Lec2-First Steps in Vision- Jan 17.docx

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Matthias Neimier

PSYB51 – Lec02 th Friday January 17 , 2013 The First Steps in Vision • Optics: how light enters the eye o Physics o Need to understand how/why we see light o How it stimulates retina in the eye • Anatomy of the eye  How it evolved o Accommodation • Retina  Point where perception starts  Very complex o Natural inhibition  Certain cells that inhibit certain colours o Dark and light adaptation  Different from accommodation o Retinitis pigmentosa: a retinal disease  Retina affected by genetic based disease • Receptive fields o Sensory cells have these o Stimulated somehow • Filters o Cells considered as filters • Contrast o Concept of change o Contrast in time o Space – light and dark o Luminous contrast A Little Light Physics • What is light? o Form of electro-magnetic wave • What can happen to it? o Our brain interprets these waves into colour o Whether it passes through an object or bounced off a surface • Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy • Light can be absorbed, diffracted, reflected, transmitted, or refracted • Absorbed: Energy (e.g., light) that is taken up and transformed to other forms of energy o Taken up into a surface o Ex. Solar panels • Transmitted: convey light from one place to another through a transparent medium o Ex. Filters for photo cameras o Red filter = absorbs any light but the red light o Black surfaces absorb more light  it is always hotter • Reflected: Energy that is redirected when it strikes a surface o Purkinje reflections, eye movements  Light hits the surface of your eye and it is reflected • Diffracted: Bent, or having waves that are spread out, (e.g., waves of sound or light, as they encounter an obstacle, e.g., pass through a narrow aperture) o Passes through small apertures o Spreads out depending on light wave lengths o Looks like a rainbow, but not o Enters the pupil to some extent, and the brain sorts it out • Refracted: Energy that is altered as it passes into another medium, (e.g., light entering water from the air) o Depth of a swimming pool  Looks shallower than it is o Lenses  Based on refraction o As light travels into water from air, it changes direction o Rainbows are based on refraction The Human Eye • Why is sensing light a “good idea”? o Why is it useful for us to have vision?  So we can assess the environment  To find food  To avoid danger  Facial expression o We get tired  Circadian rhythm  In the absence of light, we get tired  Light is a source of growth  Photoreceptors in the eyes specifically used to inform hypothalamus to tell us when to sleep or not • Light can tell us about the rhythms of life (days/ years) o The sun But it wasn’t always like that… • Wasn’t always the most important sense • Mammals emerged when dinosaurs ruled the earth • Small creates that survived at night o Hid away in the dark at night o Nocturnal animals, bad eyesight o Keen sense of smell, hearing • Photopic vision not so important for our ancestors • Vision became dominant modality later than in other species o Our vision is not as good as many animals The Human Eye • Why do we sense light? • How did the eye evolve? • What is the anatomy of the eye? • How is light modulated by the eye? o How accommodation works • Evolution of the eye  Not completely resolved how/when it evolved o Eye spots  Started with this o Light sensitive protein: opsin o Some set of photo receptors • Different nerves that sense light o Photosensitive cells • Because of aperture, light becomes selective o Depressed/folded area allows limited directional sensitivity • Pinhole eye allows finer directional sensitivity and limited imaging o Water-filled chamber  Can hold a lot of bacteria o To protect it from the outsider, there is a transparent humor • Distinct lens develops o Eye that’s quite functional • Iris and separate cornea develop o Makes the eye even better • The human eye is made up of various parts: o Know different parts of the eye o Cornea o Aqueous humor  Water in front of the lens o Iris (controls pupil size – fast adaptation of light)  Pinhole camera effect of eye o Pupil – gap in the iris o Lens – hooked up in a circular fashion surrounded by… o Ciliary muscles o Zonules of Zinn  Connect the lens with the ciliary muscles o Vitreous humour – biggest part of the eye, round shape comes from the pressure it puts on the eye  Inflated balloon o Retina o Sclera  White of your eye o Cornea  Has fovea • Pit o Optic nerve + blood vessels that happen in the eye o Optic disc – area where retina has no photoreceptors Eyes that see Light • Refraction is necessary to focus light rays. This is done by the four optical components. o The ciliary muscles change the shape of the lens, and thus alter its refractive power: Accommodation  eye needs to have images of the outside world that is acute, source of light • star is a spot of light that is projected as a spot in the retina o need to have refraction for this to be preformed • performed by the cornea (much more important for refraction)and the lens (flexible) o cornea= constant