T2 L8: Physiology of Vision

Question 1

How does the Fovea appear through an Ophthalmascope?

Answer 1

As a yellow spot

Question 2

How much ray bending is done by the Cornea?

Answer 2

2/3. The rest is done by the lens

Question 3

What is Hypermetropia and what causes it?

Answer 3

Long sightedness caused by a short eyeball or a lens system that is too weak

Question 4

What is Myopia and what causes it?

Answer 4

Short sightedness caused by the eyeball being too long or the system being too strong

Question 5

What is refractive power measured in?

Answer 5

Diopters (D)

Question 6

What are rods for in the retina?

Answer 6

Detecting dim light

Question 7

What are cones for in the retina?

Answer 7

For detecting bright light and colour

Question 8

What are Muller cells for?

Answer 8

Responsible for the homeostatic and metabolic support of retinal neurons

Question 9

What are Amacrine cells for?

Answer 9

The major carriers of rod signals to the ganglion cells in the retina

Question 10

How many receptors converge into a single optic nerve?

Answer 10

About 125 (There are 125 times for receptors that neurones in the eye)

Question 11

How many rods are there compares to cones in the retina?

Answer 11

120 million rods and 5 million cones

Question 12

What is Rhodopsin?

Answer 12

The photosensitive pigment in rods

Question 13

Describe the process by which rods detect light

Answer 13

When Rhodopsin is hit by a photon, it flips from 11-cis to all-trans.
This results in chemical changes that cause the cGMP-gated nonselective cation channels to close.
This leads to hyperpolarisation of the photoreceptor and a reduction in release of glutamate.

Question 14

What do ganglion cells respond to in the retina?

Answer 14

To contrast (on centre and off-centre) rather than changes in light intensity

Question 15

Which wavelength do red cones detect?

Answer 15

560nm

Question 16

Which wavelength do green cones detect?

Answer 16

530nm

Question 17

Which wavelength do blue cones detect?

Answer 17

420nm

Question 18

Which wavelength do rods detect?

Answer 18

500nm

Question 19

What are the 3 types of cones and what is their abundance?

Answer 19

Red (most abundant)
Green
Blue (Least abundant)

Question 20

What is colour blindness?

Answer 20

The loss or modification of one or more of the three cone visual pigments (Cone opsins)

Question 21

Where is the gene for red/green colour blindness found and what does that mean?

Answer 21

It’s on the X chromosome so blue/green colour blindness affects 7% of men and 0.5% of women

Question 22

Where is the gene for blue colour blindness found and what does that mean?

Answer 22

It found of chromosome 7 so is paired in both sexes. This type is very rare

Question 23

What is Central Achromatopsia and what causes it?

Answer 23

A rare type of colour blindness that had nothing to do with the pigments but is caused by damage to the cortical colour processing areas (V4) in the brain

Question 24

What is the function of the lateral geniculate nucleus?

Answer 24

It’s the area that receives input from both eyes to build a representation of the contralateral visual hemifield. It’s found on both sides

Question 25

What is the function of the superior colliculus in the mid-brain?

Answer 25

It’s the area where visual, auditory and somatosensory information are integrated to initiate motor commands. It’s attached to the midbrain

Question 26

What is the function of the striate nucleaus?

Answer 26

Output from both the lateral geniculate nuclei goes directly towards the striate nucleus found in the occipital love (V1)
This is the area where images from both hemispheres are combined
The area from the fovea is greatly exaggerated

Question 27

What are the 3 areas of the visual cortex and what are their functions?

Answer 27

1. Ocular dominance columns driven by the left and right eye, not both at the same time
2. Smaller orientationial columns which are organized regions of neurons that are excited by visual line stimuli of varying angles.
3. Colour blobs for colour information

Question 28

What does a hypercolumn in the visual cortex contain?

Answer 28

It contains each of the 3 areas together

Question 29

What causes left eye blindness?

Answer 29

A lesion in an area before the optic chiasm

Question 30

What is Homonymous hemianopia and causes it?

Answer 30

A defect involving either the two right or the two left halves of the visual fields of both eyes caused by a lesion on one side after the nerves reach the chiasm

Question 31

What is Bilateral hemianopia and what causes it?

Answer 31

A defect involving either the two right or the two left halves of the visual fields of both eyes caused by a lesion at the optic chiasm

Question 32

What is Scotoma and what causes it?

Answer 32

A blind spot that obstructs part of your vision caused by damage, pressure, or lesions in an area of the retina. Eg, methyl alcohol, MS, Hypertension. It can affect both eyes

Question 33

What is the dorsal stream from the occipital to the parietal cortex concerned with?

Answer 33

With location, motion, and action

Question 34

What is the ventral stream from the occipital to temporal cortex concerned with?

Answer 34

Object and face identity, and conscious perception

Question 35

What is visual agnosia and what causes it?

Answer 35

A condition in which a person can see but cannot recognize or interpret visual information. It’s caused by a lesion in the ventral steam

Question 36

What is Optic ataxia?

Answer 36

When someone can see but not act on what they see

Question 37

What is Prosopagnosia and what causes it?

Answer 37

The inability to recognise familiar faces associated with damage to the fusiform gyrus on the underside of the temporal lobe

Question 38

What is blindsight?

Answer 38

The ability to respond to visual stimuli without consciously perceiving them

Question 39

What is the Vestibulo-ocular reflex (VOR) for and what is the mechanism behind it?

Answer 39

It stabilises gaze by countering movement of the head

Mechanism: Oculomotor nuclei - vestibular nucleus - semi-circular canals

Question 40

What is the Optokinetic reflex (OKR) for and what is the mechanism behind it?

Answer 40

It stabilises the image of a moving object on the retina

Mechanism: Oculomotor nuclei - nucleus of the optic tract

Question 41

What is the pupillary reflex and why does it happen?

Answer 41

The fact that both pupils will contract if light is shone only into one

It happens because the Edinger Westphal nuclei receive signals from both eyes. So even if the nerve is damaged on one eye, the other nerve will cause contraction