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Vision

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on 3 October 2013

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Transcript of Vision

Vision
Sandra, Jasmine, Diana, Gabriela, Claudia, Erika
Human eye is capable of responding to an enormous range of light intensity, exceeding 10 units on logarithmic scale. Inevitably, eye response to the signal intensity, which determines its apparent intensity, or brightness, is not linear. That is, it is not determined by the nominal change in physical stimulus (light energy), rather by its change relative to its initial level.
In general, there is a minimum required change in signal intensity needed to produce change in sensation, and the latter is not necessarily proportional to the former. It was the father of photometry, Pierre Bouguer, who in 1760 first noted that the threshold visibility of a shadow on illuminated background is not determined by the nominal differential in their illumination level, but on the ratio between the two intensities. In other words, that eye brightness response is not proportional to light's nominal (physical) intensity, but proportional to its intensity level. This threshold ratio, which he found to be 1/64 (around 1.5%) did not change with the change of intensity level.
The Vision Process
Vision Accommodation
Color Vision
The normal ability to see any color in the spectrum of visible light.
Intensity
Intensity (energy) of the light waves translates into brightness.
The intensity of a wave of light affects the amplitude (height) of a wave.
Amplitude
the total accommodative power of the eye, determined by the difference between the refractive power for farthest vision and that for nearest vision.
Brightness of Light
How much light our eyes can stand without damaging it.
Accommodation and convergence allow us to see objects clearly both near and far without diplopia (double vision).
The light then progresses through the
pupil
, the circular opening in the center of the colored
iris
.
Accommodation
Light waves from an object enter the eye first through the
cornea
which is the clear dome at the front of the eye.
Trichromatic Theory
Created in the 1800s
Based on the premise that there are 3 classes of cone receptors sub serving color vision.
It is possible to match all of the colors in the visible spectrum by appropriate mixing of the 3 primary colors.
Color Blindness
Monochromacy
Complete color blindness, colors can be differentiated only on the basis of brightness.
Caused by the total absence of either
two
or
three
of the pigmented retinal cones.
Rod monochromacy:
Typical or complete achromatopsia(seeing in
black/white
and shades of
gray
.), total color blindness, day blindness.

May result in completely unable to distinguish colors, reduction of visual acuity, sever light sensitivity, and involuntary eye movements.
Blue-cone monochromacy:
S-cone monochromacy, incomplete achromatopsia.

Caused by loss or rearrangement of the genes encoding L and M cones.

Means only blue color receptors and rods are transmitting color/brightness info.
Cone monochromacy:
Complete achromatopsia with normal visual acuity.

Condition of having only one single type of cone in the retina.

Categorized
as blue cone(S-cone), red cone(M-cone), and green cone(L-cone) monochromacy
Cerebral achromatopsia
Typical achromatopsia.

Thought that the color receptors are working properly but the info is not reaching the brain/not being processed. Not inherited but an acquired color vision defect, may be caused by trauma or illness.
Dichromacy
Does not have or have weak cone cells in the retina. Unable to recognize any two of these colors: blue, green or red.
Protanopia
(unable to spot red colors)

Also have difficulty in spotting green colors, with a combination of blue and

green they are unable to tell them apart. (Color usually appears grey to

them.)
Deuteranopia
(unable to spot green color)

When a mixture of green and red is presented to them they are unable to

spot either one of them. Singly can spot red.
Tritanopia
(unable to spot green or red shades)

Unable to distinguish between yellow and blue colors. Do not have

difficulty in recognizing colors that relate to green and red shades.

Tritanopia is hereditary and is a rare disease.
Anomalous Trichromacy
Impairment of normal color vision (not complete loss).

Occurs when one of the cones is altered in its spectral sensitivity, can be red/green or blue/yellow.
Protanomacy
“red weakness”, less depth of color and brightness.
Deuteranomaly
Green sensitive cones have decreased sensitivity.

An x-linked trait, disorder caused by a gene located on the x chromosome.

Person has all three retinal pigments in the cones, but the sensitivity of the green-sensitive cones is decreased.
Tritanomaly
Rare type of anomalous trichromatic vision, which the third blue-sensitive cones have decreased sensitivity.

Red and green are unaffected, difficulty
distinguishing between yellow and blue.
Retina
is a light-sensitive layer at the back of the eye that covers about 65 percent of its interior surface.
Photosensitive cells in the retina convert incident light energy into signals that are carried to the brain by the
optic nerve
.
Rods and Cones
Fovea
It is located in the middle of the retina. It is the center of the eye's sharpest vision and the location of most color perception.
Small and highly sensitive part of the retina responsible for detailed central vision. Helps us appreciate detail and perform tasks like reading.


Macula
Optic Nerve
Is the cable of nerve fibers with carries the electrical signals from the retina to the brain for processing.

The point of departure of that optic nerve through the retina does not have any rods or cones, and thus produces a "blind spot".
The Difference
Afterimages
Type of optical illusion in which an image continues to appear briefly even after exposure to the actual image has ended.
Two major types of afterimages: Positive + Negative Images.
Opponent - Process Theory
Suggests that color perception is controlled by the activity of 2 opponent systems; a blue-yellow mechanism + a red-green mechanism.
Process of excitory + inhibitory the 2 components of a mechanism oppose each other.
Red creates positive (excitory) and Green creates negative (inhibitory.)
Rods see in black, white, and shades of gray and tell us the form or shape that something has.
Cones sense color and they need more light than rods to work well. Cones are most helpful in normal or bright light.
Lens
: Transparent tissue that bends light passing through the eye. To focus light, the lens can change shape by bending.
Positive
The colors of the original images are maintained. The afterimage looks the same as the original.
Negative
The colors you see are inverted from the original image. It can be explained by the opponent. Red image has a green after image.
Frequency of light
The amount of visible light our eyes can see.
It is measured in diopters.
Blind spot
Bipolar and Ganglion Cells
The
bipolar cells
act as the signal couriers between the photoreceptors that react to light stimuli.

The
ganglion cells
are responsible for transferring information from the eye to the brain.
It is located at the back of the eye right before the optic nerve. It is there so that the optic nerve fibers exit the back of the eye.
There are no receptors in this part of the retina.
Receptive field
It is the region of the retina where the action of light alters the firing of the neuron.
Lateral geniculate nucleus
It receives the visual information from the retina.
Optic Chiasm
The X-shaped structure formed at the point below the brain where the two optic nerves cross over each other.
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