Transcript After Images

After Images
• In the retina of your eyes, there are 3 types of color
receptors (cones) that are most sensitive to either red,
blue or green.
• When you stare at a particular color for too long, these
receptors get "tired" or "fatigued.“
• When you then look at the white background, the
receptors that are tired do not work as well. The
information from all of the different color receptors is
not in balance and you see the color "afterimages."
• As you probably noticed, you vision quickly returns to
normal.
http://faculty.washington.edu/chudler/after.html
Ocular Dominance
• Ocular dominance, sometimes called eye dominance or
eyedness is the tendency to prefer visual input from one eye to
the other. It does not necessarily correspond to handedness.
• Approximately two-thirds of the population is right-eye
dominant and one-third left-eye dominant; however in a small
portion of the population neither eye is dominant.
• Dominance does appear to change depending upon direction of
gaze due to image size changes on the retinas.
• In normal binocular there is an effect of parallax (difference in
apparent position for objects viewed along to lines of site), and
therefore the dominant eye is the one that is primarily relied on
for precise positional information. This may be especially
important in sports which require aim, such as archery, darts or
shooting sports.
Depth Perception
• Depth perception allows us to see the world in 3D or estimate the
distance between people/objects in all directions
• Accurate depth perception relies on stereoscopic vision or
stereopsis.
• The primary ways we determine depth are:
– The size a known object has on your retina - Knowing the size of an object
due to previous experience helps our brains calculate the distance based on
the size of the object on the retina.
– Moving parallax – Stand face to face with someone and move your head
side to side. The person in front of you moves quickly across your retina, but
the objects that are further away don’t move very much at all. This helps
your brain calculate how far or close something is from you.
– Stereo vision - Since our eyes are about two inches apart, each eye receives
a different image of an object on its retina, especially when an object is close
up. When the object is far away, this method doesn’t work as well since
objects appear more identical when further away from your eyes.
Monocular Cues of Depth
• Interposition – Interposition cues occur
when there is an overlapping of objects
• Linear perspective – When objects of
known distance appear smaller and
smaller, it’s interpreted as these objects
being further away.
• Aerial perspective - The relative color
and contrast of objects gives us clues to
their distance. When scattering light
blurs the outlines of objects, the object
is perceived as distant.
• Light and shade - Shadows and
highlights can provide information
about an object’s depth and dimensions.
• Monocular movement parallax - When
our heads move side to side, objects at
different distances move at different
speeds, or relative velocity. Closer
objects move in the opposite directions
of the head movement, and farther
objects move with our heads.
Interdependence of Taste & Smell
• It is estimated that seventy to seventy-five
percent of what we perceive as taste actually
comes from our sense of smell.
• Taste buds allow us to perceive only bitter,
salty, sweet, and sour flavors. It’s the odor
molecules from food that give us most of our
taste sensation.
• When you put food in your mouth, odor
molecules from that food travel through the
passage between your nose and mouth to
olfactory receptor cells at the top of your nasal
cavity.
Importance of Olfaction
• Smell is our most primal. Animals rely on this sense to
survive.
• Although a blind rat might survive, a rat without its sense
of smell can’t mate or find food. Smells can also signal
danger, fear, or dread.
• Although humans don’t rely on smell for mating or finding
food, the sense of smell communicates many of the
pleasures in life--the aroma of a apple pie in the oven,
fresh-cut grass, a flower garden.
• Smell is actually a very complex sense. To identify the
smell of a flower, the brain analyzes over 300 odor
molecules. The average person can discriminate between
4,000 to 10,000 different odor molecules.
Tongue Map
• Although taste buds are
concentrated along the
perimeter of your tongue and
some chemoreceptors are
more sensitive to one taste
over another, they are not
clustered by sensitivity into
specific regions
Thermoreceptors: Relative Temp
• When you put your finger into cold water, cold receptors
depolarize quickly, then adapt to a steady state level which is
still more depolarized than the steady-state. Warm receptors
do the opposite: hyperpolarize quickly, then adapt to a
slightly hyperpolarized state.
• When you move your finger to cold to warm water, cold
receptors (which are already slightly depolarized), don't
respond very strongly. Warm receptors do, and the response
is stronger than normal, because they are slightly
hyperpolarized. The brain perceives the warm water as hot
because it is receiving more information from hot receptors
than from cold.
Thermoreceptors: Density
• Cold receptors start to perceive cold sensations when the
surface of the skin drops below 95 º F. They are most
stimulated when the surface of the skin is between 77 º
F and 41 º F . The receptors stop firing below 41 º F,
which accounts for the numbness experienced when
subject to cold temperature for extensive periods of time.
•
• Hot receptors start to perceive hot sensations when the
temperature of the skin’s surface is between 86 º F and
113 º F. Beyond 113 º F, pain receptors take over to avoid
damage being done to the skin and underlying tissues.
• Although thermoreceptors are found all over the body,
cold receptors are found in greater density than hot
receptors. The highest concentration of thermoreceptors
is in the face and ears.
