The Visual System: essentials of Sight

Question 1

What is sensation?

Answer 1

Sensation is the stimulation of sense organs.

Question 2

What is perception?

Answer 2

Perception is the selection, organization, and interpretation of sensory input.

Question 3

For people to see, there must be light.. What is light?

Answer 3

Light is a form of electromagnetic radiation that travels as a wave, moving, naturally enough, at the speed of light.. Light waves vary in amplitude (height) and in wavelength (the distance between peaks). Amplitude affects mainly the perception of brightness, while wavelength affects mainly the perception of color.

Question 4

What is the lens?

Answer 4

The lens is a transparent eye structure that focuses the light rays falling on the
retina.

Question 5

What is accomodation?

Answer 5

Accommodation occurs when the curvature of the lens adjusts to alter visual focus.

Question 6

What is the iris?

Answer 6

The iris is the colored ring of muscle surrounding the pupil, or black center of the eye.

Question 7

What is the pupil?

Answer 7

The pupil is the opening in the center of the iris that helps regulate the amount of light passing into the rear chamber of the eye.

Question 8

What happens when the pupil contracts?

Answer 8

When the pupil constricts, it lets less light into the eye, but it sharpens the image falling on the retina.

Question 9

What happens when the pupil dilates?

Answer 9

When the pupil dilates (opens), it lets more light in, but the image is less sharp.

Question 10

What is the retina?

Answer 10

The retina is the neural tissue lining the inside back surface of the eye; it absorbs light, processes images, and sends visual information to the brain.

Question 11

What is the blind spot?

Answer 11

The axons that run from the retina to the brain converge at a single spot where they
exit the eye. At that point, all the fibers dive through a hole in the retina called the optic disk. Since the optic disk is a hole in the retina, you cannot see the part of an image that falls on it. It is therefore known as the blind spot.

Question 12

What are cones?

Answer 12

Cones are specialized visual receptors that play a key role in daylight vision and
color vision.

Question 13

What are rods?

Answer 13

Rods are specialized visual receptors that play a key role in night vision and peripheral vision.

Question 14

Are rods and cones equal in number.

Answer 14

Rods outnumber cones by a huge margin; humans have about 100 million rods, but only about 6 million cones

Question 15

What is the fovea?

Answer 15

The fovea is a tiny spot in the center of the retina that contains only cones; visual acuity is greatest at this spot

Question 16

What is the optic chiasm?

Answer 16

The optic chiasm, the point at which the axons from the inside half of each eye cross over and then project to the opposite half of the brain

Question 17

Describe the visual pathways through the brain.

Answer 17

1. Input from the right half of the visual field strikes the left side of each retina and is transmitted to the left hemisphere (shown in blue).
2. Input from the left half of the visual field strikes the right side of each retina and is transmitted to the right hemisphere (shown in red).
3. The nerve fibers from each eye meet at the optic chiasm, where fibers from the inside half of each retina cross over to the opposite side of the brain.
4. After reaching the optic chiasm, the major visual pathway projects through the lateral geniculate nucleus (LGN) in the thalamus and onto the primary visual cortex.
5. A second pathway detours through the superior colliculus and then projects through the thalamus and onto the primary visual cortex

Question 18

Where are visual signal processed?

Answer 18

Visual signals are processed in the LGN and then distributed to areas in the occipital lobe that make up the primary visual cortex.

Question 19

After visual input is processed in the primary visual cortex, it is often routed to other
cortical areas for additional processing. These signals travel through two streams thathave sometimes been characterized as the what and where pathways.

What does the ventral stream process?

Answer 19

The ventral stream processes the details of what objects are out there (the perception of form and color)

Question 20

After visual input is processed in the primary visual cortex, it is often routed to other
cortical areas for additional processing. These signals travel through two streams thathave sometimes been characterized as the what and where pathways.

What does the dorsal stream process?

Answer 20

the dorsal stream processes where the objects are (the perception of
motion and depth)

Question 21

What is subtractive colour mixing?

Answer 21

Subtractive color mixing works by removing some wavelengths of light, leaving
less light than was originally there.

Question 22

What is additive colour mixing?

Answer 22

Additive color mixing works by superimposing lights, putting more light in the
mixture than exists in any one light by itself.

Question 23

What is the trichromatic theory?

Answer 23

The trichromatic theory holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths.

Question 24

What does colour blindness encompass?

Answer 24

Color blindness encompasses a variety of deficiencies in the ability to distinguish
among colors.

Question 25

What are complimentary colours?

Answer 25

Complementary colors
are pairs of colors that produce gray tones when mixed together.

Question 26

What is an afterimage?

Answer 26

afterimage—
a visual image that persists after a stimulus is removed.

Question 27

What is the opponent process theory?

Answer 27

The opponent process theory holds that color perception depends on receptors that make antagonistic responses to three pairs of colors.

Question 28

What are the 3 pairs of opponent colours?

Answer 28

The three pairs of opponent colors he hypothesized were red versus
green, yellow versus blue, and black versus white.