4.2—the visual system

Question 1

4.2 Learning Objectives

Question 2

4.2 Focus Questions

Question 3

How the Eye Gathers Light

Answer 3

• wavelength: the distance between peaks of a wave; long wavelengths correspond to our perception of reddish colours and short wavelengths correspond to our perception of bluish colours.
  
  • if a large portion of the lightwaves are clustered around one wavelength, you’ll see an intense, vivid colour.
  • if there are a large variety of wavelengths being viewed at the same time, the colour wlil appear washed out.
• amplitude: the height of a wave; low amplitude waves are seen as dim colours, whereas high-amplutide waves are seen as bright colours.

Question 4

The Structure of the Eye (Image)

Answer 4

Question 5

Sclera

Answer 5

the white, outer, surface of the eye.

Question 6

Cornea

Answer 6

is the clear layer that covers the front portion of eye and also contributes to the eye’s ability to focus.

Question 7

Pupil

Answer 7

regulates the amount of light that enters by changing its size; it dilates (expands) to allow more light to enter and constricts (shrinks) to allow less light into the eye.

Question 8

Iris

Answer 8

a round muscle that adjusts the size of the pupil; it also gives the eyes their characteristic colour.

Question 9

Lens

Answer 9

• lens: a clear structure that focuses light onto the back of the eye.
• accomodation: the lens changing its shape to ensure that the light entering the eye is refracted in such a way that it is focused when it reaches the back of the eye.
• transduction: when light reaches the back of the eye, it stimulates a layer of specialized receptors that convert light into a message that the brain can then interpret.

Question 10

Retina

Answer 10

lines the inner surface of the eye and con- sists of specialized receptors that absorb light and send signals related to the properties of light to the brain.

Question 11

Photoreceptors

Answer 11

• photoreceptors: where light will be transformed into a neural signal that the brain can understand.
• having the photoreceptors wedged into the back of the eye protects them and provides them with a constant blood supply, both of which are useful to your ability to see.
• ganglion cells: gather information from the photoreceptors, which will alter the rate at which the ganglion cells fire; they then send information out of the brain through the optic nerve.

Question 12

Optic Nerve

Answer 12

• optic nerve: a dense bundle of fibres that connect to the brain.
• optic disc: an area on the retina with no photoreceptors.
• blind spot: a space in the retina that lacks photorecepters (as a result of the optic disk).
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Question 13

Rods

Answer 13

• rods: photoreceptors that occupy peripheral regions of the retina; they are highly sensitive under low light levels.
• limited to the periphery of the retina.
• have a ten-to-one ratio with ganglion cells.
• explains why colourful stimuli are often perceived as sharp images while shadowy grey images are perceived as being hazy or unclear.

Question 14

Cones

Answer 14

• cones: photoreceptors that are sensitive to different wavelengths of light that we perceive as colour.
• cones tend to be clustered around the fovea.
• have a one-to-one ratio with ganglion cells.

Question 15

Fovea

Answer 15

the central region of the retina.

Question 16

Dark Adaptation

Answer 16

is the process by which the rods and cones become increasingly sensitive to light under low levels of illumination.

Question 17

Trichromatic Theory (Young-Helmholtz Theory)

Answer 17

maintains that colour vision is determined by three different cone types that are sensitive to short, medium, and long wavelengths of light.

Question 18

Opponent-Process Theory

Answer 18

• opponent-process theory: we percieve colour in terms of opposing pairs: red to green, yellow to blue, and white to black.
• a cell that is stimulated by red is inhibited by green; when red is no longer perceived (as when you suddently look at a white wall), a “rebound” effect occurs.

Question 19

Common Visual Disorders

Answer 19

• nearsightedness: (or myopia) occurs when the eyeball is slightly elongated, causing the image that the cornea and lens focus on to fall short of the retina.
• farsightedness: (or hyperopia) occurs when the length of the eye is shorter than normal, causing the image to be focused behind the retina.

Question 20

Optic Chiasm

Answer 20

• optic chiasm: the point at which the optic nerves cross at the midline of the brain.
• ipsilateral: half of the nerve fibres travel to the same side of brain.
• contralateral: half ot the nerve fibres travel to the opposite side of the brain.
• the left half of your visual field is initially processed by the right hemisphere of your brain, whereas the right half of your visual field is initially processed by the left hemisphere of your brain.
• having both eyes send some information to both hemispheres increases the likelihood that some visual abilities will be preserved.
• lateral geniculate nucleus (LGN): specialized for processing visual information; fibres from this nucleus send messages to the visual cortex.
• feature detection cells: these cells respond selectively to simple and specific aspects of a stimulus, such as angles and edges.
  
  • researchers can map the areas feature detection cells respond by measuring the firing rates of groups of neurons in the visual cortex in lab animals.

Question 21

The Ventral Stream

Answer 21

• the ventral stream of vision extends from the visual cortex in the occipital lobe to the anterior (front) portions.
• the division of our visual system serves for object recognition.
• prosopagnosia: face blindness; individuals are able to recognize voices and other defining features of individuals, but not faces.
• perceptual constancy: the ability to perceive objects as having constant shape, size, colour despite changes in perspective.
  
  • shape constancy: we judge the angle of the object relative to our position.
  • colour constancy: allows us to reocgnize an object’s colour under varying levels of illumination.

Question 22

The Dorsal Stream

Answer 22

• the dorsal stream of vision extends from the visual cortex in our occipital lobe upwards of the parietal lobe.
• it locates an object in space and allows you to interact with it.
• e.g. you need the dorsal stream to recognize that the object in front of you is a cup, the liquid inside is caffeine, and that you can drink it. you decide you want to have a sip, and thus require your arm to move your hand to grasp the mug.

Question 23

Binocular Depth Cues

Answer 23

are distance cues that are based on the differing perspectives of both eyes.

Question 24

Convergence

Answer 24

occurs when the eye muscles contractso that both eyes focus on a single object.

Question 25

Retinal Disparity

Answer 25

• retinal disparity: the difference in relative position of an object as seen by both eyes, which provides information to the brain about depth.
• stereoscopic vision: overlapping fields of vision, which most primates have.

Question 26

Monocular Cues

Answer 26

• monocular cues: are depth cues that we can perceive with only one eye.
• one is accommodation.
• motion parallax: used when you or your surroundings are in motion.