The Eyes Have It
The Eye and Interpretation by the Brain
www.handprint.com/HP/WCL/colortop.html
http://en.wikipedia.org/wiki/Human_eye
http://en.wikipedia.org/wiki/Optical_illusion
http://en.wikipedia.org/wiki/After_image
Objectives:
Students will examine:
 Optional activity: The form and function of the eye by:
o Dissecting a sheep eye
 How their eyes work by
o Conducting tests to explore
 Visual acuity
 The blind spot and astigmatism
 The size of the fovea
 Color vision
 Depth perception
 Accommodation and Near point
 Peripheral vision
 Afterimages
 How their brain understands what they see by
o Examining a variety of optical illusions
Background Informaton:
Eyes are organs that detect light. Different kinds of light-sensitive organs are found in a variety
of animals. The simplest eyes do nothing but detect whether the
surroundings are light or dark, which is sufficient for the
entrainment of circadian rhythms but hardly can be called vision.
More complex eyes can distinguish shapes and colors. The
visual fields of some such complex eyes largely overlap, to
allow better depth perception (binocular vision), as in humans;
and others are placed so as to minimize the overlap, such as in
rabbits and chameleons.
The human eye.
The compound eyes of a dragonfly.
In the human eye, light enters the pupil and is focused on the retina by
the lens. Light-sensitive nerve cells called rods (for brightness) and
cones (for color) react to the light. They interact with each other and
send messages to the brain that indicate brightness, color, and contour.
The first proto-eyes evolved among animals 540 million years ago.
Almost all animals have eyes, or descend from animals that did.
In most vertebrates and some mollusks, the eye works by allowing light
to enter it and project onto a light-sensitive panel of cells known as the
retina at the rear of the eye, where the light is detected and converted into electrical signals. The
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visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically
roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a
focusing lens and often an iris which regulates the intensity of the light that enters the eye. The
eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing
vision is achieved by telescoping the lens—similar to how a camera focuses.
Compound eyes are found among the arthropods and are composed of many simple facets which
give a pixelated image (not multiple images, as is often believed). Each sensor has its own lens
and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged
hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very
sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only
a few facets, each with a retina capable of creating an image, creating multiple-image vision.
With each eye viewing a different angle, a fused image from all the eyes is produced in the brain,
providing very wide-angle, high-resolution images.
Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the
world's most complex color vision system. Trilobites, which are now extinct, had unique
compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they
differ from most other arthropods, which have soft eyes. The number of lenses in such an eye
varied, however: some trilobites had only one, and some had thousands of lenses in one eye.
Compound eye of Antarctic krill
Some of the simplest eyes, called ocelli, can be found in animals
like snails, who cannot actually "see" in the normal sense. They do
have photosensitive cells, but no lens and no other means of
projecting an image onto these cells. They can distinguish between
light and dark, but no more. This enables snails to keep out of
direct sunlight. Jumping spiders have simple eyes that are so large,
supported by an array of other, smaller eyes, that they can get
enough visual input to hunt and pounce on their prey. Some insect
larvae, like caterpillars, have a different type of simple eye
(stemmata) which gives a rough image.
Diagram of major stages in the eye's evolution
Biologists use the theory of evolution to explain the origin and
development of eyes, as well as of organs in general.
The common origin (monophyly) of all animal eyes is established
by shared anatomical and genetic features of all eyes; that is, all
modern eyes, varied as they are, have their origins in a proto-eye
evolved some 540 million years ago. The majority of the
advancements in early eyes are believed to have taken only a few
million years to develop, as the first predator to gain true imaging
would have touched off an "arms race", or rather, a phylogenetic
radiation from the species with that first proto-eye, among the
descendents of which, there may well have been an "arms race". Prey animals and competing
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predators alike would be forced to rapidly match or exceed any such capabilities to survive.
Hence multiple eye types and subtypes developed in parallel.
Vision in various animals shows adaptation to environmental requirements. For example, birds
of prey have much greater visual acuity than humans, and some can see ultraviolet light. The
different forms of eyes in, for example, vertebrates and mollusks are often cited as examples of
parallel evolution, despite their distant common ancestry.
The earliest eyes, called "eyespots", were simple patches of photoreceptor cells, or light-sensitive
proteins in unicellular organisms, physically similar to the receptor patches for taste and smell.
These eyespots could only sense ambient brightness: they could distinguish light and dark, but
not the direction of the lightsource. This gradually changed as the eyespot depressed into a
shallow "cup" shape, granting the ability to slightly discriminate directional brightness by using
the angle at which the light hit certain cells to identify the source. The pit deepened over time,
the opening diminished in size, and the number of photoreceptor cells increased, forming an
effective pinhole camera that was capable of slightly distinguishing dim shapes.
Anatomy of the mammalian eye
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posterior compartment
ora serrata
ciliary muscle
ciliary zonules
canal of Schlemm
pupil
anterior chamber
cornea
iris
lens cortex
lens nucleus
ciliary process
conjuntiva
inferior oblique muscule
inferior rectus muscule
medial rectus muscle
retinal arteries and veins
optic disc
dura mater
central retinal artery
central retinal vein
optical nerve
vorticose vein
bulbar sheath
macula
fovea
sclera
choroid
superior rectus muscule
retina
