T2L10 physiology of vision Flashcards
overview of how eye works
- 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
retina through ophthalmoscope
- 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
fovea
small yellow spot in far right
bending the light components
cornea does 2/3 of the bending
lens does 1/3 but can adjust to focus (accommodation)
long sightedness
hypermetropia
- eyeball too short or lens too weak
- cant see close
- converging lens
short sightedness
myopia
- eyeball too long or lens too strong
- cant see far
- diverging lens
structure of retina
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
light receptors
rods: 120 mil
- dim light
cones: 5 mil
- 3 types
- bright lights and colours
visual processing layers
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
rhodopsin and its chromophore - retinal
- 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
the ganglion response
- 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
colour vision (_____)
colour vision (trichromacy)
red/blue/green cones
red>green>blue
colour blindness
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
central achromatopsia
damage to cortical colour processing areas
central visual pathways
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
responses of cells in primary visual cortex
simple cell responses are constructed from rows of ganglion cell on and off centre fields
ie a basic outline of shapes n shit
visual cortex organisation
organised in 3 overlapping patterns
- ocular dominance columns
- smaller orientational columns - orientation
- colour blobs - kept separate from orientation and passed to other regions such as v4
1 hypercolumn contains one set of everything
s16
causes of partial vision loss
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
dorsal and ventral streams in the cortex
dorsal stream: (occipital to parietal cortex)
- location
- motion
- action
ventral stream: (occipital to temporal cortex)
- object and face identity
- conscious perception
visual agnosia
a condition in which a person can see but cannot recognize or interpret visual information, due to a problems in VENTRAL STREAM
prosopagnosia
inability to recognise familiar faces
damage to temporal lobe
blindsight
- destruction of striate cortex
visual reflexes
- vestibulo-ocular reflex (VOR)
- stabilises gaze by countering head movement - optokinetic reflex
- stabilises image of moving object on retina - 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