amount of refraction o lens can change its refractory power depending on distance of object to the eye  thin or thin • light from a very far spot (stars in the sky) enters and hits your eyes o lens need to have very little refractory power = very thin o see that star as one spot • ciliary muscles relax so aperture of lens becomes wider • ciliary muscles contract to become smaller while the lens relaxes into its normal shape • Cornea, aqueous and vitreous humors help refract light, but the refractive power of these 3 structures is fixed so they can’t bring close objects into focus. This job is performed by the lens. Optics of the Human Eye • (No) problems with refraction: • Eyes can work very well until 16 years old o Then it’s downhill from then on • Emmetropia o Eye has the ability to accommodate/adjust refractory power to whatever is necessary  Close and far  Normal shape of the eye • Myopia o Eyeball is longer than is what the lens can do (adjust refractory power) o Light gets bundled at a point before the retina o When light hits retina, it has already fanned out  the light that you see is blurred, little puddle of light o Can’t see clear in the distance o What I have… o Eyeball is elongated  Relative to the ability of the lens o Sclera is too round  causes myopia • Myopia with correction o Concave lens that undoes some of the refractory power of which your eye has too much • Hyperopia o Opposite of myopia o Lens is too weak o Light gets bent at point beyond the retina o Effect , things looked blurred as point is behind the retina o Happens with objects that are close by o Lens gets less flexible o Less and less relaxed o Would need convex lens to see something o What mom has … • These problems are caused by the lens not fitting the needs of our eyes Astigmatism • Same contrast? • Has to do with the cornea o Strongest mechanism for refraction • Misshaped cornea causes problems • Cornea is supposed to be spherical, if it looks like a rugby shape… then you have astigmatism • Very common o What I also have • If you take off your glasses at night, you see a lot of stars instead of a dot of light Eyes that see Light cont’d • Let’s take a look • Which layers and cells does the retina contain? • Two pathways in the retina? • The receptive field of ganglion cells • Using the ophthalmoscope, you can view the back surface of a person’s eyes: fundus o Eye doctors will see this if they use the device. • Check out whether everything is fine with the retina • Retina is part of the brain • Three things to notice o (Macula) Fovea (dark spot in the middle)  Most acute vision o Optic disk (yellow dot to the side)  No photoreceptors there  Used to rule out glaucoma  All sorts of stuff enters and exits the eye • Blood vessels meet at this point • Axons of cells o Blood vessels The Blind Spot • Optic disk is where the blind spot is • Brain chooses to ignore the blind spot • B) Can see a continuous bar rather than a hole The Retina • Light comes from the lower direction in the image • Light passes through a jungle of cells before it stimulates the cells that are able to sense light • Ganglion cells are not sensitive to light themselves o They are stimulated by the cells that are sensitive to light • Photoreceptors are in the back • Eyes evolved in different ways o Compound eye • Squids have eyes that have photo receptors at the front of the eye  no blind spots • Have photoreceptors: rods & cones • Bipolar cells  ganglion cells horizontal cells  amacrine cells Retinal Information Processing • Photoreceptors: Cells in the retina that initially transduce light energy into neural energy. Duplex retinas: o Rods: photoreceptors that are specialized for night = scotopic vision (90 million)  Night time vision o Cones: Photoreceptors that are specialized for daylight = photopic vision, fine visual acuity and color (4-5 million)  Day time vision  Much more acuity than rods o Some animals have mostly rod retinas, (e.g., rats, owls) or only cones (some lizards)  Nocturnal animals have mostly rods  Lizards have fantastic colour vision = only cones o 3 photoreceptor for circadian rhythm Photoreceptor Density across the Retina • Different photoreceptors in the eye • Two different curves • Red=cones • Blue=rods • Cones spike at 0 (distance from fovea/directly inside fovea) • Largest amount of photoreceptors (especially cones) inside the fovea o Very small, fine resolution • Mostly in the centre, we don’t have blue cones • Count of cones drop quite quickly as we move out to the side • @ 10 visual degrees from the fovea: low count of cones • Difficult to read small letters out of the corner of your eye • Rods are not present in the fovea at all o Fovea reserved for reading and daylight • Looking at dim star at night, easier to see it out of the corner of your eye o Use peripheral vision o More rods • Optic disk has no cones nor rods • Nasal portion of retina (where optic disk exits the eye) o But blind spot is on the right side o Why is the optic disk opposite of the blind spot?  Right eye projects into the left hemisphere
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