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The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent
damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a
transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's
refractive index, and allowed functionality outside of water. The transparent protective cells
eventually split into two layers, with circulatory fluid in between that allowed wider viewing
angles and greater imaging resolution, and the thickness of the transparent layer gradually
increased, in most species with the transparent crystallin protein.
The structure of the mammalian eye can be divided into three main layers or tunics whose names
reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic.
 The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball
consisting of the cornea and sclera. The sclera gives the eye most of its white color. It
consists of dense connective tissue filled with the protein collagen to both protect the
inner components of the eye and maintain its shape.
 The vascular tunic, also known as the tunica vasculosa oculi, is the middle vascularized
layer which includes the iris, ciliary body, and choroid. The choroid contains blood
vessels that supply the retinal cells with necessary oxygen and remove the waste products
of respiration. The choroid gives the inner eye a dark color, which prevents disruptive
reflections within the eye.
 The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory which
includes the retina. The retina contains the photosensitive rod and cone cells and
associated neurons. To maximise vision and light absorption, the retina is a relatively
smooth (but curved) layer. It has two points at which it is different; the fovea and optic
disc. The fovea is a dip in the retina directly opposite the lens, which is densely packed
with cone cells. It is largely responsible for color vision in humans, and enables high
acuity, such as is necessary in reading. The optic disc, sometimes referred to as the
anatomical blind spot, is a point on the retina where the optic nerve pierces the retina to
connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is
thus "blind". In addition to the rods and cones, a small proportion (about 2% in humans)
of the ganglion cells in the retina are photosensitive through the pigment melanopsin.
They are generally most excitable by blue light, about 470 nm. Their information is sent
to the SCN (suprachiasmatic
nuclei), not to the visual center,
through the retinohypothalamic
tract, not via the optic nerve. It is
these light signals which regulate
circadian rhythms in mammals
and several other animals. Many,
but not all, totally blind
individuals have their circadian
rhythms adjusted daily in this
way.
The mammalian eye can also be divided
into two main segments: the anterior
segment and the posterior segment. Diagram of a human eye; note that not all eyes have the same anatomy
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The posterior segment is the back two-thirds of the eye that includes the anterior hyaloid
membrane and all structures behind it: the vitreous humor, retina, choroid, and optic nerve. On
the other side of the lens is the second humour, the vitreous humour, which is bounded on all
sides: by the lens, ciliary body, suspensory ligaments and by the retina. It lets light through
without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some
animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount
of light each photosensitive cell perceives, allowing the animal to see better under low light
conditions.
Light from a single point of a distant
object and light from a single point of a
near object being brought to a focus
In many species, the eyes are inset
in the portion of the skull known as
the orbits or eyesockets. This placement of the eyes helps to protect them from injury.
In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) away from the
eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also
wash away the tear fluid—along with it the protective lipid layer—and can alter corneal
physiology, due to osmotic differences between tear fluid and freshwater. This is made apparent
when swimming in freshwater pools, as the osmotic gradient draws "pool water" into the corneal
tissue (the pool water is hypotonic), causing edema, and subsequently leaving the swimmer with
"cloudy" or "misty" vision for a short period thereafter. It can be reversed by irrigating the eye
with hypertonic saline which osmotically draws the excess water out of the eye.
In many animals, including humans, eyelids wipe the eye and prevent dehydration. They spread
tears on the eyes, which contains substances which help fight bacterial infection as part of the
immune system. Some aquatic animals have a second eyelid in each eye which refracts the light
and helps them see clearly both above and below water. Most creatures will automatically react
to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering
the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a
reflex.
In many animals, including humans, eyelashes prevent fine particles from entering the eye. Fine
particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to
tears and subsequent blurred vision.
The structure of the mammalian eye owes itself completely to the task of focusing light onto the
retina. This light causes chemical changes in the photosensitive cells of the retina, the products
of which trigger nerve impulses which travel to the brain.
The retina contains two forms of photosensitive cells important to vision—rods and cones.
Though structurally and metabolically similar, their function is quite different. Rod cells are
highly sensitive to light allowing them to respond in dim light and dark conditions, however,
they cannot detect color. These are the cells which allow humans and other animals to see by
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moonlight, or with very little available light (as in a dark room). This is why the darker
conditions become, the less color objects seem to have. Cone cells, conversely, need high light
intensities to respond and have high visual acuity. Different cone cells respond to different
wavelengths of light, which allows an organism to see color.
The differences are useful; apart from enabling sight in both dim and light conditions, humans
have given them further application. The fovea, directly behind the lens, consists of mostly
densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading,
bird watching, or any other task which primarily requires staring at things. Its requirement for
