Physiology of Vision

Question 1

What does the retina contain?

Answer 1

A sheet of light responsive photoreceptor cells

Question 2

What does each cell of the retina have?

Answer 2

A distinct receptive field within the visual field

Question 3

What does light coming from the receptive field generate?

Answer 3

An electrical response in photoreceptor cell = phototransduction

Question 4

Where is the electrical response to light transmitted to?

Answer 4

Across the retina and visual structures terminating in the cortex

Question 5

Where does perception of light occur?

Answer 5

The cortex

Question 6

What is the direct pathway for signal transmission in the retina?

Answer 6

Photoreceptors - bipolar cells - ganglion cells

Question 7

What are the lateral connections of the retina which influence signal processing?

Answer 7

Horizontal and amacrine cells

Question 8

What is the role of horizontal cells?

Answer 8

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

Question 9

What is the role of amacrine cells?

Answer 9

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

Question 10

What is the dark current the basis of?

Answer 10

Phototransduction

Question 11

What happens with the dark current in the dark?

Answer 11

PNa = PK so Vm is therefore between ENa and EK

Question 12

What happens with the dark current in the light?

Answer 12

PNa is reduced so Vm equals EK and hyperpolarises

Question 13

What kind of change does the dark current cause?

Answer 13

Local and graded

Question 14

What does light convert 11-cis-retinal to?

Answer 14

All-trans-retinal (activated form)

Question 15

What effect does the greater convergence in the rod system have?

Answer 15

Increases sensitivity while decreasing acuity

Question 16

What kind of light do rods see in?

Answer 16

Dim light

Question 17

What kind of light do cones see in?

Answer 17

Normal daylight

Question 18

What is centre surround organisation also known as?

Answer 18

Lateral inhibition

Question 19

What is the purpose of centre surround organisation?

Answer 19

Emphasises area of contrast = sharpens boundary between objects of different luminance

Question 20

How is light intensity detected in the visual system?

Answer 20

Detects local differences in light intensity, not the absolute amounts of light

Question 21

What type of gluR is involved in the on and off pathways?

Answer 21

On = inotropic gluR
Off = metabotropic gluR

Question 22

What effect does lateral inhibition have on ganglion cells?

Answer 22

Modifies the receptive fields of ganglion cells to have centre surround organisation

Question 23

What cells are primarily involved in centre surround organisation?

Answer 23

Horizontal and amacrine cells

Question 24

What does centre surround organisation help with?

Answer 24

Aids with localisation = exaggerates difference in stimulus intensity detected by adjacent neurons

Question 25

How do horizontal cells interact with groups of surround neurons?

Answer 25

Interconnect and sample the total amount of excitation in the surround and responds by releasing GABA

Question 26

What happens if there is low surround detected by horizontal cells?

Answer 26

Less GABA is released = response of centre on bipolar cell is thus higher

Question 27

How is retinal output processed?

Answer 27

Parallel processing = simultaneous input from two eyes, info compared in cortex

Question 28

Where does info about light and dark come from?

Answer 28

On centre and off centre ganglion cells

Question 29

What are the three different basic types of retinal ganglion cells?

Answer 29

M cells = movement
P cells = form and colour
Nonm-nonP cells = colour

Question 30

How is a binocular visual field created?

Answer 30

Each eye sees a part of the visual field (monocular visual field) but their visual fields overlap extensively

Question 31

Where do nerve fibres from the nasal half of each retina cross over?

Answer 31

The optic chiasm

Question 32

What do the two optic tracts allow?

Answer 32

Right and left visual fields to reach the left and right hemispheres separately

Question 33

What percentage of nerve fibres cross over?

Answer 33

60% cross over = nasal retina

40% don’t cross over = temporal retina

Question 34

Where is the entire visual field mapped?

Answer 34

Precisely mapped on to the primary visual cortex = has visuotopic organisation

Question 35

Is the scaling of the visual field on to the cortex constant?

Answer 35

No = about half of the cortex is dedicated to info from the relatively small fovea

Question 36

What does the visuotopic organisation of the primary visual cortex allow?

Answer 36

Mapping of lesions in the visual system

Question 37

Where do inputs to the striate cortex come from?

Answer 37

Magnocellular LGN neurons = layer IVCa
Parvocellular LGN neurons = layer IVCbeta
Konicellular LGN axons = bypass layer IV to synapse in layers II and III

Question 38

Where are binocular neurons found in the striate cortex?

Answer 38

Mostlyy layer IV

Question 39

What receptive fields are located in the striate cortex?

Answer 39

Cortical receptive fields

Question 40

How do the cortical receptive fields display orientation selectivity?

Answer 40

Respond to light/dark bars or edges only if in a particular orientation = peak frequency depends on angle

Question 41

Where are ocular dominance column located?

Answer 41

Present in VI

Question 42

Why is ocular dominance important?

Answer 42

Connections from the two eyes compete with each other

Question 43

Why are retinal synapses in the LGN unaffected by deprivation?

Answer 43

They are monocular

Question 44

What can monocular deprivation in the cortex during critical stages in development lead to?

Answer 44

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