vision Flashcards
what’s the role of the iris ?
changes size accommodate image in the pupil
in what order does light travel through the eye ?
cornea
lens
vitreous humour
retina
what does it mean when we foveate ?
bring image to central part of the retina
why is there many blood vessels in the eye ?
high metabolic activity
what is special about the lens ?
gelatinous material
designed to change shape
what moves the lens ?
cililary muscles which are attached through zonular fibres
what is the measure of lens focussing power ?
dioptere
average human eye 57-60 diopteres
what determines lens strength ?
curvature and refractive index
where does refraction usually occur ?
cornea
48 ou tof 57 D
further refraction caused by lens
what is meant by accommodation ?
focal length is changed by altering lens shape
what happens when the ciliary muscle contracts ?
loosens zonular fibres
causes lens to squish spherically
relieves ligament tension on lens
increases lens power
shortens focal length for closer objects
what happens when ciliary muscles relax ?
tightens zonular fibres
make sense more flat
what happens when our lens is in focus ? - far away object (4)
relaxed ciliary muscles
tension on zonular fibres
flattened lens
light rays nearly parallel
what happens when the lens is out of focus ? (2)
relaxed ciliary muscles
light rays from near objects diverge
what happens when the lens is in focus? - close up object (4)
contraction on ciliary muscles
relaxed zonular fibres
rounded lens
accommodation
what is myopia ? (4)
short sightedness
eye is too long
image focusses before hitting retina
concave lens helps reverse some refraction to bring focal point to retina
what is hypermetropia ? (4)
long sightedness
eye is to short
image focussed behind the eye
glasses lenses are thicker in middle , helps bring image forward
what is presbyopia ? (3)
when lens siezes up with age
doesn’t bulge when ciliary muscles contract
near point moves away
what does the pupil do in relation to light ? (3)
diameter changes light levels 2-8mm
can alter amount of light captured by 16 times
only a small portion of how the eyes adapt to light
what are the pupil diameter muscles and what do they do ?
sphincter pupillae- contracts making pupil smaller
dilator - contracts making pupil bigger
why do we see better with a smaller pupil ? (3)
higher quality image
greater depth of filed
compensates for myopia
why is a larger pupil size less effective ?
more scattered light and glare
increased spherical aberration - blurry
what does reduced sensitivity to light do ?
reduced quality of image
what is an ophthalmoscope ?
tool that shines light to back of retina
has an adjustable lens that can bring retina to focus
what does it mean if the fovea has a high density ofphot receptors
higher aquity - image quality
what re the 5 retinal cells ?
rods
cones
horizontals
bipolars
ganglion
which retinal cells can carry an AP ?
ganglion
what is special about the structure of the retina and how light travels through?
the photoreceptors are on the outside
light has to travel through the structures to reach them
its adjacent to the pigment epithelium which minimises reflectance
what’s special about the foveal retina ?
structures int he foveal retina are pushed to one side which reduces light scattering
increases acuity
what is the role of black pigment epithelium
minimises reflectance
what’s is the structure of rods and cones ?
folds of membrane that create disks
the disks contain photopigment
what is relevant in term son rods and cones density ?
acuity can be defined by the pointspread function
- determines the minimum separation of 2 points before the are perceived as 2 separate entities
what is photopigment ?
2 molecules ( retinal + opsin) combien to create photopigment
this is called rhodopsin in rod cells
what is photopigment bleaching ?
rhodopsin interacts with a light photon causing configurational change
retinal and opsin then part - bleached
how is the cell membrane hyperpolarised? (5)
opsin activates phosphodiesterase (PDE)
PDE converts cGMP to GMP
this opens Na channels
closure of the Na channels cause hyperpolariastion
due to K leaking out
how does the firing rate of the ganglion cell change ?
rod/cone hyperpolariastion causes LESS neurotransmitter release
(glutamate)
this modulates the membrane potential of the bipolar cell
inhib/excite
what is scotopic vision ? (4)
low light vision
rods only
high sensitivity
low acuity
what is photopic vision ? (6)
high luminance
cones only
rod cells wont work - bleached
low sensitivity
high acuity
foveal + peripheral
what are the 4 adaptations to luminance ?
pupil size
switch between rods and cones
dark adaptation
field adaptation
what is dark adaptation?
bleaching/ regeneration of photopigment
less light is needed to bleach which aids regeneration to protects light
what is field adaptation ?
quick light adaptation
automatic gain control within the photoreceptor
calcium release mechanism in the photoreceptor
adaptation is a constant trade of between what ?
sensitivity vs acuity
which is more sensitive: rods or cones ?
rods
by 1 order of magnitude
where does more sensitivity come from in terms of cells ?
high convergence of rods onto ganglion cells via bipolar cells
degree of convergence depends on the light conditions
if only cones are used for colour vision in mesonic and scotopic vision, does it get better or worse ?
colour vision gets worse if only cones are used
explain the distribution of rods and cones in the retina (4)
foveal vision is dependent on cones
centre of the eye has high density of both rods and cones
cones in periphery are poor compared to rods
foveal acuity at night is poor
what is meant by acuity ?
our ability to see sharp and defined shapes at certain distances
explain the processing of info after the photoreceptor (4)
retinal processing reduces things of interest in our scenery
changes both spacial and temporal imagery/field
brain can recognise shapes through spatial awareness of edges
helps us identify what we look at
what is the concept of the receptive field ? (4)
ganglion cell is final output of retina
each ganglion responds to many photoreceptors
some photoreceptors excite/inhibit ganglion cell
forms centre surround shape - causes lateral inhibition
what is lateral inhibition ?
mediated by horizontal cells
retinal ganglion cells are set up to detect edges
respond better to both light and dark (shadows) rather than pure light and pure darkness
what is a contrast illusion ?
an illusion where the perceived brightness or identical visual targets seems to differ depending on its background and ratio of brightness
looks diff but is the same lol
what is the theoretical 1 channel receptor system in terms of colour ?? (4)
responds to stimulus intensity
one degree of freedom
no discrimination except brightness
monochrome system- limits ability to judge light
what is the theoretical 2 channel receptor system in terms of colour ?? (5)
responds to 2 aspects of stimulus intensity
2 degrees of freedom
can discriminate colours
outputs are described by 2 numbers
can be ambiguous
what’s the colour triangle ?
assumes constant luminance of all colour wavelengths
any colour can be dialled by combo of red, green and blue
what are the 2 factors that colour can be defined by ?
saturation - strength of colour
hue - defines colour itself
what does no wavelength stimulate in terms of colour ?
green cones
no need to activate red+blue
where does colour processing mainly take place ?
retina
how do ganglion cells respond to colour?
responds to combination of red, green or blue
doesn’t respond indiv
to activate the blue photoreceptor what happens ?
yellow inhibits blue
to activate the green photoreceptor what happens ?
red inhibits green
what are the types of colour disorders ?
pronatopes - no red
dichromats - no green
tritanopes - no blue
what is meant by the brain is contralateral in terms of visual info ?
optic nerve splits at the optic chiasm
left eye - right hemisphere of brain and vice versa
how is info related to the visual cortex of the brain ?
via the lateral geniculate nucleus - in thalamus
how is location of blindness useful ?
can help localise the anatomical lesion
blindness in one eye- optic chiasm defecit
part blind in both eyes - visual cortex / optic chiasm
