Week 2 Lecture 4: Visual Processing

Question 1

Where are photoreceptors and where are they found?

Answer 1

Photoreceptors are light-sensitive neurons that convert light energy into electrical energy in cell (phototransduction) and can be found in the retina

Question 2

What are the two types of photoreceptors in the retina?

Answer 2

Rods (low light) and cones (bright light and colour vision) (more rods than cone)

Question 3

Do rods and cones fire action potential?

Answer 3

No, although they are neurons, they do not fire action potentials but instead respond to stimuli with graded membrane potentials

Question 4

What is the structure of rods and cones?

Answer 4

• In the outer segment, the membrane folds into a disk-like layers that contain visual pigments that respond to light
• In the inner segment, there is the nucleus and organelles for protein synthesis
• In a basal layer, there is a synapse that releases glutamate

Question 5

How do photoreceptors detect light?

Answer 5

Using membrane-bound visual pigments, pigment molecules change shape when light hits them, this conformational change starts a signalling cascade that hyperpolarizes the cell, reducing its release of glutamate

Question 6

When are photoreceptors most active?

Answer 6

In darkness photoreceptors produce more glutamate

Question 7

What pigment is present in rods?

Answer 7

Rhodopsin

Question 8

Where are photoreceptors most densely packed?

Answer 8

The macula, a central disk, especially in its central pit the fovea

Question 9

What is the fovea used for?

Answer 9

Detailed vision

Question 10

Are there photoreceptors distributed all over the retina?

Answer 10

No, there are no photoreceptors in the blindspot, the hole where the axons carrying visual information leave the eye (optic nerve)

Question 11

Which are more sensitive, cones or rods?

Answer 11

Rods are more sensitive, can detect single photons.

Question 12

Do cones only operate in bright light conditions? Do rods only operate in dim conditions?

Answer 12

Yes for both, for rods specifically, in daylight, rhodopsin is broken down so light cannot be sensed. When light go dim, rod dark adapt (they rebuild their rhodopsin stores)

Question 13

How rods and cones distributed in the retina?

Answer 13

Cones are found primarily in the fovea and rods are found in the peripheral retina (not the fovea)

Question 14

What are the three layers of neurons in the retina?

Answer 14

Photoreceptors, bipolar neurons, ganglion cells.

Question 15

Where is convergence greatest in retina? Where is it least prevalent?

Answer 15

Convergence is greatest in the peripheral retina and least in the fovea where there are some receptors that synapse 1:1 with bipolar neurons

Question 16

What are receptive fields?

Answer 16

the region of the retina where light affects the cell’s activity

Question 17

What are center-surround receptive fields?

Answer 17

Property of bipolar neurons and ganglion cells, light affects either round center region or doughnut-shaped surround

Question 18

What is the difference between on-center and off-centre receptive fields

Answer 18

On-center cells are excited by light in the center of their field and inhibited by light in their surround while off-center cells are excited by light in their surround and inhibited by light in their center

Question 19

What do bipolar cell respond strongly to?

Answer 19

Contrast, when the light is uniform, the effects of the center and surround cancel each other out, leaving the cell at its resting level of activity therefore contrast is required

Question 20

How do bipolar cells respond to contrast?

Answer 20

With graded potentials (do not fire action potentials)

Question 21

What are ganglion cells?

Answer 21

Bipolar cells synapse onto them, they fire action potentials and have center-surround receptive fields (detect contrast)

Question 22

What can be inferred from the Chevruel illusion?

Answer 22

That the accentuation of contrast perceived is a result of the bipolar and ganglion cells that sense contrast

Question 23

How does the size of receptive fields of ganglion cells differ in different parts of the retina?

Answer 23

The receptive field of ganglion cells near the fovea are smaller than receptive fields of ganglion cells in the periphery because less bipolar receptors project onto them

Question 24

What is the consequences of differing receptive fields of ganglion cells across the retina?

Answer 24

Ganglion cells in the periphery are sensitive to light (rods are in the periphery) and poor at reporting spatial detail since the receptive field are larger, blends info from many receptors
Ganglion cells in the fovea are NOT sensitive to light but have decent spatial resolution since the receptive field of ganglion cells is smaller

Question 25

How are ganglion cells classified?

Answer 25

Based on how their signals are used in the brain

Question 26

What are the classifications of ganglion cells?

Answer 26

Magnocellular: large, provide info that is used to infer movement, phasic
Parvocellular: provide info that is used to infer form and fine detail, phasic-tonic
Melanopsin: photoreceptors, with their own visual pigment (melanopsin), projects to suprachiasmatic nucleus (center for circadian rhythms)

Question 27

How does visual information leave the retinas?

Answer 27

The axons of ganglion cells form the optic nerve (cranial nerve 2) which leaves the eye through the blind spot

Question 28

What happens at the optic chiasm?

Answer 28

Nerves from the nasal half of each retina crosses over to other side of the brain

Question 29

Why do nerves cross over?

Answer 29

It is necessary for all the info from the respective visual hemifield to come together in the respective hemisphere

Question 30

What are the nerve bundles that emerge from the optic chiasm called?

Answer 30

Optic tracts

Question 31

Where do the optic tracts end up?

Answer 31

In the two lateral geniculate nuclei (LGN) in the thalamus which project via optic radiation to the primary visual cortex (V1)

Question 32

How many neurons are in the LGN and why is it significant?

Answer 32

There are 2 million neuron in the two LGN. This is significant because it is the same as the number of ganglion cells

Question 33

Where is the V1 located?

Answer 33

The occipital lobe

Question 34

How are visual area of the brain organized?

Answer 34

Retinotopically, neurons close together in visual areas (LGN, V1) get info from close-together regions in the retina