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PSYB51H3 (301)
Chapter 2

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University of Toronto Scarborough
Matthias Niemeier

PSYB51H3- Chapter 2: The First Steps in Vision From Light to Neural Signals A little light Physics:  Light is made up of waves when it moves around the world, and is made up of photons when it is absorbed  Wave-> an oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium  Photon-> A quantum of visible light or other form of electromagnetic radiation demonstrating both practice and wave properties  Once it reaches the atmosphere the starlight’s photons will be absorbed by dust and vaporized water etc. o And some will be scattered by these particles o if the light strikes a light- coloured surface it will be reflected o light that is neither reflected or absorbed is transmitted through the surface o as light travels from air into glass some of the light rays are bent or refracted as it is transmitted Eyes That See Light:  An eye can form images of the outside world, enabling animals that possess eyes to use light to recognize objects  Eye Structure: o Cornea-> provides a window to the world because it is transparent-> most photons are transmitted though  It is transparent because it is made of highly ordered arrangements of fibers and because it contains no blood vessel or blood which absorb light  This is the part of the eye that contact lens sit on o Aqueous Humor-> the watery fluid in the anterior chamber of the eye o Crystalline Lens-> the lens inside the eye that enables the changing of focus o Pupil-> the dark, circular opening at the center of the iris in the eye, where light enters the eye o Iris-> The coloured part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding and contracting the pupil o Vitreous Humor-> The transparent fluid that fills the vitreous chamber in the posterior part of the eye o Retina-> A light- sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve Focusing Light on the Retina  Accommodation-> the process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects)  Presbyopia-> Literally “old sight” o The loss of near vision because of insufficient accommodation  Cataract-> An opacity of the crystalline lens o Caused by the irregularity of the crystallins o Most are discovered after age 50, and the prevalence of cataracts increases with age so that by age 70 almost everyone has lost some transparency  Emmetropia-> the condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball  Myopia-> Nearsightedness, a common condition in which light entering the eye is focused in front of the retina and distant objects cannot be seen sharply  Hyperopia-> Farsightedness, a common condition in which light entering the eye is focused behind the retina and accommodation is required in order to see near objects clearly  Astigmatism-> A visual defect caused by the unequal curving of one or more of the reflective surfaces of the eye, usually the cornea The Retina  Transduce-> To convert from one form of energy to another (e.g. from light to neural electrical energy, or from mechanical movement to neural electrical energy)  Fundus-> The back layer of the retina—what the eye doctor sees through an ophthalmoscope  Photoreceptors-> where the transduction of light energy into neural energy begins o A light-sensitive receptor in the retina Retinal Information Processing  Rod-> a photoreceptor specialized for night vision  Cone-> A photoreceptor specialized for daylight vision, fine visual activity, and color  Duplex-> In reference to the retina, consisting of two parts: the rods and cones, which operate under different conditions  Outer Segment-> the part of a photoreceptor that contains photopigment molecules  Inner segment -> the part of a photoreceptor that lies between the outer segment and the cell nucleus  Synaptic Terminal-> the location where axons terminate at the synapse for transmission of information by release of a chemical transmitter  Chromophone-> the light-catching part of the visual pigments of the retina  Rhodopsin-> the visual pigment found in rods o Each cone has one of the3 pigments which respond to long, medium and short wavelengths  Melanopsin-> a photopigment that is sensitive to ambient light  Photoactivation-> activation by light o Initiates a biochemical cascade of events eventually resulting in the closing of channels in the cell membrane that normally allow ions to flow into the rod outer segment o Closing these channels alters the balance of electrical current between the inside and outside of the rod outer segment, making the inside of the cell more negatively charged  Hyperpolarization-> an increase in membrane potential such that the inner membrane surface becomes more negative than the outer membrane surface  Graded potential-> an electrical potential that can vary continuously in amplitude o Photoreceptors respond with graded potentials on to bi-polar cells NOT AP’s  Eccentricity-> the distance between the retinal image and the fovea o The cones are most concentrated in the center of the fovea and their density
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