Visual system- optics and transduction Flashcards
How much of our cerebral cortex does our visual system use
over 1/3
What actually is light
Electromagnetic radiation
What is the wavelength range of visible light
400-700nm
What does the wavelength of light determine
Hue- red light has the longest wavelength, violet the shortest
What does the wave amplitude of light determine
Intensity or brightness
What primarily determines the width of the visible light spectrum
The spectral absorbances of the photopigments in the eye
What allows us to view the back of the eye
Opthalmoscope or fundus camera
What is the macula
A spot at the centre of the retina, focused on clear straight-ahead vision
What is within the macula
The fovea- very high cone density, used for high acuity vision
What is the optic disk
Where cells of retina send their axons together in the optic nerve through the optic disk to the brain
What is the retina
Light sensing part of the eye
What causes our blind spot
The optic disk has no rods and cones as it is where the optic nerve leaves the eye
Why don’t we see our blind spot
The brain fills in the hole with surrounding patterns
Where is the aqueous humour vs vitreous humour
Aqueous humour- between the iris and the inner surface of the cornea
Vitreous humour- between the lens and optic nerve
What does the aqueous humour do
Carries O2 and nutrients to the structures within the eye, bathes them, and removes waste products (blood would prevent transparency)
What is vitreous humour
Gelatnious substance that accounts for 80% of the eye’s volume, fills the space between the lens and optic nerve
What does the vitreous humour do
Helps maintain the spherical shape of the eye, contains phagocytic cells that remove blood and debri that might interfere with light transmission
What is choroid
A pigment within the pigmeted epithelium that helps absorb any stray light to prevent it reflecting back into the eye and distorting the image
Where are the eyes suspended
In orbits of the skull
What are the extraocular muscles
Attached to the sclera- we use them to move our eye all the time eg tracking objects allows light to come into the fovea for high acuity vision
What do light rays pass through as they travel through the eye
Transparent structures- the cornea, lens, fluid compartments
What keeps the vascularity of the eye to a minimum
Cleansing function of aqueous humour, function of the choroid in nourishing the retinal layers
Where in the eye do blood vessels avoid
Avoid the fovea at the centre of the macula that provides the greatest visual acuity
What features minimise optic distortion
Transparet structures, minimised vascularity, no blood vessels in fovea, pigmented epithelium
What does it mean that human vision is frontal facing and foveally dominated
When we move our eyes we are bringing the object of interest to bear on the fovea
What is vergence
Simultaneous movement of both eyes with the functino of pointing the fovea of each eye on an area of visual interest
What is convergence
Inward rotation of both eyes so fixate on an object
What is divergence
Outward rotation of both eyes to focus on an object
How are rods and cones distributed in the retina
Cones are in the fovea of retina, rods are very high in periphery of the fovea but completely absent in the fovea (Osterberg, 1935)
What are saccadic eye movements
Small jerky eye movements as you move your eyes eg across a page- allows you to focus on small bits of interest in the scene with greater resolution
Where are photoreceptors located in the eye
At the back of the eye at the back of a 3 neuron relay (so behind bipolar, ganglion cells etc)
Why are photoreceptors at the back of the 3 neuron relay
They need pigmented epithelium to continually replace disks (every 12 days) of photoreceptors that are shed as they move to the tip of the outer segment, and to regenerate bleached photopigment molecules
What is the diameter of the fovea
1.5mm (5 degrees of visual angle)
How are cones organised in the centre of the fovea to allow close packing
Those in the center are tapered
How does the arrangement of retinal nerve bodies in the fovea allow a direct path for light
Retinal nerve cell bodies either side of the fovea are shifted aside, so light has a direct path to photoreceptors
What does it mean to say light refracts
It changes direction as a result of entering a medium with a different density
What is the purpose of refraction of light in the eye
Bend the light entering the eye so it forms a single point on the back of the retina
What formula gives the optical refractive power of the eye
P= uf
u is refractive incidence of the media of the eye
f= focal length of the eye
What real numbers are plugged into the formula to give the optical refractive power of the eye
f= 22mm u= 1.33 P= 60 diopters
What is the refractive power of the cornea
42 diopters (non-adjustable)
What is the refractive power of the lens
18 diopters (adjustable via accomodation)
What is spatial resolution
How well fine details can be resolved
What is visual acuity
A measure of spatial resolution, the angle subtended at the eye of two points which are perceived as separate
How can visual acuity be tested
Snellen chart- test in bright light with high contrast, achromatic letters, with glasses if needed
What is normal visual acuity
20/20- at 20 feet, someone can read a line of letters labeled 20/20
Letters with gaps about 1 min of arc can just be read
What is 1 min of arc
1/60 degree
What is visual acuity at best
Gaps of 0.5min of arc on the retina can be resolved aka 2 microns
What determines the limits of visual acuity
Gaps of 0.5min of arc (2 microns) is the width of an individual foveal cone outer segment, hence acuity is limited by the cones, as light must hit 2 seperate cones for 2 points to be seen as separate
What does the iris do
Controls the size of the pupil and the amount of light entering the eye, reducing optical errors and increasing depth of focus
How does the iris reduce optical errors
Constriction of the pupil means less rays of light are entering the eye that can have aberrations that will cause blurring on the back of the eye