Blind Spot
• Your blind spot is located on top of your optic nerve inside
your eye. The optic nerve enters the back of your eye and
spreads nerve fibers onto the back of the eye to make up
there retina.
• The small round spot where this cable enters the back of
your eye is called the optic nerve head or optic disc. There
are no light-detecting cells on this disc. As a result, you have
a very small gap in the visual field of each eye.
• Each of your eyes has a visual field that overlaps with that
of the other to compensate for your blind spot.
• The brain is very efficient at using the information from the
other eye and other spatial information to "fill in" missing
information. In the absence of binocular input, the blind
spot can be detected.
Near Point
• Near point is the shortest distance from your eye that an
object is in focus.
• The ciliary body and elasticity of the lens allow the lens to
take on a more spherical shape to focus on close objects.
• With age, the eye gradually loses it’s ability to focus on
(presbyopia). This loss is believed to be due to changes in
the lenses curvature with continued growth, natural loss of
the lenses flexibility and progressive weakening of the
ciliary muscles.
• To get an idea of how these changes affect near-point,
consider the following average measures of near-point in
inches.
– 3.5” @ age 20
– 6.8” @ age 40
– 33” @ age 6
Visual Acuity – The Snellen Chart
• The letters on an acuity chart are formally known
as “opotypes".
• In the traditional Snellen chart, the optotypes have
the appearance of block letters, and are intended
to be seen and read as letters. However, letters
from any ordinary typographer's font. They have a
particular, simple geometry in which:
– the thickness of the lines equals the thickness of the
white spaces between lines and the thickness of the gap
in the letter "C“
– the height and width of the optotype is five times the
thickness of the line.
• Only ten C, D, E, F, L, N, O, P, T, Z are used in the
traditional Snellen chart.
Visual Acuity – The Snellen Test
• In the common acuity test, as we performed in lab, a Snellen chart is
placed at a standard distance: 20 ft in the US, or 6 metres in the rest
of the world.
• At this distance, the symbols on the line, designated 20/20 (US) or
6/6 (rest of world), is the smallest line that a person with normal
acuity can read at a distance of 20 ft.
• Three lines above, the letters have twice the dimensions of those on
the 20/20 line. The chart is at a distance of 20 ft, but a person with
normal acuity could be expected to read these letters at a distance of
40 ft. This line is designated by the ratio 20/40.
• If this is the smallest line a person can read, the person's acuity is
"20/40," meaning in a very generalized sense that this person needs
to approach to a distance of 20 ft to read letters that a person with
normal acuity could read at 40 ft.
• This person could be said to have "half" the normal acuity.
Astigmatism
• Astigmatism is a refractive error, which results in blurred
vision.
• With astigmatism, the cornea is abnormally curved, altering the
way light is bent (refracted) and focused on the retina. Since
the light rays are not clearly focused, vision is blurred.
• Astigmatism makes it difficult to see fine details, either close up
or from a distance.
• Mild astigmatism may not need to be corrected.
• The cause of astigmatism is unknown, however it is usually
present from birth, and often occurs together with
nearsightedness or farsightedness. It sometimes occurs after
certain types of eye surgery, such as cataract surgery.
• Some degree of astigmatism is actually very common. Glasses
or contact lenses will correct astigmatism, but do not cure it.
• Laser surgery can help change the shape of the cornea surface
to eliminate astigmatism, along with nearsightedness or
farsightedness.
Accommodation to Light Intensity
• The pupillary light reflex is the well-known response
in which the pupils constrict in bright light.
• If an equal amount of light shines into both eyes, the
degree of constriction is generally equal.
• If the light is directed primarily into one eye the pupil
of that eye greatly constricts (direct reflex) while the
pupil of the other eye shows a much smaller degree of
constriction (consensual reflex).
Colorblindness
• People think that colorblindness means a person only sees
black and white, but this is a big misconception. It is
extremely rare to be totally color blind (monochromasy)
• There are actually many different types and degrees of
colorblindness - more correctly called color vision
deficiencies
• 5% to 8% of the men and 0.5% of the women of the world
are born colorblind.
• 99% of these color-blind people are either protans (red
weak) and deutans (green weak). These are categorized as
dichromasy because 2 or the three cones function normally .
Protanomoly
• Protanomaly is also known as "redweakness“.
• Any redness seen in a color by a
normal observer is seen more weakly
by the protanomalous viewer, both in
terms of its "coloring power"
(saturation, or depth of color) and its
brightness.
• Red, orange, yellow, and yellow-green
appear somewhat shifted towards
green, and appear paler than they do
to the normal observer.
Deuteranomoly
• The deuteranomalous person is
considered "green weak".
• Similar to the protanomalous
person, they are poor at
discriminating small differences
in hues in the red, orange, yellow,
green region of the spectrum.
• Hues in this region because
appear somewhat shifted
towards red.
Normal vs. Protanope vs. Dueteranope