Alex- Seeing Flashcards
History- Greeks
Extramission (things come out of your eyes and that how we see). Democritus had Persian tutors. Said images are subtle atoms (no extramission), see by eyes alone and organs just conduits, all visual images are blurred. Black, red, white, green are primary and make infinite others, vision relative. Plato said observation and experiments bad, can’t predict and too hard so should give up
Arab contribution
Roman fell, Europe in dark ages, 12000 years W no good science (winter). Al hasan consolidates Greek. Said vision doesn’t come out of eye, matter doesn’t enter it. Light rats from object carry info in straight line. White light made of many colours. Perspective, illusions
Renaissance
Da Vinci painted Dolomites and saw mountains in distance bluer and paler. Did dissection, previously band. Eye connected to the brain but when projected images saw it was inverted at retina and made incorrect assumptions
Hermann con helmholtz
Wrote handbook of optics. Solved how we looked at the retina as created the ophthalmoscope which used mirrors to focus light. Sign theory: sensations symbolise their stim but not copies and learning a part of vision as babies don’t see well like lang. said eye isn’t a fine tuned instrument
Anatomy
See the pupil, irises and sclera. Pupil means little person. Cornea is main lens, aqueous humour is fluid in anterior chamber, iris the muscle, pupil is hole, lens , ciliary body makes humour, ciliary muscle controls lens, vitreous humour jelly between lens and retina. Cornea focuses light
Receptors
At retina, light passes through membranes until the receptors-evolution not creatation as not straight but photoreceptors shed so would have shed into humour. Fovea has highest acuity as shortest route to receptors due to fovea pit, fovea slope. 3 cones and 1 rod (short blue, medium green and long red) high res, colour, fovea. In low light use rods, in periphery, low acuity, more numerous . No blue at fovea and more r than g
Receptor adaptation
Adapt to the avg light via troxler fading: only show things different, used when moving inside to out (diffs around the average). Schnapf separated cones and found that in the daylight, more light is needed to get the same response so neurones can change their sensitivity
Blind spot
Optic disc where the nerve meets the eye. Visual angle is the angle something makes as it enters the eye- means things far make same image as close. Thumb 2 degrees at arm length, fist 8 degrees, blind spot 15 from contra . Brain fills it in, does same W most of periphery as bad, only good at central 2 dgs
Receptive fields
Mount castle: measured electrical signals via oscilloscope. Neurones have rf w central and inhibitory peripheral, mean spec small spots. The areas of the retina over which the activity of the cell can influence. Antagonism is called lateral inhibition
Testing rfs
Hermann grid: see black patches at the junction of black squares on a white background. Partly due to spec rf, and inhibiton when you look at the junctions. Also as more inhibition at a junction
Scintillating grid- in colour
Why we have rfs
Bandwidth from eye is limited. Removes environmental variation as removes brightness but responds to structure inputs by pattern matching. Turn world into invariant code called constrast. Combo of light adaptation at receptors and rfs and ganglion get a constant image of world
Myopia
Short sighted, most common issue. Increasing in numbers, 90% asain compared to 10% 60 years ago. Not fully genetic, more school and electronics. Failure to focus on retina, too strong so projects before it
Hyperopia
Long sightedness, eye too short/not strong enough so project behind the retina. Some genetic origin. At 40 lens hardens called presbyopia so can’t focus light
Glaucoma
Glaucoma/crushed pipes: aq humour can’t drain as angle between cornea and iris closed/trabecular meshowrk tissue is blocked. Means build up of pressure so damage nerve cells. Peripheral vision closes in but don’t notice as brain fills in
Macular degeneration
Peeling walls: degeneration of the retina and death of photoreceptors. Retina leaks and wet form is when bleeding happens treatable but dry isn’t. Miss things in centre of visual field, age related
Diabetic retinopathy
Leaking pipes, blood vessels leak blood and interfere retina in patches. Cauterise vessels to treat, mainly in periphery as not important. Spots in field but brain fills in
History of colour
Isaac Newton: went home from Cambridge due to plague. Ran light through prims and saw colours, passed through another prism and found no more splitting. Found white a mixture of wls, refraction depends on wl, object colour depends on light, cam mix primaries to produce any colour but don’t know why 3
Thomas young
Said eye must be limited as can’t have one detector per colour so we have primary couloirs . From 3 special, get 7 primitive and can make any. Trichromatic theory of vision (all colour produced by 3 receptors)
Cones in the retina
Roorda and Williams: used laser from us army to see retina by cancelling out wobbles/adaptive optics. Saw diff ppl have diff proportions of red and green even tho same vision. S is blue, m is green , l is yellow/red. Metamers: lights that look the same even tho thier spectra are diff as activate receptors in same way . Used in TVs as create metamers of real colour
Genetics
Opsin genes code for light pigments, l and m on the x chromie. S /blue are fossils of original colour as across animals and less of the,, don’t convey form or motion, on chromie 7 so rare to have mutation.
Colour blindness
Mostly missing l and m on the x so affect men more/recessive/sex linked. Absorption spectra not even, l and m closer so similar info. Retina computes 3 combos l+m, l-m and S-(m+l) which are dark/light, red/green and yellow/blue on 3 axis. In colour blindness, if all m become l then l-m dimension goes so can’t determine red/green.
Colour blindness in humans and animals
Many animals only have 2 photoreceptors l and S like human dichromate-tell light and dark and blue and yellow,but not red/green. In humans rarest is tritanopua lack S, most common anomalous trichromacy can’t tell red/green
Tests for colour blindness
Only needed due to industrial revolution as train drivers need to tell go and stop. Old method could cheat as ppl can use brightness. Use ishaira plates so red green pattern masked by random luminance so no cues
Evolution
After continents split, primates split into old and new rod. An old world monkey born W 3 photoreceptors, as opposed to 2 so could tell red green and see ripe fruit. Also supports frugivory/eating of fruit-spread seeds
In the brain
Vental stream has object I’d and form, present in fovea and responds to colour. Dorsal stream has motion, action and location- v weak to colour
