

**Rating**: moderate

**Now You See It, Now You Don't**

**Demonstrating Visual Deficiencies Inherent to the Anatomy of the Eye**

**By:** Charles R. "Chuck" Moren, Associate Professor of Aeronautical
Science,

Embry-Riddle Aeronautical University, Daytona Beach, Florida 32114.

e-mail: morenc@cts.db.erau.edu

**From**: The PUMAS Collection http://pumas.jpl.nasa.gov

©2000, California Institute of Technology. ALL RIGHTS RESERVED. Based on
U.S. Gov\'t sponsored research.

**Objective:** A demonstration of the effect of the optic disc, or
"blind spot," inherent to the optic nerve entering the posterior of the
eye (bulbus oculi).

**Application:** This** **demonstration works well at all levels of
instruction involving the anatomy of the eye. It lends itself very well
to basic anatomy and/or physiology classes. It is also appropriate for
introductory psychology classes during discussions of vision and visual
perception. I use it in my flight physiology class during discussions
concerning night vision and visual illusions. The demonstration only
takes a few minutes and is a "real attention getter."

**Equipment required:** One (1) piece of 8.5-inch by 11-inch plain paper
and one (1) pen or pencil, per student. Optional: One (1) piece of
8.5-inch by 11-inch ruled paper or graph paper, per student.

**Applicable Anatomy/Physiology:** Light, in the form of wave energy,
enters our eye and stimulates light-sensitive receptor cells called rods
and cones. The rods and cones are located at the posterior of the eye in
nerve tissue referred to as the retina. When the light reaches these
cells, the cells generate nerve impulses that travel through the optic
nerve to the occipital lobe of the cerebral cortex of the brain. The
brain then interprets the information and creates an image that we
"see."

**Optic Disc: **The optic nerve enters the posterior of the eye through
the retina. There are no rods or cones located in the area where the
optic nerve penetrates the retina. This circular area is referred to as
the optic disc. Because of the absence of rods and cones, the optic disc
causes a blind spot near the center of the retina.

**Demonstration: **On a blank sheet of paper (copy paper works great)
draw an asterisk the size of a dime in the upper left and upper right
corners of the paper. The asterisk should be about 2 inches from the top
and 2 inches from each side.

With your right hand, cover your right eye. Hold the paper in front of
your face approximately 14 inches away with the RIGHT asterisk in front
of your LEFT eye. The left asterisk will disappear in the blind spot.

You may need to vary the distance several inches or move the paper side
to side slightly to find your blind spot. Be sure to look directly at
the RIGHT asterisk.

When doing the demonstration in class, either draw a piece of paper on
the board and show the location of the asterisks or hold up a piece of
paper after you draw the asterisks on it. Demonstrate the process by
doing. The students will understand the process faster if they can
duplicate your actions. Listen for the WOWs !!

You can use lined notebook paper. Just have the students take out a
sheet of paper and proceed as above.

**Background:** Our ability to "see" is a complex process that is often
taken for granted. For those of us who have the use of all five of our
senses, the majority of information stimulus about the world around us
enters our sensory system through the sense of vision. Thus, anything
that disrupts the normal visual process can have a significant effect
upon our ability to maintain our situational awareness. Many of us have
seen illustrations in textbooks that demonstrate visual illusions. These
illustrations demonstrate that our perceptions can be tricked by visual
stimulus that is improperly interpreted by our brain. The first
significant interpretation that always occurs in the brain is the
inversion of the image. Because of the properties of the lens of the
eye, the light entering the eye is turned upside down. However, thanks
to our brain, we see the upside down stimulus as something that is right
side up. Most visual illusions are caused by the brain and not by
anything inherently wrong with our eyes. True visual illusions occur
when the brain has difficulty interpreting data received from the eye
and not by a malfunction of the eye itself. This demonstration is unique
because it shows a deficiency in the function of the eye itself.

**Retinal Disparity:** Most of us go through life without ever seeing
our blind spots. The optic nerve is not located in the exact center of
the retina. The optic nerve of the right eye enters slightly to the left
of center and the optic nerve of the left eye enters slightly right of
center. In other words, the location of the optic disc is different in
each eye. Because our eyes are not located on the exact centerline of
our body, when we look at an object, light waves reflected from that
object are projected onto the retina of each eye in a slightly different
position. This is referred to as retinal disparity. Retinal disparity
allows our brain to receive the entire image reflected from the object.
The left eye captures the portion of the image that is missed by the
optic disc of right eye, and visa versa. At short distances, retinal
disparity also provides binocular cues for depth perception.

**Practical Application/Discussion**

** Night Vision**: Cones are stimulated by bright light and do not
function at low light levels. Rods function at low light levels, after a
period of adaptation. During the adaptation period, a chemical called
rhodopsin is produced by the rods (rhodopsin is destroyed by bright
light). Night adaptation occurs over a 30-45 minute period. This is the
improvement in night vision that we recognize after sitting in a dark
theater for a period of time.

On the retina of the eye, the cones surround the optic disc in the area
known as the Fovea centralls. The rods surround the cones. At low light
levels, when the cones stop functioning, the size of the blind spot
(now, the area of the optic disc plus the non-functioning cones)
increases significantly. An unlighted object twelve inches in diameter
cannot be seen at night at a distance of 30 feet or more. At 3000 feet,
a large aircraft will be invisible. Although this demonstration does not
specifically show the effects of night vision, it can be a great
introduction to the subject. Pilots are taught a concept called
off-center viewing that can be used at night to help overcome the
effects of the increased size of the blind spot. Pilots do not look
directly at what they want to see outside of the aircraft, they look at
a point about 10 degrees to one side of where they expect to see an
object. Throughout our lives, we develop a habit of looking directly at
what we want to see, and it requires conscious effort to overcome this
center-viewing habit. This demonstration introduces the effects of the
blind spot in a way that clearly demonstrates the limitations of the
eye, and the need and importance of off-center viewing is reinforced.

** Other Precautions**: You may point out the danger of operating a
motor vehicle if one eye is covered, such as after an injury to the eye.
In addition to the deficiencies caused by the blind spot, covering one
eye also causes problems with depth perception at distances up to
approximately 50 feet.

** Perceptual Constancy**: When using lined paper (or graph paper, if
available), notice that the line(s) appears to continue through the area
from which the asterisk has disappeared. The brain has interpreted the
lines as continuing through the area because it thinks they \"should be
there,\" based upon a lifetime of learning. This phenomenon is referred
to as perceptual constancy.

**Bibliography:**

Department of the Air Force, (1976). [Physiological
Training]{.underline}. Washington, D.C.

Edwards, L. F. and Gaughran, G. R. L. (1971). [Concise
Anatomy]{.underline} (3^rd^ ed.). New York: McGraw-Hill.

Reinhart, R. O. (1996). [Basic Flight Physiology]{.underline} (2^nd^
ed.). New York: McGraw-Hill.

Wade, C. and Tavris, C. (1995). [Psychology]{.underline} (4^th^ ed.).
New York: HarperCollins.