Lecture 19- Vision 1

Question 1

Why are the special senses ‘special’?

Answer 1

Because they have a specialized organ associated with them

Question 2

What are four features of vision?

Answer 2

• Colour
• Shapes
• Depth
• Movement

Question 3

Where is sensation of vision located?

Answer 3

The brain (not eyes)

Question 4

In form is information about special senses travelling in the brain?

Answer 4

Action potentials (changes in membrane potentials)

Question 5

For the perception of vision what is the stimulus that results in action potentials? What is this process called?

Answer 5

Light. Light becoming APs in the brain is known as signal transduction.

Question 6

What is the small band of light that is visible to humans? Why is this the case?

Answer 6

Wavelengths of 400-750nm

Because we have special receptors to detect only these specific frequencies

Question 7

What are two features of light waves/ what do they tell us?

Answer 7

• Wavelength= Distance between two consecutive peaks or troughs (longer is red, shorter is violet).
• Amplitude= Height of peak. The larger the amplitude the brighter the stimulus and vice versa.

Question 8

What is the general role of the eye in vision?

Answer 8

Detects light and processes visual information. This ultimately results in visual perceptions in the brain and guides behaviour.

Question 9

What are the two basic components of the eye?

Answer 9

1. Optical component – collects and focuses light onto the plane of the
   retina
2. Neural component – converts light energy into patterned changes of
   membrane potential that the brain can decode to create visual
   perceptions

Question 10

What is the conjunctiva?

Answer 10

A thin layer of cells sits over top of cornea

Question 11

True or false tears form a layer at the surface of the eye that provides protection?

Answer 11

True

Question 12

What is the cornea? What are its important features?

Answer 12

• Sits over front of eye
• It is avascular (nutrients + getting rid of waste happens by diffusion instead of direct blood supply via vessels). This allows the front part of the eye to be transparent and thus light is not blocked.

Question 13

What is the Sclera? What is the cornea in relation to this?

Answer 13

• Whites of eyes, covers most of eye.
• The cornea is simply a modification of the sclera, it sits at the front of the eye and is transparent allow light to pass through.

Question 14

What is the role of the extraocular muscles?

Answer 14

Important in moving the eyes around in our skull (balance)

Question 15

What is the aqueous humour? What is a disease that can result?

Answer 15

-Aqueous humor is fluid filled chamber that maintains a specific pressure (15 millimoles of mercury).

-If the aqueous humor inflates this pressure level is distorted. Pressure too high=
Glaucoma.

Question 16

What is the pupil?

Answer 16

Simply a hole it allows us to see through to the back of the eye which is pigmented black explaining why we see a black circle.

Question 17

What is the lens?

Answer 17

• The lens is a fibrous capsule which contains a lot of crystalized lens cells.
• These are held in place by ciliary muscles and zonary fibers.
• It is transparent due to being avascular, so light can pass through.

Question 18

What part of the eye is ‘neural’? What is it’s function? What abnormality can occur here?

Answer 18

• Back of eye.
• Black choroid lining (seen via the pupil) absorbs all wavelengths of light (important!).
• In albino person not black so light hits back and reflects everywhere leading to visual problems.

Question 19

Where are the retinal ganglia in the eye?

Answer 19

Retina ganglia run around surface of retina and leave via the optic disc forming the optic nerve

Question 20

What is the optic disc?

Answer 20

Where there are no visual receptors: blind spot

Question 21

What is the fovea of the eye?

Answer 21

Forms depression in retina= higher visual acuity (strongest vision)

Question 22

What is refraction? What is it related to?

Answer 22

-Refraction is the “bending” of light as it passes from one substance to another
e.g., air to water
-Refraction is related to the difference between the refractive indices of
the two media, and to the curvature of the refractive surface

Question 23

Where is the majority of the eye’s refractive power located?

Answer 23

• At the cornea as this is where there is the most difference in refractive indices.
• The lens has power to change in order to find tune perception of light so the remainder of the refractive power is here.

Question 24

What unit is refractive power measured in? How is this done?

Answer 24

• Diopters.

- This is the reciprocal of focal length in meters e.g. . A 2 diopter object would convert rays 0.5m beyond the object.

Question 25

List the 3 processes that occur as part of the near response? What is the near response?

Answer 25

-Accommodation, constriction, convergence
-Near response= What happens when you shift your gaze from distant to near
objects

Question 26

What is accommodation?

Answer 26

• Contraction/relaxation of ciliary muscle to alter lens shape and change refractive power
• When looking at distant objects the ciliary muscle is relaxed due to low parasympathetic activity, so the zonular fibres are taut, and the lens is flattened.
• When gaze shifts to close objects parasympathetic activation of ciliary muscle increases, ciliary muscle contracts, tension removed from zonular fibres, lens become more spherical due to natural elasticity (eye’s total refractive power increases from about 60 up to about 75 diopters).

Question 27

Explain the problem symptom and correction for the following optic defects….
Hypermetropia?

Answer 27

• Problem= Misshapen (shortened) eyeball means light is focused beyond the plane of the retina.
• Symptom= Far-sighted (near objects appear blurry)
• Solution= Convex (convergent) lens

Question 28

Explain the problem symptom and correction for the following optic defects….
Myopia?

Answer 28

Problem= Mishapen (elongated) eyeball means that light is focused in front of the plane of the retina 
Symptom= Near-sighted (distant objects appear blurry)
Correction= Concave (divergent) lens

Question 29

Explain the problem symptom and correction for the following optic defects….
Astigmatism

Answer 29

• Problem=Aspherical curvature of the cornea/lens. Means that there is a different amount of refraction in different planes.
• Symptom= Objects appear blurry particularly at night
• Solution= Cylindrical lens

Question 30

Explain the problem symptom and correction for the following optic defects….
Presbyopia

Answer 30

• Problem=Loss of lens elasticity (old age)
• Symptom= Near objects appear blurry (near point recedes). Near point is where could hold and read clearly therefore older people often have to hold things at greater distance to see them.
• Solution= Convex (convergent) lens

Question 31

Explain the problem symptom and correction for the following optic defects….
Cataract?

Answer 31

Problem= Increase in lens opacity means no longer colorless and light can't pass through
Symptom= Loss of ability to accommodate
Solution= Surgery, and convex lens

Question 32

What term describes perfect eyesight?

Answer 32

Emmetropia

Question 33

True or false is the incidence of myopia increasing? And if so why?

Answer 33

• Yes risen from 20% to 90% of Asian 20 year olds
• Because of technology and also increase in studying means less time outdoors (being outdoors in the sun helps eyes develop)

Question 34

What is constriction if the pupil?

Answer 34

Improved depth of focus, fewer optical aberrations by excluding edges of lens

Question 35

What is convergence of the eyes?

Answer 35

Objects remain in register on corresponding parts of the two retinae

Question 36

What is the refractive power of the human eye?

Answer 36

Around 60 diopters (largely due to cornea, small input from the lens during accommodation/ near response)