What do sympathetic vs parasympathetic response cause the pupil to do
Sympathetic response- dilation of pupil
Parasympathetic response- constriction of pupil
How does the image projected onto the retina differ from the object in the world
Image on the retina is inverted due to the optics of the eye
What did Stratton say about the importance of inversion of the retinal image
Stratton (1897)- inversion is not important as long as the relative spatial positions remain preserved
What experiments did Stratton carry out into the importance of inversion
Stratton (1897)- wore goggles with inverted lenses so everything he saw looked upside down, after a few days he adjusted to things being upside down, concluding inversion is not important
What is the accomodation reflex
The increase in power of the lens caused by contraction of the annular ciliary muscle, which reduces tension in radial zonular fibres, allowing the lens to relax to a may convex state
By which system is the annular ciliary muscle innervated
Parasympathetically innervated
Accomodation reflex- what is the state of the rays of light when we look at something distant
The rays of light are entering the eye parallel, so when they hit the cornea they form a single point on the back of the eye
Accomodation reflex- what happens when a fixated object is distant
Relaxed ciliary muscles pull suspensory ligaments taut, so the lens is pulled flat and rays of light come to a single point on the retina- don’t need to be refracted a lot
Accomodation reflex- what is the state of the rays of light when we look at something close
The rays of light are already diverging when they meet the cornea, so more refraction is needed to bring them to a single point on the retina
Accomodation reflex- what happens when a fixated object is close
Ciliary muscles contract and release the tension on suspensory ligaments, allowing the lens to bulge out thicker to help refract light rays onto a single point on the retina
What stimulates the accomodation reflex
The visual cortex monitors the image- if it appears blurred, parasympathetic fibres are activated, and we automatically and instantaneously accomodate via ciliary muscle contraction
What is presbyopia
Reduced ability of the eye to focus on nearby objects aka reduced ‘near point’ as we age
What is our ‘near point’
The most contracted ciliary muscles can be ie the closest objects can be that we can still accomodate them
Approx 25cm, but declines with age
What are possible mechanisms for presbyopia
Decreased elasticity of the lens so it can’t curl enough, loss of power in ciliary muscles they can’t contract enough
What is myopia
Short-sightedness- near objects in focus but distant objects are blurred
What does the size of the blur for far ojects in short sightedness depend on
How open the pupil is, because a constricted pupil allows less myopic light rays to enter the eye
What causes distant objects to appear blurred in myopia
Light rays cross over too early, meaning when they reach the back of the eye the image is blurred
How can myopia be treated
A corrective concave lens will make light rays diverge outwards before they enter the cornea, meaning they cross over later, on a single point at the back of the eye- spectacles or contacts
Study into environment vs genes causing myopia
Xiong et al (2017)- systematic review and metanalysis found increased time outdoors is effective in preventing the onset of myopia, but is not effective in slowing progression of already myopic eyes
What is hypermetropia
Hyperopia, long-sightedness, near objects are blurred but distant objects are in focus
What causes hypermetropia
Rays of light cross over behind the retina, causing a blur on the back of the retina
How can hypermetropia be treated
Convex lens causes some convergence of light rays before they reach the cornea, so rays form a clear image and cross over at the back of the eye- spectacles or contacts
Treatment for myopia vs hypermetropia
Myopia- corrected with negative power
Hyperopia- corrected with positive power
What is the effect of the axial length of the eyeq
If the eye is too long or short, rays of light will cross over either before or after they hit the retina
How is the visual environment important in developing vision
Visual environment exerts a powerful influence on refractive state by controlling the axial length of the eye during postnatal development
Study showing a cause of eye enlargement
Wiesel and Raviola (1977)- if a monkey eye is fused shut, it grows too long, causing myopia
Study showing need for near and distance vision in early development
Hung et al (1995)- monkeys made to wear spectacle lens that simulated refracrive anomalies developed compensating ocular growth, suggesting both distance and near vision are needed within a normal environment to produce the normal eye length
What atigmatism
Common cause of blurred vision due to irregularly curved cornea, causing different degree of curvature in different planes and refractive error
How does astigmatism cause refractive error
When lights travels through the unevenly curved cornea, it comes to focus at multiple different points on the retina eg horizontal plane may focus well while vertical plane will be blurred
How can astigmatism be assesed
Astigmatism chart with lines at many different angles- for a person with astigmatism, not all the lines are in focus
How can astigmatism be corrected
Spectacles with a cylindrical component in their curved surfaces- the cylindrical lens allows the different lines to be in focus at the same time
How does posterior vitreous detachment affect vision
Flashes of light, floaters and cobwebs in the vitreous humour, can affect image quality
What causes the flashes of light in posterior vitreous detachment
As the vitreous pulls away from the retina it can stimulate the retina which causes flashes
What causes the floaters in posterior vitreous detachment
As vitreous becomes more watery, debris and clumped microscopic fibres too large to be cleared by phagocytes can float freely, casting shadows on the visual field
What causes the cobweb effect in posterior vitreous detachment
