1.7 The Visual System

Question 1

what does the visual system do

Answer 1

provide long and short range sensory information about the layout of surrounding

Question 2

how does the visual system work

Answer 2

o Converts photons into electrical signals
o Processes the signals using several pathways
 Analyses visual scenes, identifies objects and faces
 Provides info about threats
 Estimates self-motion and motion of external objects

Question 3

how is the optical apparatus is suited to focus light onto the photoreceptors

Answer 3

photoreceptors in the eye

o By adjusting the curvature of the lens, visual objects located at different distances are focussed onto the retina.

Question 4

what does the retina consist of

Answer 4

• photoreceptor cells (PRCs)
• bipolar cells
• retina ganglion cells
• horizontal cells

Question 5

what are photoreceptor cells

Answer 5

 Outermost layer

 Absorb light and convert to a neural signal (phototransduction)

Question 6

what are bipolar cells

Answer 6

 Receive synaptic signals from PRCs

Question 7

what are retina ganglion cells

Answer 7

 Innermost layer
 Input from bipolar cells
 Output from retina, with axons forming optic nerve

Question 8

what are horizontal cells

Answer 8

 Provide lateral connections

Question 9

where is the vision sharpest

Answer 9

The area of retina near the optical axis, the fovea, is where vision is the sharpest
o Corresponds to the centre of gaze when we look at something
o Density of photoreceptors, bipolar cells and ganglion cells is highest at the fovea.
o In the centre of the fovea, the foveola, cellular layers are pushed aside to allow more immediate access to PRCs

Question 10

what are the 4 regions in RPCs

Answer 10

outer segment
inner segment
cell boy
synaptic terminal

Question 11

what is the outer segment

Answer 11

at distance surface of retina

Question 12

what is the inner segment

Answer 12

located proximally

Question 13

what are the 2 types of photoreceptors

Answer 13

rods and cones

Question 14

what is the shape of a rod

Answer 14

Long, cylindrical outer segment within which the stacks of discs are separated from the plasma membrane

Question 15

what is the sensitivity of rods

Answer 15

Highly sensitive, even to a single photon so dominate vision at low light levels
o Saturate at higher light levels

Question 16

where are rods found

Answer 16

Only a single type present in primates
 100 million in human retina
o None in fovea
 We can’t see what we are directly looking at during the night

Question 17

what is the shape of a cone

Answer 17

Shorter, tapered outer segment and the discs are continuous with the outer membrane

Question 18

what is the sensitivity of a cone

Answer 18

Much less sensitive to light o Make no contribution to night vision  Considerably faster response

Question 19

what are the types of cones primates have

Answer 19

L, M and S cones

Question 20

where are cones found

Answer 20

6 million in human retina
o Concentrated in fovea
o S-cones make up 10% but none in fovea

Question 21

what type of vision is fovea specialised for

Answer 21

daytime vision

Question 22

what controls the membrane potential o RPC

Answer 22

controlled by conductances of 𝐾+ and 𝑁𝑎+ ions whose transmembrane gradients are maintained by pumps

Question 23

what happens to the ions in the dark

Answer 23

In the dark, 𝑁𝑎+ ions flow into the photoreceptor through nonselective cation channels that are activated by the second messenger cGMP

Question 24

what triggers the photocascade

Answer 24

Absorption of a photon triggers a biochemical cascade that lowers the concentration of cGMP, closing the gated channels and hyperpolarising the cell to the 𝐾+ potential

Question 25

whats the visual pigment in rod cells

Answer 25

rhodopsin

Question 26

where is the opsin component

Answer 26

embedded in disc membrane

Question 27

what is retinal

Answer 27

the light absorbing moiety  11-cis isomer covalently linked to a lysine residue of Opsin  Absorption of a photon causes it to flip from 11-cis to the all-trans configuration

Question 28

what does the activated rhodopsin do

Answer 28

diffuses within the disc membrane where it encounters a G-protein (transducin in rods)

Question 29

what does the inactive form of rhodopsin do

Answer 29

Inactive form binds GDP but interaction with rhodopsin promotes binding of GTP instead  This causes dissociation into active subunits

Question 30

what does the active transducin subunit do

Answer 30

subunit T𝛼-GTP complexes with cGMP phosphodiesterase increasing its activity o Enzyme hydrolyses cGMP -> 5’-GMP

