Contents Contents Test Objectives 1 Eye and Retina 3 Test Objectives 2 Eye and Retina 7 Test Objectives 3 Eye and Retina 9 Test Objectives 4 Eye and Retina 11 Test Objectives 5 Visual Pathways from Retina to CNS 14 Test Objectives 6 Visual Processing in Cortex 18 Test Objectives 7 What can visual cortex teach us about the nature nurture debate 20 Test Objectives 1 Eye and Retina What is light Where does it fit into the spectrum of electromagnetic radiation o Light is a form of electromagnetic radiation energy produced by vibrations of electrically charged material Light is the stimulus for vision It is only a fraction of the spectrum of electromagnetic radiation Our sensitivities range from 400 700nm wave An oscillation that travels through a medium by transferring energy from one particle or point to another without causing permanent photon A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties o 2 ways to conceptualize light Waves wavelengths are understood similar to frequencies Photons tiny particles that each consist of one quantum of energy think of as little packets of energy electromagnetic wave stream of photons Why is short wavelength electromagnetic radiation dangerous to us whereas long wavelength electromagnetic radiation is considered safe o The shorter the wavelength the higher the energy o Short wavelengths tiny enough to penetrate cell and break the chemical bond of DNA Doesn t kill the cell burst of energy strong enough to damage the DNA causing mutation Light is useful if 2 criteria are met EMR must be able to travel in a straight line through gas Must reflect light bounce off objects and into the eye Can t be too short Can t be too long o Long wavelengths are less likely to collide with DNA but even if it does collide it doesn t have enough energy to change the DNA just bounces off Which wavelengths do we see as Light Why these wavelengths Why couldn t the shorter and longer wavelength stuff work just as well o We see 400 700 nm nanometers o Light is only useful to the extent that it bounces off objects in our environment that we need to see o Different color perceptions are associated with different wavelengths of visible light Given the properties of Light what has to be different about the sensory system that detects it Which properties of Light are related to Hue color and Brightness o Your brain has created perceptual qualitative categories for color o Qualitative color system Not viewed as on a continuum Placing visual objects into categories based on color e g we don t perceive an object s color as more or less green o Brightness of a particular color is related specifically to the number of photons your eye absorbs Iris varies in diameter in order to regulate intake of photons Eyes must continuously regenerate their light absorbing capacity as it gets used up Thus the photoreceptors are the busiest of the sensory receptors When your eye takes in more photons than capacity you become blind reflecting others o Pigment refers to any molecule that absorbs some wavelengths while o 3 receptors for abundant light color vision RGB Across receptor pattern coding for color using just three broadly tuned receptors we can perceive an enormous number of different colors For example white R ON G ON B ON yellow R ON G ON B OFF The visual pigment molecules consist of an opsin a protein that detects the wavelength of light and a chromophore which captures photons of light The opsin used by rods is called rhodopsin Each cone has one of three other opsins that detect long medium or short wavelengths Photoreceptors Functional differences between rods and cones thresholds o Photons wavelength color and number brightness o Rods and cones are distributed differently throughout the retina In the periphery of the retina rods are highly concentrated and there are few cones When images fall in the periphery of the retina from out peripheral vision visual acuity is not very good Conversely in the center of the retina an area called the fovea there are few rods and a high concentration of cones This area is where visual acuity is the greatest Additionally there is an area in the retina where there are neither rods nor cones This area is called the blind spot and contains no photoreceptors because this is where the optic nerve leaves the eye to transmit visual information to the brain o Rods and Cones Differ in Sensitivity to Light note that these threshold curves are just inverted absorbance curves o Rods most sensitive to green light i e 510 nm o The amount of light required for Photopic Cone vision is generally TOO MUCH light for Scotopic Rod vision Test Objectives 2 Eye and Retina The basic structure and function of the human eye retina Anatomy of the Eye which are the moving parts Function of curved optical elements of the eye cornea lens o Light enters through the cornea which acts as a protective layer of the eyeball When it is scratched it forces the eye to produce tears which help prevent further scratching o Light then travels through a fluid filled chamber called the aqueous humor then travels through the pupil and iris These structures help regulate the amount of light that enters the eye for example in a dark room the pupil will enlarge to let in more light o After the iris light is refracted through the lens The function of the lens is to refract the light in such a way that we have the most accurate possible representation on the retina o Light is then refracted through the vitreous humor another fluid filled chamber o Finally light is projected onto the retina which spans the backside of the eyeball The retina is where photoreceptors are located How does variation in the shape of the eye lead to poor eyesight Test Objectives 3 Eye and Retina Anatomy of Retina photoreceptors bipolar cells ganglion cells o The Blind Spot s There are several cell layers that transmit visual information before it enters the brain Photoreceptor cells connect to horizontal cells and bipolar cells Bipolar cells in turn connect to amacrine cells and ganglion cells Photoreceptors are the innermost cell layer light has to bypass all other cell layers to activate photoreceptor cells Visual information is transmitted through three cell layers the photoreceptor bipolar and ganglion cells Cross talk between cells in the same layer is achieved though the horizontal cells and amacrine cells How is the trade off between detection and
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