high intensity light does cause problems for astronomers, as they cannot see dim stars, or other
celestial objects, using central vision because the light from these is not enough to stimulate cone
cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars
through the "corner of their eyes" (averted vision) where rods also exist, and where the light is
sufficient to stimulate cells, allowing an individual to observe faint objects.
Rods and cones are both photosensitive, but respond differently to different frequencies of light.
They both contain different pigmented photoreceptor proteins. Rod cells contain the protein
rhodopsin and cone cells contain different proteins for each color-range. The process through
which these proteins go is quite similar—upon being subjected to electromagnetic radiation of a
particular wavelength and intensity, the protein breaks down into two constituent products.
Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into
photopsin and retinal. The opsin in both opens ion channels on the cell membrane which leads to
hyperpolarization, this hyperpolarization of the cell leads to a release of transmitter molecules at
the synapse.
This is the reason why cones and rods enable organisms to see in dark and light conditions—
each of the photoreceptor proteins requires a different light intensity to break down into the
constituent products. Further, synaptic convergence means
that several rod cells are connected to a single bipolar cell,
which then connects to a single ganglion cell by which
information is relayed to the visual cortex. This is in direct
contrast to the situation with cones, where each cone cell is
connected to a single bipolar cell. This results in the high
visual acuity, or the high ability to distinguish between detail,
of cone cells and not rods. If a ray of light were to reach just
one rod cell this may not be enough to hyperpolarize the
connected bipolar cell. But because several "converge" onto a
bipolar cell, enough transmitter molecules reach the synapse
of the bipolar cell to hyperpolarize it.
A hawk's eye
Furthermore, color is distinguishable due to the different iodopsins of cone cells; there are three
different kinds, in normal human vision, which is why we need three different primary colors to
make a color space.
Visual acuity is often measured in cycles per degree (CPD), which measures an angular
resolution, or how much an eye can differentiate one object from another in terms of visual
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angles. Resolution in CPD can be measured by bar charts of different numbers of white–black
stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the
eye, it will subtend an angle of 1 degree, so the number of white–black bar pairs on the pattern
will be a measure of the cycles per degree of that pattern. The highest such number that the eye
can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity
of the eye.
For a human eye with excellent acuity, the maximum theoretical resolution would be 50 CPD
(1.2 minute of arc per line pair, or a 0.35 mm line pair, at 1 m). However, the eye can only
resolve a contrast of 5%. Taking this into account, the eye can resolve a maximum resolution of
37 CPD, or 1.6 minute of arc per line pair (0.47 mm line pair, at 1 m). A rat can resolve only
about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a
human has, but does not match the high acuity of the human eye's central fovea region.
A maximum resolution of the human eye in good light of 1.6 minute of arc per line pair will
correspond to 1.25 lines per minute of arc. Assuming two pixels per line pair (one pixel per line)
and a square field of 120 degrees, this would be equivalent to approximately 120×60×1.25 =
9000 pixels in each of the X and Y dimensions, or about 81 megapixels.
However, the human eye itself has only a small spot of sharp vision in the middle of the retina,
the fovea centralis, the rest of the field of view being progressively lower resolution as it gets
further from the fovea. The angle of the sharp vision being just a few degrees in the middle of the
view, the sharp area thus barely achieves even a single megapixel resolution. The experience of
wide sharp human vision is in fact based on turning the eyes towards the current point of interest
in the field of view, the brain thus perceiving an observation of a wide sharp field of view.
The narrow beam of sharp vision is easy to test by putting a fingertip on a newspaper and trying
to read the text while staring at the fingertip — it is very difficult to read text that's just a few
centimeters away from the fingertip.
Human eyes respond to light with wavelength in the range of approximately 400 to 700 nm.
Other animals have other ranges, with many such as birds including a significant ultraviolet
(shorter than 400 nm) response.
The retina has a static contrast ratio of around 100:1 (about 6 1/2 stops). As soon as the eye
moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Initial dark
adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full
adaptation through adjustments in retinal chemistry (the Purkinje effect) are mostly complete in
thirty minutes. Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 stops) is possible.
The process is nonlinear and multifaceted, so an interruption by light nearly starts the adaptation
process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may
be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.
The visual system in the brain is too slow to process that information if the images are slipping
across the retina at more than a few degrees per second. Thus, for humans to be able to see while
moving, the brain must compensate for the motion of the head by turning the eyes. Another
complication for vision in frontal-eyed animals is the development of a small area of the retina
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with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual
angle in people. To get a clear view of the world, the brain must turn the eyes so that the image
of the object of regard falls on the fovea. Eye movements are thus very important for visual
perception, and any failure to make them correctly can lead to serious visual disabilities.
Having two eyes is an added complication, because the brain must point both of them accurately
enough that the object of regard falls on corresponding points of the two retinas; otherwise,
double vision would occur. The movements of different body parts are controlled by striated
muscles acting around joints. The movements of the eye are no exception, but they have special