When the vitreous edge pulls away from the retina, light passes through it differently, and tou may see a thicker outer edge of the vitreous, appearing as cobwebs
What is posterior vitreous detachment
With age, the gelatinous substance can partially liquify and lose its shape
How do visual distortions (eg refractive errors, posterior vitreous detachment) need to be treated
Can’t be corrected at higher brain levels, necessary to generate a clear image at the eye itself
What is cataracts
Condition in which the lens becomes cloudy- patches can become bigger, causing blurry misty vision and eventually blindness
Causes of cataracts
Genetics, aging, environmental (sunlight, dehydration), injury
What can cause congenital cataracts
No of genes identified, including mutations in crystallins
Why does cataracts cause blurry vision
Light is spread around and obstructed by the cloudy patches rather than having a clear passage through the eye
Treatment of cataracts
Removing the lens surgically and replacing it with an artificial lens
How is aqueous humour circulated through the eye
Aqueous humour produced and comes out in front of the eye in front of the lens, then drains out through the canal of Schlemm
What is circulation of aqueous humour important in maintaining
Normal intraocular pressure (IOP) around 16.5mm Hg
What causes glaucoma
If the eye can’t drain aqueous fluid well enough, pressure inside the eye rises, reducing blood flow and squeezing and damaging the cells that form the optic nerve
What cells together form the optic nerve
Retinal ganglion cells
Why is nighttime vision often lost first in glaucome
Info from rods is sent on the outside of the optic nerve, meaning glaucoma damages rods first (involved in nighttime vision)
How does glaucoma affect vision
Reduces the visual field through damage to the optic nerve that is permanent- one of the most common causes of blindness
What elevated IOP is glaucoma associated with
Over 21mmHg
How can glaucoma be treated
Can be treated to reduce intraocular pressure to prevent permanent vision loss- medication, laser treatment or surgery
What is retinitis pigmentosa
RP is an inherited degenerative eye disease that causes severe vision impairment
What causes retinitis pigmentosa (RP)
Progressive degeneration of initially the rod photoreceptor cells in the retina, and later the cones
How does retinitis pigmentosa impair vision
First sign is loss of peripheral vision and night vision, eventually leads to tunnel vision and total blindness
What do animal models causing the degeneration of photoreceptors in retinitis pigmentosa (RP)
Reduced ability of retinal pigment epithelial (RPE) cells to phagocytose receptor outer segments leads to accumulation of debris and seperation of photoreceptors from RPE cells- causes their degeneration
Study in animal model showing how RP may be treated
Lorach et al (2018)- surgical removal of debris when about half the photoreceptors are lost allows the remaining photoreceptor cells to renew their outer segments and survive 3 times longer than in untreated eyes
How is the retina’s output not a faithful reproduction of the light falling on it
It is specialised to detect differences in light intensity on different parts of the retina, image processing is already underway
What is the pupillary light reflex
in bright light, your pupils contract
What controls the pupillary light reflex
Connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils
What does it mean to say the pupillary light reflex is consensual
Shining a light into only one eye causes constriction of the pupils of both eyes- if not, often a sign of seriously neurological disorder involving the brain stem
What are the benefits of the pupil constricting and dilating
Allows light/dark adaption, pupil constriction allows increased depth of focus
How does the process of information compression and sorting into parallel pathways begin
In the retina, 130 million photoreceptors are refined into an output of 1 million ganglion cells
How many rods and cones are in the retina
125 million rods, 7 million cones
What cells send visual info to the brain
Retinal ganglion cells
Which cells in the retina use graded potetials vs APs
All neurons in the retina use graded potentials, except the ganglion cells that use APs
What are the 6 types of retinal neurons
Retinal ganglion cells, amacrine cells, horizontal cells, bipolar cells, rods and cones
What order do electrochemical signals travel through the retinal neurons
Photoreceptors ->bipolar cells -> retinal ganglion cells (->LGN)
What is optics
The study of light rays and their interactions
What determines the angle of refraction
The size of the difference betweeen the speed of light in 2 media- the greater the difference, the greater the angle of refraction
What is the cornea
The glassy transparent external surface of the eye, continuous with the sclera (white of the eye) which forms the tough wall of the eyeball
Why aren’t the extraocular muscles visible
They are behind the conjunctiva, a membrane that folds back from the inside of the eyeballs and attaches to the sclera
Where is the retina thinnest
In the fovea
How do you describe the different areas of the retina
Part of the retina closer to the nose than the fovea is nasal, part near the temple is temporal, above fovea is superior, below fovea is inferior
Why is the aqueous humour in the cornea the main contributor to refraction
The aqueous humour has simiilar refractive properties to water so light travels significantly slower in the cornea than air, while there is less refraction after this point as the aqueous humour/lens/vitreous humour are all composed largely of water
What is the focal distance
The distance between the refractive surface and where the paralllel light rays converge- the tighter the cornea curvature, the shorter the focal distance
What is the diopter
Refractive power of the eye- the reciprocal of the focal distance
Around how close to objects have to be to the eye for the lens to change shape to focus the object
Closer than 9m from the eye
What is the visual field
The total amount of space that can be viewed by the retina when the eye is fixated straight ahead
