Vision

Question 1

Retinal ganglion cells - what do they respond to?

Answer 1

Spontaneously active - their rate increases or decreases in response to changes in visual input.

Question 2

What do ON cells respond to?

Answer 2

Increases in incident light levels

Question 3

What do OFF cells respond to?

Answer 3

Decreases in incident light levels

Question 4

What do parvocellular (P- cells, midget cells, x-cells) do?

Answer 4

They convey colour detail.
They have small receptive fields.
They have tightly packed mosaics.
They have fine grained detection.
Detailed, more sustained, high res.

Question 5

What do magnocellular (m-cells, parosol, y-cells) do?

Answer 5

Respond quickly to change (prefer movement).
Large receptive fields.
Broadly spaced mosaics.
Coarse grained detection.
lo-res, blurred, rapid transient info transfer.

Question 6

Where is the density of all moasaics (and therefore acuity) highest?

Answer 6

At the fovea (macula lutea) but it decreases towards the periphery.

Question 7

What is the optic nerve head?

Answer 7

Where the axons leave the retina - blind spot.

Question 8

Are RGC light responsive?

Answer 8

No, but photoreceptive RGCs are. They directly respond and mediate infor about overall light levels to the brain.

Question 9

How is adaptation to a huge range of lighting levels achieved?

Answer 9

Patterns of incident light fall on the retina and are converted to a voltage in the neural retina. (Light - change in Vm of photoreceptor membrane - bipolar cells - RGCs - first cells to generate APs.

Question 10

Explain the different photoreceptors…

Answer 10

Rods - no colour info BUT greater sensitivity.

Cones (x3) colour info - red, green, blue sensitive, not active in low light (scotopic) conditions.

Question 11

What do bipolar and horizontal and amacrine cells do?

Answer 11

Bioolar - ON and OFF types - link photoreceptor output to RGC.
Horizontal and amacrine - compare inputes between neighbouring photoreceptors and bipolar cells - enhance contrast.

Question 12

How is an image formed?

Answer 12

The lens focuses the image on the retina - the anterior surface of the cornea and the posterior surface of the lens - both static therefore focus the image on the retinal surface

Question 13

Explain accommodation?

Answer 13

If focussing on an image closer than 7m…ciliary body contracts which relieves the natural ension of the suspensory ligaments and the front surface of the lens bulges.

Question 14

Different RGCs have different targets.

Explain those of the LGN…

Answer 14

In the thalamus.
P and M cells.
Iinputs from RGC classes remain segreated.
Inputs from either eye remain segrgated but aligned.
Visual signals to the primary visual cortex
Opportunity to shut down visual transmission.

Question 15

Different RGCs have different targets.

Explain those of the superior colliculus

Answer 15

In the midbrain, M cells.

Visual info processed for regulating eye movements, proprioception.

Question 16

Different RGCs have different targets.

Explain those of the pretectum

Answer 16

Photoreceptive ganglion cells.

-pupillary reflexes and circadian rhythms.

Question 17

What would the consequence be of…

• an aneurysm of the opthalmic artery
• blockage of anterior choroidal artery
• pituitary tumor

Answer 17

pressue on optic nerve and mononocular blindness.

ischemia of optic tract - right/left homonoymous hemianopia.

bitemporal loss.

Question 18

Explain the characteristics of rods.

Answer 18

Have an elongated outer segment.
Most sensitive at low iluminescence.
Poor at temporal discrimination.
Low spatial resolution.
More membrane disks than cones (1000x more sensitive to light).

Question 19

Explain the characteristics of cones.

Answer 19

Conical outer segment.
Most sensitive at high iluminescence.
Good at temporal discrimination.
High spatial resolution.
Colour differentiation.

Question 20

Explain what the photosensitive photopigment membrane disks contain and how they act.

Answer 20

Contain rhydopsin. Protein: opsin. And cis-trans retinal (vit A derivative).
Capacity to switch from cis to trans when a photon is absored.
The energy for this photoisomeration is dependant on the way that cis-retinal is bound to opsin.
The isomeration can sometimes happen randomly due to thermal energy,

Therefore - 6-7 photons are required to activate a single rod.

Cones need many more photons to activate them.

Question 21

Explain the biochemical cascade that links rhydopsin to a change in Vm via cGMP. And how amplification can act…

Answer 21

Rhydopsin interacts with (700) transducin (a G protein) which interacts with (2:1) PDE…hydrolyses (many) cGMP (decrease [cGMP]) therefore channels close = hyperpolarisation.

In cones - ;less channels close due to less amplification.

Question 22

How do ON and OFF cells respond to light.

Answer 22

Light - decrease NT release.
Normally - ON cells, hyperpolarise in response to glutamate. Therefore, decreased glutamate = decreased hyperpolarisation = depolarisation = increase APs.

Normally off cells depolarise in response to glutatmate. Therefore decreased glutamate = decreased depolarisation = hyperpolarisation = decrease APs.

Question 23

Explain centre surround organisation of bipolar cells.

Answer 23

On cells - light on receptive centre and dark surround = good.

OFF cells - dark centre and light surround = good.

Centre surround antagonises centre.

Question 24

Define the visual field

Answer 24

Location on the sensory surface which the optimal stimulus will modulate the firing of a neurone.

Question 25

What is Meyer's loop?

Answer 25

Carries information for the superior portion of the contralateral visual field.

Question 26

With respect to the calcarine sulcus, where are different parts of the visual field represented?

Answer 26

Lower visual field represented ABOVE sulcus, | Upper visual field represented BELOW calcarine sulcus.

Question 27

Going through each area of the retinogeniculostriate pathway in turn, say which areas are monocular or biocular.

Answer 27

Retina: biocular LGN: monocular V1: Alternating eye specific ocular dominance columns. Other cortical layers - binocular which allows depth perception (stereopsis).

Question 28

Explain the different layers of the LGN and which side information comes from, what sort of retinal cells, Lgn vell type and the layer number.

Answer 28

``` 1 - contralateral, M-cell (parosol), Magnocellular. 2 = ipsilateral, M-cell, Magnocellular, 3= ipsilateral, P-cell, parvo. 4 = contra, P, parvo 5 - ipsi, P, parvo 6 - contra, P - parvo. ```

Question 29

Explain the basic circuit as common to all cortical areas.

Answer 29

Input from 4. Feedforward to 2/3, feedback 5 and descending output from 6. Vertical relay from 4 to 2/3 to 5 to 6. Horizontal connections aswell.

Question 30

Which layers of the V1 have pyramidal cells and which have stellate cells.

Answer 30

All except 4C have pyramidal cells. Pyramidal cells are the only cellls which send output out of cortex.

Question 31

What is the representation of the central region of the fovea in the cortex?

Answer 31

It is over represented because of the high number of RGCs (esp cones).

Question 32

Explain the ventral stream

Answer 32

From V4 (responds to colour) and striate cortex to the inferior temporal lobe = object recognition. Can lead to cerebral achromatopia where everything seems mucky!

Question 33

Explain the dorsal stream

Answer 33

From the middle temporal area (motion appreciation and response to direction) and striate to the parietal lobe. Motion analysis. Can lead to cerebral akinetopsia...eg, keep filling a cup of tea!

Question 34

What is the optimal stimuli for bipolar and cortical cells?

Answer 34

Bipolar - spot of light/dark Cortical cells - simple; bar of light, correct width and correct orientation. - complex - bar of light, correct width, orientation not crucial.

Question 35

What is an orientation tuning curve?

Answer 35

Show the optimal orientation for cortical cell response.