advantages not shared by skeletal muscles and joints, and so are considerably different.
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the
inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the
muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an
almost pure rotation, with only about one millimeter of translation. Thus, the eye can be
considered as undergoing rotations about a single point in the center of the eye. Once the human
eye sustains damage to the optic nerve, the impulses will not be taken to the brain. Eye
transplants can happen but the person receiving the transplant will not be able to see. As for the
optic nerve, once it is damaged it cannot be fixed.
Rapid eye movement, or REM for short, typically refers to the stage during sleep during which
the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique
form of eye movement.
Saccades are quick, simultaneous movements of both eyes in the same direction controlled by
the frontal lobe of the brain.
Even when looking intently at a single spot, the eyes drift around. This ensures that individual
photosensitive cells are continually stimulated in different degrees. Without changing input,
these cells would otherwise stop generating output. Microsaccades move the eye no more than a
total of 0.2° in adult humans.
The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during
head movement by producing an eye movement in the direction opposite to head movement, thus
preserving the image on the center of the visual field. For example, when the head moves to the
right, the eyes move to the left, and vice versa.
The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular
reflex as it requires the brain to process incoming visual information and supply feedback.
Following an object moving at constant speed is relatively easy, though the eyes will often make
saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in
adult humans.
It is more difficult to visually estimate speed in low light conditions or while moving, unless
there is another point of reference for determining speed.
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The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for
example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a
short moment (through smooth pursuit), until the tree moves out of the field of vision. At this
point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the
tree (through a saccade).
When a creature with binocular vision looks at an object, the eyes must rotate around a vertical
axis so that the projection of the image is in the centre of the retina in both eyes. To look at an
object closer by, the eyes rotate 'towards each other' (convergence), while for an object farther
away they rotate 'away from each other' (divergence). Exaggerated convergence is called cross
eyed viewing (focusing on the nose for example) . When looking into the distance, or when
'staring into nothingness', the eyes neither converge nor diverge.
The two eyes converge to point to the same object.
Vergence movements are closely connected to
accommodation of the eye. Under normal
conditions, changing the focus of the eyes to look at
an object at a different distance will automatically
cause vergence and accommodation.
To see clearly, the lens will be pulled flatter or
allowed to regain its thicker form.
In many countries, stuffed cow's eyes are
considered a delicacy. They are made by first
removing the vitreous humor, lens, cornea, and iris,
then are usually boiled. Cow eyes are often stuffed
with varieties of coleslaw, beef, and even cream
cheese.
Seal eyes are eaten by the Inuit, providing a source
of zinc in their diet.
A delicacy in western Norwegian cuisine is the singed head of a sheep or lamb (smalahovud),
where also the eyes are eaten.
Optical illusion
An optical illusion. Square A is exactly the same
shade of grey as square B. See Same color illusion
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An optical illusion (also called a visual illusion) is characterized
by visually perceived images that are deceptive or misleading.
The information gathered by the eye is processed by the brain to
give a percept that does not tally with a physical measurement of
the stimulus source. There are two main types of illusion physiological illusions that are the effects on the eyes and brain of
excessive stimulation of a specific type - brightness, tilt, color,
movement, and cognitive illusions where the eye and brain make
unconscious inferences.
Physiological illusions, such as the afterimages following bright lights or adapting stimuli of
excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the
effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color,
movement, etc. The theory is that stimuli have individual dedicated neural paths in the early
stages of visual processing, and that repetitive stimulation of only one or a few channels causes a
physiological imbalance that alters perception.
A scintillating grid illusion. Shape position and color contrast converge
to produce the illusion of grey blobs at the intersections.
An example movie which produces distortion illusion
after watching the movie then looking away.
See: http://en.wikipedia.org/wiki/Optical_illusion
The Hermann
grid illusion and
Mach bands are
two illusions
that are best
explained using
a biological
approach. Lateral inhibition, where in the
receptive field of the retina light and dark
receptors compete with one another to
become active, has been used to explain why
we see bands of increased brightness at the
edge of a color difference when viewing
Mach bands. Once a receptor is active it
inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann
grid illusion the grey spots appear at the intersection because of the inhibitory response which
occurs as a result of the increased dark surround. Lateral inhibition has also been used to explain
the Hermann grid illusion, but this has been disproved.
Cognitive illusions
Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading
to "unconscious inferences", an idea first suggested in the 19th century by Hermann Helmholtz.
Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox
illusions, or fiction illusions.
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1. Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the
alternative interpretations. The Necker cube is a well known example; another instance is
the Rubin vase.
2. Distorting illusions are characterized by distortions of size, length, or curvature. A
