Vision 090505113755 Phpapp02

COME TO YOUR SENSES
VISION

Vision
Light comes into our eye through an opening in the center of the iris called the pupil. The lens of the eye is adjustable and focuses the light and the cornea (which is not adjustable) also helps focus the light and it is then projected onto the retina at the rear surface of the eye. The rear surface of the eye is lined with visual receptors. The highest concentration of receptors specialized for particularly detailed vision is known as the fovea. The receptors send action potentials to the brain along the optic nerve. The point at which the optic nerve leaves the eye has no receptor cells and is called the blind spot. We cannot see anything with the part of the eye where the blind spot is. The message about vision is carried to the primary visual cortex in the occipital lobe of the brain.

ANATOMY OF THE EYE

Vision
The receptors that are located in the back of the eye in the retina include the bipolar cells which send their messages to ganglion cells, located closer to the center of the eye than the bipolar cells. The ganglion cells bind together and form the optic nerve. Another set of receptors are amacrine cells, and these send their messages to bipolar cells, other amacrine cells and to ganglion cells. Horizontal cells are also receptor cells, but they are cells that inhibit rather than excite.

Vision
In addition to the bipolar, ganglion, amacrine, and horizontal cells, the retina also is made up receptor cells called rods and cones. There are more rods at the edges of the retina, and they respond to faint light and seeing in the dark. There are more rods than cones in the brain, but they are smaller. Cones are clustered in and around the fovea and are more useful in bright light, and they are critical for color vision. Cones have a more direct route to the brain than rods do.

Rod (left) and cone (right)
Both rods and cones contain a chemical that releases energy when hit by light called photopigments. Photopigments have a sensitivity to different wavelengths of light. The different lengths of light waves are what allow us to perceive distinct colors.

Color Vision
For humans, the shortest light waves are seen as violet and light waves that get longer and longer are seen as blue, then green, then yellow, then orange, and the longest are seen as red.

Color Vision
There are three major theories about how it is that we see color and also shades of color because a neuron in the visual system can only vary the frequency of action potentials. A single action potential can’t respond to both brightness and color, for example. And we don’t have separate receptors for different colors.

Color Vision
The first theory is called thetrichomatic theory of color vision or the Young-Helmholtz theory (after the people who developed the theory). This theory suggests that we have three kinds of cones: cones that respond best to the short wavelength blue part of the visual color spectrum, cones that respond best to the medium wavelength green part of the visual color spectrum, and cones that respond best to the long wavelength red part of the visual color spectrum. We see different shades of color depending on how active cones sensitive to each particular color are because they are firing at the same time.

Color Vision
The second theory is called the opponent-process theory. This theory is based on the fact that if you stare at an image of one of the colors a specific cone is sensitive to for a minute or so and then look at a white surface or white piece of paper, the color should be replaced by its opposite. For example, in the picture below, if you stare at the red square for a time and look away, you should see a blue afterimage. This theory proposes that we see colors based on paired opposites: red versus green, yellow versus blue, and white versus black. This theory proposes that shades of color occur both because of the active firing of some cones but also a decrease in firing of cones sensitive to a different color.

Paired Opposites

Color Vision
The third theory is called the retinex theory. This theory explains the idea of color constancy, that is the ability to recognize the color of an object even when light changes. For example, if someone wearing a bright shirt walks into a tunnel where it is pretty dark, the shirt will look more gray than yellow, but we know the color of the shirt has not actually changed. Also, we see how bright an object is by comparing it with other objects. This theory suggests that the visual cortex compares information from many parts of the retina to decide the brightness and color of objects. This theory suggests that vision requires thinking about what we are seeing and having some experience with vision to be able to make judgments about objects, including what the object is, what color it is, and how bright it is.

Color Blindness
Some people don’t see all of the colors in the light spectrum because they have a condition known as color vision deficiency or color blindness. Color vision deficiency is caused by differences in our genes. Some people are missing one or two kinds of cones, and some have all three kinds of cones, but one kind of cone has some abnormalities.
The most common type of color blindness occurs because a person has the same kind of photopigments in their long wavelength and medium wavelength cones instead of different ones. This causes them to have trouble telling the difference between red and green.

Vision and the Brain
The optic nerve leaves the eye and forms the optic tract to the brain. At a place called the optic chiasm, the optic nerve sends part of its signal to the same side of the brain and part of its signal to the opposite side of the brain. Before the signal from the optic nerve gets to the brain, the thalamus and some other brain areas below the cerebral cortex help route the information to the primary visual cortex in the occipital lobe of the brain.

The primary visual cortex manages the first stage of visual processing and is responsible for most of the visual information of which we are consciously aware. Some other areas of the cortex process visual information about shape, other areas about movement, and still other areas about brightness and color. Areas in the temporal cortex of the brain process information about what something is and where something is that we are looking at. All of these areas of the brain work together so that we see a complete object when we are looking at it, even if different information about the object is processed in different places in our brains.

Optic chiasm and route of visual information to the brain

Vision 090505113755 Phpapp02

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