Perception Week Six Notes
Perception Week Six Notes 222
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This 4 page Class Notes was uploaded by Sarah Kincaid on Thursday October 13, 2016. The Class Notes belongs to 222 at Boston University taught by Rucci in Winter 2016. Since its upload, it has received 9 views. For similar materials see Psychology of Perception in Psychology at Boston University.
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Date Created: 10/13/16
9 The retina Refraction: - necessary to focus light rays - accomplished by cornea and lens Accommodation: - process in which lens changes shape, altering its refractive power - Distance from object determines how much we need to bend ray of light for it to come into focus Emmetropia: - happy condition of no refractive error (contacts/glasses if you don't do that naturally) - Rays are converging at same point (cornea is working with lens & refractory index to bend the rays of light to the right amount) - This doesn't happen for most of us o Most of us are myopic today (esp. Asia) The system is not refracting right - the eye is too large, the rays intersect and make vision blurry/blob nearsighted Refraction doesn't work correctly because eye is too long o During childhood, most people are hyperopia The eye is too short, so the rays of light coming from single point far away are bent not enough to converge on surface of retina farsighted Image is out of focus For some reason, our eyes continue to grow through childhood and grow until we are myopic With hyperopia, we can use different strength lenses to correct With contacts/glasses, our refraction is corrected To correct vision with surgery, change curvature of cornea Astigmatism - refractory power isn't uniform across all possible orientations How does lens contract and relax? - Ciliary muscle has strings, Zonules of Zinn, the lens is floating there attached by strings o when relaxed, strings are tense and lens is thin (when not using ciliary muscle) o When you want to focus on objects close by, contract muscles, release strings, and lens springs into thicker bulb (bends light smaller) Diopter: - unit of refractive power that is equal to reciprocal of focal length (in meters) of a given lens (e.g. to focus image of object at 0.5 m you need 2 diopters) - Diopter = 1/distance = 1/0.5 = 2 diopters - Diopter tells you how powerful lens is in bending light; bigger number = more bending What’s Presbyopia? - During the natural aging process, the muscle still contracts but the lens gets hard - A hard lens = harder for the muscles to change lens shape - This means that close objects are no longer focused clearly on retina o Ex. Why dad has to hold menus at arm’s lengthLose elasticity? Why does this happen? - Bc of how it grows? - No one knows! Camera analogy for the eye F-stop: iris/pupil - regulates amount of light coming into eye Focus - lens - changes shape to change focus Film - retina - records image Camera only records image, visual system also interprets scene! Where will first notice accommodation isn’t working well? - In a dimly light place (i.e. a restaurant) - When it is dark, the pupil is open more so the lens must work well for images to be in focus - When it is bright, the pupil is really small, so everything is in focus (don’t need the lens to adjust) What is another common problem of lens? - becoming opaque (i.e. cataracts) Why is this a problem? - Lens is transparent because crystallins are packed very regularly - Cataracts opacities in lens caused by irregularity in crystallins o Opacities absorb and scatter light, so less light reaches retina, resulting in blurry images - Congenital cataracts are present at birth and severely affect visual development Famous Painting: Impression, soleil levant (1872) Claude Monet This painting of his gave name to impressionism Visual system of humans - how light is perceived by us When he was younger, he painted colorful, happy pics When older, he did much darker, muted pics Suffered from terrible cataracts - his color perception was badly altered With ophthalmoscope see back of eye we can see… Fundus (back of eye) o Photoreceptors (tubules that catch light) Bright circle: blind spot - all fibers from neurons come together and bring away info - optic nerve dark region: fovea - highest acuity Capillaries Regions that behave very differently Retina Entry point of visual system We don't know a lot on how it works General structure o Built opposite of what you would expect o Light comes from fundus and goes through layers of neurons before reaching receptors (counter-intuitive, damn evolution) Light going from inside eye towards outside Need to be close to pigment epithelium = provides nutrients All these neurons are transparent and thus do not affect image quality Have blood vessels - where they are, there is no image o From receptors to bipolar cells to ganglion cells (vertical pathway bringing info from retina) o Bipolar cells connected to amacrine and ganglion cells o Ganglion cells have axons that leave o There are vertical and horizontal cells Summary: Light comes from bottom up through all neurons to photoreceptors, and send to bipolar to ganglion to bundle of fibers that form blind spot and info leaves eyes! 2 types of photoreceptors: cells in retina that initially transduce light energy into neural signals Rods - night vision - work best in low lighting - no color processing Cones: - daylight vision - finer acuity - color - work best with lots of light Why do we have two? - because they allow us to function in difference environments (dark, bright, etc) o Duplex retina in humans o Others don’t have that (owls - just night, chameleons - just day) - Rods & cones have special molecules containing a protein, opsin, that changes shape when struck by photon Photon – smallest unit of light Capturing a photon: when light hits eye, process of photo activation begins Photon has right energy (wavelength) and opsin changes structure to receive photon Rods & Cones Structure 3 regions: outer segment, inner segment, synaptic terminal Visual pigment molecules are generated in inner segment Pigments are incorporated into membrane of outer segment Pigments contain an opsin & a chromophore (do not need to remember opsin or chromophore) Each cone contains one of three pigments Rods contain rhodopsin. Cones contain three different opsins Melanopsin is used by receptors w/in ganglion cells which send their signals to suprachiasmatic nucleus to regulate the circadian rhythm o Not just photoreceptors can receive light, ganglion cells with melanopsin also receive light and use it to regulate day & night for body Most are rods Cones have different wavelengths - depending on pigment Photo activation - biochemical cascade of events leading to change of glutamate concentration for rods & cones (only difference is which photons they absorb, due to dif pigments) Photon is absorbed by visual pigment Pigment changes shape Channels that normally allow Na+ to enter the cell close Cell hyperpolarizes (inside become s more negative) Less glutamate is released at synaptic terminal (graded potentials) - when capture a photon, send out less neuromotivators (more activity in dark) Following bipolar cell will become hyperpolarized/depolarized depending on its type All in a few ms! Layout of photoreceptors Humans have ~90 million rods and ~4-5 million cones Distributions change with eccentricity (distance from fovea) Cones are very densely packed in fovea - at center, need lots of cones for fine details Move a few cm from fovea, and cone density drops dramatically At night, you cannot see straight ahead - need cones for that, only use rods at night (not center of gaze) Rod density increases the as you move further from fovea (20 degrees outside fovea is max density of rods) If you want to see a star clearly at night, look to the side of it
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