striking example is the Café wall illusion. Another example is the famous Müller-Lyer
illusion.
3. Paradox illusions are generated by objects that are paradoxical or impossible, such as the
Penrose triangle or impossible staircases seen, for example, in M. C. Escher's Ascending
and Descending and Waterfall. The triangle is an illusion dependent on a cognitive
misunderstanding that adjacent edges must join.
4. Fictional illusions are defined as the perception of objects that are genuinely not there to
all but a single observer, such as those induced by schizophrenia or a hallucinogen. These
are more properly called hallucinations.
Explanation of cognitive illusions
Perceptual organization
Your right brain tries to say the color but your left brain
insists on reading the word.
Look at the chart and say the COLOR
not the word
YELLOW
BLUE
ORANGE
BLACK
RED
GREEN
PURPLE
YELLOW
RED
ORANGE
GREEN
BLACK
BLUE
RED
PURPLE
GREEN
BLUE
ORANGE
Duck-Rabbit illusion
Left - Right Conflict
To make sense of the world it is necessary to organize incoming
sensations into information which is meaningful. Gestalt
psychologists believe one way this is done is by perceiving
individual sensory stimuli as a meaningful whole.
Gestalt organization can be used to explain many illusions
including the Duck-Rabbit illusion where the image as a whole
switches back and forth from being a duck then being a rabbit and
why in the figure-ground illusion the figure and ground are
reversible.
Reversible figure and ground
In addition, Gestalt theory can be used to explain the illusory
contours in the Kanizsa Triangle. A floating white triangle, which
does not exist, is seen. The brain has a need to see familiar simple
objects and has a tendency to create a "whole" image from
individual elements. Gestalt means "whole" in German. However,
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another explanation of the Kanizsa Triangle is based in evolutionary psychology and the fact that
in order to
Kanizsa triangle
survive it was important to see form and edges. The use of perceptual organization to create
meaning out of stimuli is the principle behind other well-known illusions including impossible
objects. Our brain makes sense of shapes and symbols putting them together like a jigsaw
puzzle,formulating that which isn't there to that which is believable.
Depth and motion perception
Illusions can be based on an individual's ability to see in three dimensions even though the image
hitting the retina is only two dimensional. The Ponzo illusion is an example of an illusion which
uses monocular cues of depth perception to fool the eye.
In the Ponzo illusion the converging parallel lines tell the brain that
the image higher in the visual field is further away therefore the
brain perceives the image to be larger, although the two images
hitting the retina are the same size. The Optical illusion seen in a
diorama/false perspective also exploits assumptions based on
monocular cues of depth perception. The M. C. Escher painting
Waterfall exploits rules of depth and proximity and our understand
of the physical world to create an illusion.
Ponzo Illusion
Like depth perception, motion perception is responsible for a number of sensory illusions. Film
animation is based on the illusion that the brain perceives a series of slightly varied images
produced in rapid succession as a moving picture. Likewise, when we are moving, as we would
be while riding in a vehicle, stable surrounding objects may appear to move. We may also
perceive a large object, like an airplane, to move more slowly, than smaller objects, like a car,
although the larger object is actually moving faster. The Phi phenomenon is yet another example
of how the brain perceives motion, which is most often created by blinking lights in close
succession.
Color and brightness constancies
In this illusion, the second card from the
left seems to be a stronger shade of pink in
the top picture. In fact they are the same
colour, but the brain changes its assumption
about colour due to the colour cast of the
surrounding photo.
Simultaneous Contrast Illusion.
The horizontal grey bar is the same
shade throughout
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.
Perceptual constancies are sources of illusions. Color constancy and brightness constancy are
responsible for the fact that a familiar object will appear the same color regardless of the amount
of or colour of light reflecting from it. An illusion of color or contrast difference can be created
when the luminosity or colour of the area surrounding an unfamiliar object is changed. The
contrast of the object will appear darker against a black field which reflects less light compared
to a white field even though the object itself did not change in color. Similarly, the eye will
compensate for colour contrast depending on the colour cast of the surrounding area.
Object consistencies
Like color, the brain has the ability to understand familiar objects as having a consistent shape or
size. For example a door is perceived as rectangle regardless as to how the image may change on
the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow
the rules of shape constancy and may change when the perspective is changed. The Shepard
illusion of the changing table is an example of an illusion based on distortions in shape constancy
(http://www.cut-the-knot.org/Curriculum/Geometry/Shepard.shtml for this example).
Illusions
An optical illusion. The two circles seem to move when the viewer's head is
moving forwards and backwards while looking at the black dot.
Floor tiles at the Basilica of St. John
Lateran in Rome. The pattern creates an
illusion of three-dimensional boxes.
Artists have worked with optical illusions, including M.C. Escher, Bridget Riley, Salvador Dalí,
Giuseppe Arcimboldo, Marcel Duchamp, Oscar Reutersvärd, and Charles Allan Gilbert. Also
some contemporary artists are experimenting with illusions, including: Octavio Ocampo, Dick
Termes, Shigeo Fukuda, Patrick Hughes, István Orosz, Rob Gonsalves and Akiyoshi Kitaoka.
Optical illusion is also used in film by the technique of forced perspective.
Cognitive processes hypothesis
The hypothesis claims that visual illusions are due to the fact that the neural circuitry in our
visual system evolves, by neural learning, to a system that makes very efficient interpretations of
usual 3D scenes based in the emergence of simplified models in our brain that speed up the
interpretation process but give rise to optical illusions in unusual situations. In this sense, the
cognitive processes hypothesis can be considered a framework for an understanding of optical
