Vision: Central Pathways

Question 1

Central projections of retinal ganglion cells

Answer 1

• optic nerve carries fires from only one eye
• optic chiasm - split of fibres (60% cross, 40% stay = decussation)
• optic tract carries fibres from both eyes: this is important because it allows us to represent a side of space, not an eye (left visual field goes to r hemisphere, right visual field to l hemisphere)
• major targets are the:
• -> thalamus (dorsolateral geniculate nucleus / LGN)
• -> pretectum (pupillary light reflex)
• -> hypothalamus

Question 2

Why a decussation at the optic chiasm?

Answer 2

Retinotopic mapping: the decussation at the optic chiasm allows information about space to stay together regardless of which eye detects it.

Temporal retina stays on the same side, nasal retina (60%) goes to the other side.

Peripheral locations fall only on one retina
- Left periphery –> nasal L retina
- Right periphery –> nasal R retina
More central locations fall on both retina

Question 3

The image on the retina is flipped

Answer 3

flipped vertically and horizontally

Question 4

Fovea

Answer 4

centre of the eye, when youre fixating it has really high spatial acuity because it has cones packed densely.

With both eyes open, the fovea is directed at the same image, and the centre of the fovea is the boundary between neurons that will cross the chiasm (nasal retina), and those that will stay on the temporal side

Question 5

You are looking straight ahead and you see, in your peripheral vision, an object to your left, all the way to the side. It will be registered by:

Answer 5

the left nasal retina

Question 6

Thalamus -

Answer 6

dorsolateral geniculate nucleus (LGN)

Once the info has crossed over, it heads to its major target to the LGN. Then it passes through these optic radiations to V1 (primary visual cortex = striate cortex = brodmann’s area 17)

Most of the info travels through the LGN to V1

Question 7

Optic radiations from LGN to V1

Answer 7

some go through the temporal lobe (meyer’s loop) and carry infor from the upper visual field (below)

some go through the parietal lobe and carry information about the lower visual field

Question 8

Visuotopic organisation

Answer 8

Primary visual cortex lies on the banks of the calcarine sulcus.

Central vision (fovea) represented at caudal portion (occipital pole) - disproportionally large area –> supports our high acuity

Peripheral vision more anterior - less cortical magnification –> less acuity

Upper visual field = lower calcarine
Lower visual field = upper calcarine

Question 9

Damage to the right optic nerve before the optic chiasm would

Answer 9

prevent input from the right eye (right eye blindness)

Question 10

damage at the optic chiasm:

Answer 10

this is where nasal retinal fibres cross - therefore damage at B should affect vision of space which normally falls on the nasal retina (lose periphery vision)

Question 11

damage at the right optic tract

Answer 11

carries info from both eyes about the left side of space

therefore blindness to the left side of space

Question 12

damage at optic radiation (left)

Answer 12

effects the right side of vision - if its meyer’s loop its the upper left

Question 13

Pretectum pupillary light reflex

Answer 13

• reduction in pupil diameter with increasing light
• the temporal side of retina in left eye gets combined with the nasal retina on right eye - they both come to left pretectum (this ensures that if ur left eye gets stimulated by light, your right also has a connection)
• the pretectal neurons connect to the Edinger-Westphal nuclei –> which has axons in the ocular motor nerve that terminate on neurons in the ciliary ganglion
• the constrictor muscle reduces the amount of light entering the eye, preventing the saturation of those photoreceptors

Question 14

Pupillary light reflex circuit

Answer 14

Pretectum gets info from both eyes (side of space)
Pretectum connects to Edinger-Westphal nuclei (both sides)
Travels in oculomotor nerves
Via ciliary ganglion on both sides –> controls constrictor muscle in iris –> changes diameter of both pupils

Question 15

Scenario where light is shone in left eye

Answer 15

• Retina; photoreceptors hyperpolarise, releasing less transmitter onto bipular cells
• This frees the on-centre bipolar cell to depolarise, which releases more transmitter onto the on-centre ganglion cells, which fire action potentials along the axon (which travels in the optic nerve)
• The type of ganglion cells that project to the pretectum and hypothalamus have their light sensitive pigment called melanopsin
• gets to the optic chiasm, offshoot of fibres to the pretectum
• activity in these fibres activate the cells of both the EW nuclei, which activates ciliary ganglia, which stimulates the pupilary constrictor muscles

Question 16

Input for circadian rhythms

Answer 16

• hypothalamus suprachiasmatic nucleus (SCN)
• offshoot just behind the chiasm
• input from ganglion cells that hold melanopsin gives info about environmental illumination
• melanopsin is a novel photopigment - the opposite of rods and cones, meaning it depolarises by light

Question 17

Superior colliculus

Answer 17

orients responses

coordinates head and eye movements towards targets

Question 18

What is special about the visual cortex

Answer 18

6 layers, but the 4th layer is divided into 3

layer 4 is important in terms of inputs and outputs for vision

• -> 4C has spiny stellate neurons (gets LGN info and passes it to other layers)
• -> 4B sends axons to extrastriate areas

Question 19

Neural responses in V1

Answer 19

V1 neurons respond to light-dark bars/edges and have a preferred orientation.

Orientation tuning (but also motion direction tuning, spatial freq tuning, temporal frequency tuning) 
Scenes are encoded in combined activity of neurons across the visual field

Question 20

Vertical columns

Answer 20

respond to similar areas of space and similar orientations

Question 21

Adjacent columns

Answer 21

neurons have slightly shifted receptive fields and progression in orientation selectivity

Question 22

Neurons responding to stimulation of both eyes first occur in V1

Answer 22

LGN neurons keep input from the eyes separate - now we have input from both eyes going to the LGN but kept in separate layers

Cortex: special layer 4 - they stat separate by synapsing on neurons that are organised in these ocular dominance columns

Question 23

Stereopsis

Answer 23

perception of depth from binocular disparity

Question 24

Magno- parvo- and koniocellular pathways keypoints

Answer 24

a) Retina
- M ganglion cells - large bodies, more dendrites, thicker axons
- P ganglion cells = smaller bodies, less dendrites, thinner axons

b) LGN
M cells –> 2 ventral layers = Magnocellular layers
P cells –> 4 more dorsal layers = parvocellular layers

c) V1
Magnocellular axons –> upper part of layer 4C
Parvocellular axons = lower part of 4C

Question 25

Magnocellular pathway

Answer 25

M ganglion cells = larger receptive field, faster conduction velocities, transient responses, insensitive to different wavelengths of light

MOTION, not colour

Question 26

Parvocellular pathway

Answer 26

P ganglion cells = smaller receptive fields, slower ocnduciton, sustained responses, sensitive to differences in wavelengths
COLOUR, not motion