Physiology of Vision

Question 1

Describe the direct (vertical) for signal transmission of the retina.

Answer 1

Photoreceptors > Bipolar cells > Ganglion cells.

Question 2

Describe the role of horizontal cells in the retina.

Answer 2

• Receive input from photoreceptors and project to other photoreceptors and bipolar cells.

Question 3

Describe the role of amacrine cells in the retina.

Answer 3

• Receive input from bipolar cells and project to: ganglion cells, bipolar cells and other amacrine cells.

Question 4

Light converts 11-cis-Retinal to what?

Answer 4

all-trans-Retinal (activated form).

Question 5

Rhodopsin is made up of what?

Answer 5

Opsin + 11-cis-Retinal.

Question 6

Another name for all-trans-RetinOl?

Answer 6

Vitamin A.

Question 7

What facilitates seeing in dim light?

Answer 7

Rods.

Question 8

What facilitates seeing in normal day light?

Answer 8

Cones.

Question 9

Convergence in the rod system results in what?

Answer 9

• Increasing sensitivity to light.

- Decreasing visual acuity.

Question 10

What forms the basis of colour vision?

Answer 10

• Cones.

- Different opsins for discrete wavelengths.

Question 11

Short-wavelength cones see which colour?

Answer 11

Blue.

Question 12

Medium-wavelength cones see which colour?

Answer 12

Green.

Question 13

Long-wavelength cones see which colour?

Answer 13

Red.

Question 14

Rods see which colour?

Answer 14

None.

Question 15

Describe rods of the retina.

Answer 15

• Achromatic.
• Peripheral retina.
• High convergence.
• High light sensitivity.
• Low visual acuity.

Question 16

Describe cones of the retina.

Answer 16

• Chromatic.
• Central retina: fovea.
• Low convergence.
• Low light sensitivity.
• High visual acuity.

Question 17

Describe lateral inhibition.

Answer 17

• Increases contrast and sharpness in visual response by exaggerating the difference in detected stimulus intensity by adjacent neurons.
• Aids in localisation.

Question 18

Lateral inhibition improves what?

Answer 18

Stimulus localisation.

Question 19

Horizontal cells connect “surround” neurons around the centre of each ganglion. This samples excitation in the surround and responds by releasing what?

Answer 19

GABA.

• If low surround, less GABA released.

Question 20

What function does centre-surround organisation and lateral inhibition serve?

Answer 20

Emphasises areas of contrast i.e. sharpening boundary between objects of different luminance.

Question 21

Our visual system detects local differences in light what?

Answer 21

Intensity, not absolute amounts of light.

Question 22

Lateral inhibition modifies receptive fields of ganglion cells to have what?

Answer 22

Centre-surround organisation.

Question 23

Describe parallel processing of retinal output.

Answer 23

Simultaneous input from both eyes relays information to be compared in the cortex to allow for judging the depth and distance of an object.

Question 24

Describe “on-centre” cells of the retinal ganglia.

Answer 24

Stimulated when centre of its receptive field is exposed to light. Inhibited when the surround cells are exposed to light.

Question 25

Describe “off-centre” cells of the retinal ganglia.

Answer 25

• Stimulated when surround cells are exposed to light.

- Inhibited when centre of receptive field is exposed to light.

Question 26

Each eye sees what part of the visual space?

Answer 26

• Monocular visual field (+/- 45o).

NB the visual fields overlap to create a binocular visual field (+/-45o).

Question 27

The retina is divided in half relative to what?

Answer 27

The fovea.

Question 28

The retina is divided into what?

Answer 28

• Nasal and temporal hemiretina.

Question 29

Nerve fibres from the nasal half of each retina cross over where?

Answer 29

The optic chiasm.

Question 30

Which optic tracts cross over at the optic chiasm?

Answer 30

Nasal (60%).

The temporal (40%) do not.

Question 31

Around half of the visual cortex is dedicated to information from what?

Answer 31

The fovea.

Question 32

The visual cortex is said to have what kind of organisation?

Answer 32

Visuotopic.

Question 33

With regards to input to the Striate cortex.

Magnocellular LGN neurons project to where?

Answer 33

Layer IVC-alpha.

Question 34

With regards to input to the Striate cortex.

Parvocellular LGN neurons project to where?

Answer 34

Layer IVC-beta.

Question 35

With regards to input to the Striate cortex.

Koniocellular LGN axons project to where?

Answer 35

Bypass layer IV to synapse in layers II and III.

Question 36

How may the synapses of koniocellular LGN axons in layers II and III of the striate cortex be observed?

Answer 36

As “blobs” following cytochrome oxidase staining.

Question 37

The first binocular neurons are found where?

Answer 37

In the striate cortex - most layer III neurons are binocular (but not layer IV).

Question 38

Cortical receptive fields contain what type of neurons?

Answer 38

• Orientation selective neurons.

Question 39

Cortical receptive fields respond to what?

Answer 39

Light/dark bars or edges - ONLY if in a particular orientation.

Question 40

Peak frequency of Cortical receptive fields depends on what?

Answer 40

Angle and preferred orientation.

Question 41

Retinal synapses in LGN are not affected by deprivation, why?

Answer 41

Because they are monocular.

Question 42

In the cortex, monocular deprivation during a “critical period” in development results in what?

Answer 42

Active afferents in one eye and lower activity from the other.

Question 43

Sensory deprivation early in life can alter the structure of what?

Answer 43

The cerebral cortex.