illusions as the signature of the empirical statistical way vision has evolved to solve the inverse
problem.
13
Research indicates that 3D vision capabilities emerge and are learned jointly with the planning of
movements. After a long process of learning, an internal representation of the world emerges that
is well adjusted to the perceived data coming from closer objects. The representation of distant
objects near the horizon is less "adequate". In fact, it is not only the Moon that seems larger
when we perceive it near the horizon. In a photo of a distant scene, all distant objects are
perceived as smaller than when we observe them directly using our vision.
The retinal image is the main source driving vision but what we see is a "virtual" 3D
representation of the scene in front of us. We don't see a physical image of the world. We see
objects; and the physical world is not itself separated into objects. We see it according to the way
our brain organizes it. The names, colors, usual shapes and other information about the things we
see pop up instantaneously from our neural circuitry and influence the representation of the
scene. We "see" the most relevant information about the elements of the best 3D image that our
neural networks can produce. The illusions arise when the "judgments" implied in the
unconscious analysis of the scene are in conflict with reasoned considerations about bite.
Afterimage
An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in
one's vision after the exposure to the original image has ceased. One of the most common
afterimages is the bright glow that seems to float before one's eyes after staring at a light bulb or
a headlight for a few seconds. The phenomenon of afterimages may be closely related to
persistence of vision, which allows a rapid series of pictures to portray motion, which of course
is the basis of animation and cinema.
Afterimages come in two forms, negative (inverted) and positive (retaining original color). The
process behind positive afterimages is unknown, though thought to be related to neural
adaptation. On the other hand, negative afterimages are a retinal phenomenon and are well
understood.
Negative afterimages are caused when the eye's photoreceptors, primarily those known as cone
cells, adapt from the over stimulation and lose sensitivity. Normally the eye deals with this
problem by rapidly moving the eye small amounts, the motion later being "filtered out" so it is
not noticeable. However if the color image is large enough that the small movements are not
enough to change the color under one area of the retina, those cones will eventually tire or adapt
and stop responding. The rod cells can also be affected by this.
When the eyes are then diverted to a blank space, the adapted photoreceptors send out little
signal and those colors remain muted. However, the surrounding cones that were not being
excited by that color are still "fresh", and send out a strong signal. The signal is exactly the same
as if looking at the opposite color, which is how the brain interprets it.
Ewald Hering explained how the brain sees afterimages, in terms of three pairs of primary
colors. This opponent process theory states that the human visual system interprets color
information by processing signals from cones and rods in an antagonistic manner. The opponent
color theory suggests that there are three opponent channels: red versus green, blue versus
14
yellow, and black versus white. Responses to one color of an opponent channel are antagonistic
to those to the other color. Therefore, a green image will produce a red afterimage. The green
color tires out the green photoreceptors, so they produce a weaker signal. Anything resulting in
less green, is interpreted as its paired primary color, which is red.
Positive afterimages, by contrast, appear the same color as the original image. They are often
very brief, lasting less than half a second, and may not occur unless the stimulus is very bright.
The cause of positive afterimages is not well known, but possibly reflects persisting activity in
the visual system, suggesting that the experience of a stimulus can vary with the intensity of the
stimulus. As in most circumstances only very bright stimuli produce positive afterimages, a
stimulus which elicits a positive image will usually trigger a negative afterimage via the
adaptation process. To experience this phenomenon, look at a bright light and then look away. At
first you should see a fading positive afterimage, likely followed by a negative afterimage that
may last for much longer.
In a visual disturbance called palinopsia, patients have an increased propensity for seeing
afterimages, having both a reduced amount of time required to form an afterimage, and an
increased duration of the afterimage. Positive afterimages are particularly noticeable, such that
even routine eye movement is often accompanied by flickers of what the eye has scanned over
(called "tracers"). However, increased negative afterimages are also experienced by palinopsia
sufferers. It is unknown if the negative afterimages encountered in palinopsia are formed by the
same process described above, although what little research that exists regarding the phenomena
suggests that it is brain-related, and not eye-related. Palinopsia can be a persistent condition, but
it is also experienced periodically by migraine sufferers.
Preparing for your class:
It is important that the materials are not placed with the students. It will distract the students, and
you will spend your entire 90 minutes trying to control them. Instead, there are continers to for
each experiment. After you have spent a minute or two discussing the experiement and the
students’ results, you can hand out the materials for the next experiment as they put the materials
back into the container. Then, the distraction is limited to exchanging the materials. If your
students are very distracted, pick up the materials, discuss the results, give the directions for the
next activity, and then hand out the new materials.
This lesson plan is designed for 90 minutes working with your students. If your class is meeting
for 60 minutes, you will need to choose if you will only present one hour of these activities and
the following week, present another kit, or complete the last 30 minutes during your next session.
15
For each student:
 1 dissection trays
 1 pair scissors
 1 dissecting probes
 1 tweezers
 2 gloves
 1 goggle (in teacher kit)
 1anatomy of eye handout
 1 pencils
Set up stations: use lab tape for charts
 Visual Acuity

