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 "red-

weakness“. • 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 Protanope Deutanope

Normal vs. Protanope vs. Dueteranope