Block 2 - Visual perception - Lecture 1 - The eye

Question 1

Human eye anatomy?

Answer 1

look at a diagram
- Light reflected from objects in the environment enter the eye through the pupil and is focused by the cornea to form sharp images of the object on the retina which contains the receptors for vision
- There are 2 kinds of visual receptors: rods and cones

Question 2

What is muscular degeneration?

Answer 2

• deterioration of the retina in the macula (fovea)
• the fovea contains only cones
• the peripheral retina contains both rods and cones (more rods)
• irreversible blindness, loss of central vision

Question 3

What is light?

Answer 3

• electromagnetic energy
• measured in wavelengths
• light is the means that enable us to perceive objects
• vision is based on visible light

Question 4

What are the 2 transformations of the visual process?

Answer 4

1. light waves -> retinal object representation
2. retinal object representation -> electrical signal

Question 5

1st transformation?

Answer 5

• the transformation of light (reflected by an object) into a retinal image (retinal object representation)
• the level of light refraction depends on the thickness of the lens

Question 6

What is accommodation?

Answer 6

• process by which the eye changes optical power to focus on an object as its distance varies
• prevents a person from seeing an object as blurred
• enables you to bring both near and far objects into focus

Question 7

What do ciliary muscles do?

Answer 7

relax/ contract to change the thickness of the lens (bends light to fall onto the fovea)

Question 8

What is the far point?

Answer 8

the maximum distance of an object from the eye for which a clear image of the object can be seen

Question 9

What is the near point?

Answer 9

the minimum distance of an object from the eye for which a clear image of the object can be seen, increases with age

Question 10

What happens during accommodation to a far object?

Answer 10

• ciliary muscles relax
• slim lens (little curvature, little focusing power)
• light is bent only a little

Question 11

What happens during accommodation to a near object?

Answer 11

• ciliary muscles contract
• thick lens (strong curvature, strong focusing power)
• light is bent a lot

Question 12

What is Myopia (nearsightedness)?

Answer 12

• a common visual problem
• far objects are out of focus because the lens is too thick or the eyeball is too long
• requires concave correction lenses
• concave correction to diverge light before it enters the eye

Question 13

What is Hyperopia (farsighted)?

Answer 13

• a common visual problem
• near objects are out of focus usually because the eyeball is too short (lens cannot get any thicker)
• requires convex correction lenses
• convex correction to converge light before it enters the eye

Question 14

What is transduction?

Answer 14

• when the retinal image is transformed into electrical energy
• is carried out by receptors

Question 15

What are the 2 photo receptors?

Answer 15

1. rods
2. cones

Question 16

What are the 7 differences between rods and cones?

Answer 16

1. absolute no.
2. density and distribution in the retina
3. ability to dark adaptation
4. absolute sensitivity
5. acuity
6. spectral sensitivity
7. colour vision

Question 17

Differences in the no. of rods and cones?

Answer 17

• there are 120 million rods and 6 million cones in each human eye

Question 18

Differences in the density and distribution of rods and cones?

Answer 18

• the density of rods is much greater than the density of cones

Question 19

Differences in dark adaptation between rods and cones?

Answer 19

• cones = light adapted + operate at high luminance
• rods = dark adapted + operate at low luminance
• dark adaptation (ability to detect a light in the dark) is achieved by rods because they are more sensitive to light

Question 20

What is the difference between light sensitivity in rods and cones?

Answer 20

• is higher in rods than cones

Question 21

What is the difference between visual acuity between rods and cones?

Answer 21

• is better with cone than rod vision

Question 22

What is the difference between spectral sensitivity between rods and cones?

Answer 22

• rods are more sensitive to shorter wavelengths
• cones are more sensitive to longer wavelengths
• purkinje shift = increased sensitivity to short wavelengths in dark-adapted eye

Question 23

What is the difference between colour vision in rods and cones?

Answer 23

• rod vision = no colour sensation
• cone vision:
  -> S cones = respond to short wavelengths = blue
  -> M cones = respond to medium wavelengths = green
  -> L cones = respond to long wavelengths = red

Question 24

What are the 2 types of vision?

Answer 24

1. photopic vision
2. scotopic vision

Question 25

What is photopic vision?

Answer 25

• cone-dominated
• foveal and peripheral vision
• light-adapted vision
• high visual acuity
• most sensitive to long wavelengths
• basis of colour vision

Question 26

What is scotopic vision?

Answer 26

• rod-dominated
• peripheral vision
• dark-adapted vision
• low visual acuity
• sensitive to short wavelengths
• no colour sensation

Question 27

What is mesonic vision?

Answer 27

rod and cone vision together

Question 28

What is neural convergence?

Answer 28

• occurs when 1 neuron receives signals from many other neurons
• cones have better visual acuity because they have less convergence