Snellen Eye Chart on wall

masking tape (to mark 20 ft.)

index card
 Astigmatism & blind spot

Astigmatism test chart on wall

10 foot masking tape mark

Index card

Blind spot diagram

Metric ruler
 Visual Mapping

Visual map (back of data sheet)

Masking tape

Index card

Metric ruler

Pencil (from teacher kit)
 Color Vision
 Holmgren-type color vision test
 Depth Perception
 Depth perception tester
 Black/white background card
 Metric ruler
 Accomodation
 Snellen eye chart on wall
 Black marker





masking tape (to mark 20 ft.)
Near Point
 Metric ruler
 Sharpened pencil wrapped in white
paper with tip exposed
 Index card
Peripheral Vision
 10 black cards
 10 white cards
 10 red cards
 10 blue cards
 Metric ruler
After images
 Red transparent vinyl
 Green transparent vinyl
 White paper
 Kings from deck of illusions cards
 Color pencils
Illusions
 Deck of illusion cards minus the
kings
Set up demonstration:
 The Kings from all three decks of
cards (12 cards total)
 1 white piece of paper per student
Set aside:
 1 eyeball per student or pair of students
soaking in water
 Paper towels laid out to put dissection
equipment to dry
 Biohazard bag
 Sponges, dish soap
 Student data sheets
16
The Demonstration











Hot Dog Fingers and Afterimages– 3 minutes
You do not need any volunteers for this demonstration.
Ask students to stand and face an object that is located across the room (for example, if the
classroom clock is on the far side of the room, use that.
Ask the students to focus on the clock, and keep their eyes focused there.
Instruct the students to put their hands up, index fingers pointing and the rest of the fingers
curled under.
The students bring their hands at arm-length between the clock and their eyes, keeping the
focus on their eyes.
When the fingers touch, they should “see” a hotdog between their fingers.
Ask them to close one eye, and the hotdog disappears.
Direct the students to place the card face up on one side of their white paper.
Stare at the card for 30 seconds (Instructor times and talks to the students while timing – “If
your eye wanders away from the card, just bring it back to the center of the card.”
Tell the students to look at the blank side of the paper, and to keep looking. They must keep
staring at the blank side, because it may take a second or two for the afterimage to appear to
them.
Introduction to Eyes
Eyes: – 5 minutes or less
In this introduction, you discuss with your students that they will be exploring how your eyes
work, and how your brain makes sense of the world
BRIEFLY describe that we will dissect an eye, clean up, and then explore how our eyes see.
We will be using different delicate scientific instruments to further explore eyes
Activity 1 Eye Dissection
Optional Eye Dissection: (30 minutes including clean-up)
 Students must wear gloves and goggles for the entire dissection. Remind the students that
they must keep their gloves on, and unless a glove is defective, or if they cut or tear it during
the dissection, it will not be replaced.
 Identify the parts of the outside eye. Ask students to label each structure, sclera, cornea, iris,
pupil, fat, muscle, and nerve cord (see diagram).
 Using the sharp point of the scissors, poke into the eye at the cornea/sclera interface and cut
all around. The cornea should lift off the eye.
 Directly under the cornea is the aqueous humor – a jelly-like liquid that keeps the shape of
the cornea. Remove the aqueous humor.
 Direct the students to remove the iris – the color part of the eye. Note that the pupil is a hole.
 Under the iris is the lens, a pearl-like structure. This anchored by muscles that can contract
and stretch to focus near and far. Hold up the lens and look through it. What do you see?
17







