Physiology of Vision

Question 1

What two parts of the eye focus an image onto the retina?

Answer 1

Cornea and Lens

Question 2

What can alter the focus of an image?

Answer 2

The shape of the lens

Question 3

Where in the eye is unwanted light absorbed?

Answer 3

A pigment layer behind the retina

Question 4

What can be used to view inside the eye of a patient, and what can be seen?

Answer 4

Ophthalmoscope

Optic disc and fovea can be seen

Question 5

What is the optic disc?

Answer 5

Where the optic nerve leaves the eye and also where blood vessels enter/leave the retina.

Question 6

What ratio of refraction is done by the different parts of the eye?

Answer 6

• 1/3 by the lens

- 2/3 by the cornea

Question 7

What are refractive errors?

Answer 7

Problems with focusing

Question 8

What are the two types of refractive errors?

Answer 8

Hypermetropia and Myopia

Question 9

What is hypermetropia?

Answer 9

Long-sightedness which occurs when the eyeball is either too short or the lens system is too weak.

Question 10

What is myopia?

Answer 10

Short-sightedness which occurs when the eyeball is either too long or too strong.

Question 11

How can refractive errors be corrected?

Answer 11

Via spectacles

Question 12

What is refractive power measured in?

Answer 12

Diopters

Question 13

How is refractive power (diopters) calculated?

Answer 13

Reciprocal of focal length (1 / focal length)

Question 14

How is the retina structured?

Answer 14

Back to front so ganglion cells and blood vessels are in the light path to the photoreceptors (except in the fovea)

Question 15

How many rods and cones are there, and what type of cells are they?

Answer 15

120 million rods and 5 millions cones

They are receptors

Question 16

What is the difference between rods and cones?

Answer 16

• Rods detect dim light

- Cones detect bright light and colours

Question 17

How many types go cones are there?

Answer 17

3

Question 18

What is the optic radiation?

Answer 18

What carries the output of the lateral geniculate nuclei to the striate cortex

Question 19

What are the direct layers of the retina?

Answer 19

• Receptors
• Bipolars
• Ganglion cells

Question 20

What are the transverse layers of the retina?

Answer 20

• Horizontal cells

- Amacrine cells

Question 21

What is the ganglion cell response?

Answer 21

Because ganglion cells respond weakly to changes in overall light intensity, they instead respond to local contrast (light on dark background or dark on light background).

Question 22

What does colourblindness result from?

Answer 22

A loss or modification of one (or more) of the three visual cone pigments.

Question 23

The genes for which colour pigments are on the X chromosome?

Answer 23

Green and Red

Question 24

The gene for the blue pigment is found in which chromosome?

Answer 24

Chromosome 7

Question 25

Is blue colourblindness more rare or less rare than red/green colourblindness, and why is this the case?

Answer 25

More rare because it is on chromosome 7 and there are two of them, whilst there is only one X chromosome

Question 26

What is central achromatopsia?

Answer 26

A much rarer form of colourblindness which is caused by damage to the cortical colour processing areas (V4)

Question 27

What does the optic nerve from each retina divide into?

Answer 27

Into right and left halves

Question 28

What is the optic chiasm?

Answer 28

X shaped structure where the two left halves and two right halves from the retinas combine to form optic tracts.

Question 29

What do the optic tracts relay into?

Answer 29

The lateral geniculate nuclei of the thalamus

Question 30

What do parts of each optic tract relay into?

Answer 30

Superior colliculus of the midbrain

Question 31

Where does the output of each geniculate nuclei go to, and what then happens?

Answer 31

The striate cortex, where the input from each half is combined.

Question 32

Where is the striate cortex located?

Answer 32

Occipital lobe

Question 33

What are the three overlapping patterns in the primary visual cortex?

Answer 33

1) Occular dominance columns 2) Oriental columns (which are smaller) 3) Colour blobs

Question 34

What is a hypercolumn?

Answer 34

An area of the primary visual cortex which contains a complete set of all three overlapping patterns

Question 35

What is scotoma?

Answer 35

A partial loss of vision (blind spot)

Question 36

What can cause scotoma?

Answer 36

- Lesion in the visual cortex - Retinal damage - Pressure from a tumour that is restricting the optic nerve, chiasm or tract

Question 37

What are the two streams in the cortex?

Answer 37

- Dorsal Stream: Goes from the occipital lobe to the parietal lobe and is concerned with location, motion and action - Ventral Stream: Goes from the occipital lobe to the temporal lobe and is concerned with object/face identity and conscious perception

Question 38

What is prosopagnosia?

Answer 38

Inability to recognise familiar faces

Question 39

What causes prosopagnosia?

Answer 39

Damage to specific parts of the temporal lobe (usually to fusiform sulcus which is located on the underside of the temporal lobe

Question 40

What is a hemifield?

Answer 40

The 170 degree range of vision that one eye can see when facing directly forward

Question 41

What is the vestibulo-occular reflex?

Answer 41

When the direction of the eyes remains constant whilst the head is moving

Question 42

What is the optokinetic reflex?

Answer 42

When the eye fixes a gaze on a moving object

Question 43

What is the pupillary reflex?

Answer 43

The contraction of both pupils when one eye gets illuminated. This is because the pretactal nuclei and Edinger Westphal nuclei receive signals from both eyes.

Question 44

What would prevent the pupillary reflex?

Answer 44

Damage to one of the oculomotor nerves, which would prevent the pupil in the affected eye from contracting.

Question 45

What part of the eye is able to have the clearest vision, and why is this the case?

Answer 45

Fovea because it has the highest density of cone cells