Lecture 10 & 11: McDearmid

Question 1

How does sensory input affect the development of the visual system - hypotheses?

Answer 1

Nature: sensory input = no effect
Nurture: sensory factors have an effect

Question 2

What did Roger Sperry discover about the brain?

Answer 2

Left side brain dominant for language and speech
Right side dominant for visual and spatial processing
- Both sides can actually do anything
- Problem if too sides cant communicate

Question 3

Describe the wiring of the visual nerves in adult newts

Answer 3

Nerves which regenerate
Right and Left eye information is transmitted contralaterally
Monocular vision –> each eye = independent
Information processed in optic tectum
Build a retinotopic map- point in space represented on retina.
Ganglion cells which connects specific locations to tectum.
Anterior goes to posterior, Dorsal to ventral

Question 4

Why are images innverrted ?

Answer 4

Light must be focused at the back of the eye with a point of convergence –> invertion of image
Brain then converts to correct orientation.

Question 5

How can you test whether the environment influences how ganglion cells map onto tectal cells?

Answer 5

How: lesion projections –> disrupts retinal map –> allow porjections to regrow and then test vision?
Two experiments were conducted.

Question 6

What were the conclusions of experiment 1- cutting the optic nerve

Answer 6

After regrowth the vision restored to normal, retinotopic map restored.
- Projections regrow to original targets

Question 7

What were the conclusions of experiment 2- disorganising the information in the retina

Answer 7

How- cut optic nerve, rotate eyes 180 (upside down) and allow the nerves to regrow.
Image on the retina is then the ‘correct’ way round instead of inverted, brain then inverts the image as the retinal axons wire to default locations (no learning displayed) - connections formed innately

Question 8

How are axon projection locations determined.

Answer 8

Axon guidance molecules - ephrins

Connections formed innately.

Question 9

What do congenital cataracts show above developmental plasticity in the visual system?

Answer 9

If removed early into life –> no permenant damage
If removed later in life –> disrupted vision
Suggestive of a critical window

Question 10

How does the mammlian visual system work

Answer 10

Binocular vision - one composite image with increased depth of vision

• axons from both eyes are sent contrallaterally and ipsilaterally
• both go to lateral geniculate nucleus- acts as a relay
• Then axons innervate visual cortex

Question 11

How does the visual system work in cats?

Answer 11

First few days, only detect changes of intensity, takes a few days for distinguishing of objects and patterns

Question 12

What do experiments involving shutting cats eyes show?

Answer 12

Shut one eye after birth –> leave closed for over 12 weeks –> permenant blindness due to critical window
- do the same to an adult –> No effect

Question 13

How does the critical visual work in cats?

Answer 13

Lack of light exposure affects neurons between lateral geniculate nucleus and visual cortex.
- Layer 4 of visual cortex = ordered innervation from LGN

Question 14

How is layer 4 of the visual cortex ordered?

Answer 14

Eye-specific columns

• Ocular dominance columns (ODCs)
• Preferentially recieve input from one eye

Question 15

How are ODCs labelled?

Answer 15

Transneuronal labelling- tracing of projections from each eye
Retrograde labelling - injection of radioactive proline into single eye

Question 16

How does retrograde labelling work

Answer 16

Crosses the LGN synapse due to being amino acid travels towards visual cortex
Detected by photographic film –> shows pathways
Removed by serial sections

Question 17

Describe ODCs

Answer 17

Light and dark stripes which represent left and right eye innervation - light being the injected eye
Formed during the critical window

Question 18

How do ODC change with sensory input?

Answer 18

Normally: left and right innervation is more intermingled
One eye closed: those ODCs narrow, open eye ODCs increase in territory - can;t be altered afterwards
Action potentials inhibited in one eye: same response as having the eye closed.
Remove one eye: other eye does not sort into ODCs
–> both eyes required to form ODCs

Question 19

Summarise the information about ODCs

Answer 19

ODC formation is dependent on activity, and requires two eyes to be present.
Two eyes compete for territory with A.P firing
- reduced activity –> ODCs narrow
- no competition –> no sorting / ordering

Question 20

Why do only active neurons produce ocular dominance columns.

Answer 20

” neurons that fire together, wire together”

- activity dependent trophic factor release from post-synaptic cell

Question 21

What are ODCs like in amphibians?

Answer 21

No ODCs due to lack of competition in optic tectum
(amphibian version of visual cortex)
If you add a 3rd eye –> competition –> ODCs form
–> all that is required is afferent input from different pathways to the same target.

Question 22

What happens if you redirect the visual pathways to the auditory cortex?

Answer 22

Auditory cortex now responds to visual cues –> ODCs, retinotopic map, specific responses to orientation and direction and velocity

• -> suggestive of plasticity in brain regions
• -> can still distinguish auditory and visual information

Question 23

What can be concluded from these experiments about plasticity in the brain?

Answer 23

Genetic factors are fundamental but experience also impacts on the final result e.g. critical periods
NATURE AND NURTURE.