Squeeze the eyeball, and a glob of jelly-like material called the vitreous humor. It should
squeeze out with a glop. This also helps to keep the shape of the eye. Gross and cool!
Ask the students to hold the eye and catch light on the interior surface. What do they see?
(rainbow colors) This is the tapetum. Humans do not have a tapetum, many mammals do. It
helps the animal to collect light when it is dark out. That is why your cat’s eyes will glow at
night. It is the light reflecting off the tapetum.
You will need to cut the eye open to get back to the tapetum and retina. The easiest way is to
have the students cut all around the eye, exposing the interior.
Ask students to remove the tapetum with their tweezers.
Ask the students to locate the optic nerve on the inside and outside of the eye.
Ask student to find a small depression at the back of the eye (it will be direct line from the
cornea through the pupil). This is the macula. Within the macula is the fovea, the area of the
highest concentration of cones.
Ask students to take all of the eye material (cornea, vitreous humor, aqueous humor, lens,
iris, tapetum, fat, and eyeball halves) and put them in the garbage bag you will take with you.
Ask students to take all of their equipment (scissors, tweezers, and 2 dissection probes) and
clean those with soap and water. Ask the students to take their dissection trays and clean the
pad and the tray separately.
Activity 2 Visual Perception Stations
Stations (50 minutes, 5min/station with eye dissection OR 80 minutes, 8min/station )
There are 10 stations. Students work with a partner, and they will have 5 (or 8) minutes per
station. If any station seems more popular than the others, stand by it and move students along.
Students collect data. Let the students know that you will be timing them, 5 (or 8) minutes per
station, but if they finish early, they can move ahead a station if it is unoccupied. They must
move quickly through each one. If they finish early, there are additional optical illusion cards for
them while they are waiting. Be sure to direct that the students take turns. For example on the
Depth Perception, one student tests the right eye with a black background. The partners switch
and the other person tests their right eye with a black background. That way, even if the students
don’t have enough time to complete this station, both partners had the opportunity to explore it.
The Visual Perception stations are part of a kit produced by Carolina Biological. Directions are
scanned and included here. When I present station classes, I go to each station, pick up the
materials as I tell the students what to do at that particular station. In addition, place the
directions at each station. Some of your students will have a difficult time listening, and won’t
know what to do when you let them get started. Hopefully, it won’t be the entire class!
Position yourself close to the Depth perception and Visual Mapping stations. These are more
complex directions, and may need additional assistance for the student to understand what to do.
18
19
20
21
22
NOTE:
Only use the cards with the, (on the back of the
card) “Are there gray spots at the
intersections?” illusion at this station. The
cards from the third deck, “How many circles
do you see?” can be disbursed throughout the
room. If students finish early, they can spend
some time with these.
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24
25
26
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28
back
front
front
back
29
30
Wrap-Up
(2-5 minutes)
Pick one of the stations, and discuss the results especially if all the students were able to
complete one of them. If possible, collect class data, and find the average. For example on the
Visual Acuity (station 1), you can ask, “Who has vision better than the class average? Who has
vision worse than the class average? Is the class average high or lower than 20/20?”
The students love to discuss the optical illusions, and you can always talk about what we see and
our brain having trouble with that, so it “makes a choice” as to what it will see.
31
32
The Five Senses
Learned Behavior
Our brain interprets the information that our five senses collect. In this first activity, divide
students into teams of two. One member solves the maze while the other member times it. The
two switch places. Repeat this three times, recording how long it takes each time. Go on to
other activities, and at the end of the lesson, repeat this activity. Did the students time go down,
remain the same, or improve between the beginning of the class and after 30 or 45 minutes?
Repeat this activity in one week and record the time.
Repeat this activity in one month, and record the time.
33
34
Do not turn over the directions until told.
First, answer the first question below:
Do not turn over the directions until told.
First, answer the first question below:
How fast do you think you can solve this maze?
How fast do you think you can solve this maze?
___________________________________
___________________________________
If you use the classroom clock:
Ending Time:
________________
Ending Time:
________________
Starting Time:
________________
Starting Time:
________________
How fast?
________________
How fast?
________________
35
Reaction and Touch
Take action to time your reaction!
Materials Needed




ruler with centimeter marks
table
paper and pen for charting results
a different colored sticker or pen for each person
Instructions
Has anyone ever said, "Think fast!" and then thrown something at you? How quickly or slowly
you react is called your reaction time.
1. To measure your reaction time, ask a friend to help.
2. You will need the reaction time chart below.
3. Then draw a graph to record your results. Along the left side of the paper (the y axis)
write the times from the reaction time chart in separate rows. Across the bottom of the
paper (the x axis) write "Trial 1", "Trial 2", and "Trial 3" in three separate columns. You
will record each other's reaction times on this graph to compare them when you finish
testing.
4. Now, sit in a chair with your arm resting on a table so that your wrist hangs off the edge.
5. Your friend will hold the ruler so that it dangles above your hand. Make sure the end of
the ruler is hanging between your thumb and finger.
6. When your friend lets go of the ruler, try to catch it between your thumb and finger as
quickly as you can.
7. Compare the marking on the ruler where your
fingers caught it to the reaction time chart.
Your reaction time is how long it took for
your eyes to tell your brain that the ruler was
falling and then for your brain to tell your
fingers to catch it.
8. Make a mark on your graph next to the
matching reaction time over the Trial 1
column.
9. Try catching the ruler twice more, marking the results on your graph each time. Give
your friend (or friends) a chance to test their reaction times.
Who has the best reaction time? Do your reaction times improve with practice? Did your
reaction times vary a lot or were they pretty much the same from trial to trial? Are older kids
faster than younger kids? How about your parents?
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