Question 31

what controls the activity of the cGMP-gated channels in the plasma membrane of out segment

Answer 31

conc of cGMP | o Lowered cGMP upon photon interaction causes 𝑁𝑎+ channels to close and reduce the inward current into the outer segment

Question 32

what happens with the dark current

Answer 32

There is an inward 𝑁𝑎+ current and an outward 𝐾+ current in darkness o PRC membrane potential ~−40 𝑚𝑉 o Cell’s synaptic terminal continually releases neurotransmitter glutamate

Question 33

what happens to Na current flow when there is light

Answer 33

o PRC membrane potential about that of potassium potential ~−70 𝑚𝑉 o Hyperpolarisation slows release of glutamate from photoreceptor terminal and so initiates a neural signal

Question 34

what does the photoreceptor layer produce

Answer 34

(Low level visual processing) produces a simple representation of the visual scene o Neurones in bright regions are hyperpolarised and those in dark regions depolarised

Question 35

what does the retinal circuit do

Answer 35

must edit the information before it is conveyed to the brain o This can be seen physically as the optic nerve has 1% as many axons as there are receptor cells

Question 36

what are the local interneurons is the retina

Answer 36

they generate graded membrane potentials - horizontal cells - bipolar cells - amacrine cells

Question 37

local interneuron - horizontal cells

Answer 37

collect signals from several neighbouring PRCs

Question 38

local interneuron - bipolar cells

Answer 38

send signals to amacrine or ganglion cells

Question 39

local interneuron - amacrine cells

Answer 39

horizontal processing layer

Question 40

retinal ganglion cells

Answer 40

axons form optic nerve and send AP to brain

Question 41

what do retinal ganglion cells (RCGs) have

Answer 41

receptive fields that are physically represented by a compact area on the retina o Comprises of a centre region and a surround region o These two regions produce antagonistic responses

Question 42

RGCs centre-sound anagonism is due to...

Answer 42

lateral connections by horizontal cells

Question 43

what happens when RCGs are on-centre

Answer 43

 Increased firing rate when light incident on centre of receptive field  Decreased firing rate when light incident on surround of receptive field  If both parts of receptive field covered, a weaker response occurs

Question 44

what happens when RCGs are off-centre

Answer 44

 Decreased firing rate when light incident on centre of receptive field  Increased firing rate when light incident on surround of receptive field

Question 45

RGCs transmit signals to the bain via three visual pathways...

Answer 45

``` o Pretectum  Pupillary reflex o Superior colliculus  Control of eye movements o Lateral geniculate nucleus (LGN)  Visual perception ```

Question 46

what does the LGN act as

Answer 46

the primary relay for the visual system | - part of the thalamus

Question 47

what is LGN

Answer 47

lateral geniculate nucleus

Question 48

how are the pathways that process light intensity and colour information organised

Answer 48

retinotopically organised | outputs to the primary visual cortex

Question 49

where are simple cells

Answer 49

primary visual cortex

Question 50

what do orientation selective neurones have

Answer 50

have receptive fields divided into ON and OFF sub-regions o Larger than those of RGCs o Respond optimally if a light bar of specific orientation enters the ON region

Question 51

what is the characteristic response to a moving bar

Answer 51

o Discharge briskly when a bar of light leaves an OFF region and enters ON o Highly selective for the position of a line or edge in space

Question 52

what does convergent synaptic connections

Answer 52

o Individual simple cell may receive inputs from three LGN cells whose receptive fields are arranged in space o The receptive fields are summed to create an orientation selective receptive field.

Question 53

how are orientation-selective cells are arranged in...

Answer 53

orientation columns o These contains cells that are selective for a specific orientation o Together they cover the entire range of 360°

Question 54

cortex is retinotopically arranged so...

Answer 54

each column can be seen as a set of orientation filters analysing contours within a certain patch of the visual field. o This information is passed to higher centres

Question 55

how do we perceive faces

Answer 55

Certain combinations of contours are processed by cells in the inferior temporal cortex to be perceived as faces o Recordings show how the contours of a stimulus need to be arranged to cause an increase in cell spiking activity.

Question 56

what is responsible for the cell's response to faces

Answer 56

Selective combination of simple cell outputs converging onto a cell in the inferior temporal cortex