T2L10 physiology of vision

Question 1

overview of how eye works

Answer 1

• cornea and lens focus image on the retina
• focus varied by changing the shape and power of lens
• iris acts as diaphragm; varies diameter by 4x hence intensity by 16x
• behind retina pigment layer absorbs unwanted light

Question 2

retina through ophthalmoscope

Answer 2

• main feature is optic disk (optic nerve leaves, vessels enter and leave)

fovea- a small yellow spot on the far right. It is 1-2 degrees in diameter

Question 3

fovea

Answer 3

small yellow spot in far right

Question 4

bending the light components

Answer 4

cornea does 2/3 of the bending

lens does 1/3 but can adjust to focus (accommodation)

Question 5

long sightedness

Answer 5

hypermetropia

• eyeball too short or lens too weak
• cant see close
• converging lens

Question 6

short sightedness

Answer 6

myopia

• eyeball too long or lens too strong
• cant see far
• diverging lens

Question 7

structure of retina

Answer 7

V light V

ganglion cell
muller cell (supports glial cell)
amacrine cell
bipolar cells
horizontal cells
rod
cone
pigment cell

evolved back to front baso
see s7

Question 8

light receptors

Answer 8

rods: 120 mil
- dim light

cones: 5 mil
- 3 types
- bright lights and colours

Question 9

visual processing layers

Answer 9

3 direct layers:

• receptors
• bipolar cells
• ganglion cells

2 transverse layers:

• horizontal cells
• amacrine cells

signal processing including lateral inhibition

125:1 convergence to optic nerve

Question 10

rhodopsin and its chromophore - retinal

Answer 10

• when hit by a photon the retinal in the RHODOPSIN molecule flips from 11 cis to all trans

this sets off events leading to hyperpolarisation

Question 11

the ganglion response

Answer 11

• receptors respond strongly to light intensity
• ganglion cells respond very weakly to light intensity

instead they respond to LOCAL CONTRAST:

• light on a dark background
• dark on a light background

basic pattern is either on centre or off centre s9
- due to lateral inhibition

Question 12

colour vision (_____)

Answer 12

colour vision (trichromacy)

red/blue/green cones

red>green>blue

Question 13

colour blindness

Answer 13

a result of loss or modification of one of the 3 cone visual pigments

genes for red and green are on X chromosome
» males 7%, females 0.5%

blue pigment is on chromosome 7
m=f, v rare

Question 14

central achromatopsia

Answer 14

damage to cortical colour processing areas

Question 15

central visual pathways

Answer 15

SEE S14

optic nerve from each eye divide into L and R halves

• in optic chiasm the L halves from each eye combine and the R halves combine
• optic tracts relay in lateral geniculate nuclei of thalamus
• part of each trac

learn the diagram s14

Question 16

responses of cells in primary visual cortex

Answer 16

simple cell responses are constructed from rows of ganglion cell on and off centre fields

ie a basic outline of shapes n shit

Question 17

visual cortex organisation

Answer 17

organised in 3 overlapping patterns

1. ocular dominance columns
2. smaller orientational columns - orientation
3. colour blobs - kept separate from orientation and passed to other regions such as v4

1 hypercolumn contains one set of everything

s16

Question 18

causes of partial vision loss

Answer 18

lesion in front of chiasm:
- singular whole eye blindness in one eye

lesion in left behind chiasm:
- loss of both left sides (homonymous hemianopia)

lesion in middle of chiasm
- loss of both outsides (bitemporal hemianopia)

damage to fibres behind chiasm in left side

• spots of blindness on both left sides (scotoma)
• retinal damage, lesions in visual cortex, pressure from tumours restricting optic nerve, chiasm, optic tract etc

Question 19

dorsal and ventral streams in the cortex

Answer 19

dorsal stream: (occipital to parietal cortex)

• location
• motion
• action

ventral stream: (occipital to temporal cortex)

• object and face identity
• conscious perception

Question 20

visual agnosia

Answer 20

a condition in which a person can see but cannot recognize or interpret visual information, due to a problems in VENTRAL STREAM

Question 21

prosopagnosia

Answer 21

inability to recognise familiar faces

damage to temporal lobe

Question 22

blindsight

Answer 22

• destruction of striate cortex

Question 23

visual reflexes

Answer 23

1. vestibulo-ocular reflex (VOR)
   - stabilises gaze by countering head movement
2. optokinetic reflex
   - stabilises image of moving object on retina
3. pupillary reflex
   - if one eye is illuminated both pupils should contrict
   - damage to one optic nerve will prevent that pupil constricting, but the other one should still work
   - damage to one oculomotor nerve will prevent contraction in that eye, but stimulation of either eye should cause contraction in the other one