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Chapter 3

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Bruce Mc Kay

Chapter 3  Introduction to Vision Focusing Light Onto the Retina Light: The Stimulus for Vision  Vision is based on visible light, which is a band of energy within the electromagnetic spectrum: a continuum of electromagnetic energy that is produced by electric charges is radiated as waves.  Wavelength: the distance between the peaks of the electromagnetic waves.  Visible Light: the energy within the electromagnetic spectrum that humans can perceive, has wavelengths ranging from 400-700 nm, where 1 nanometer = 10 –9 meters. The Eye  Light is reflected from objects in the environment and enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina, which contains receptors for vision.  Two types of receptors: rods and cones, that contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals.  The signals then emerge from the back of the eye in the optic nerve, which conducts signals toward the brain. Light Is Focused by the Eye  Needs to be focused onto the retina, the transparent covering on the eye accounts for 80% of the eyes focusing power.  Far away objects, the rays appear parallel, and focus at point, closer objects make the rays on an angle.  Accommodation: Allows you to bring both near and far objects into focus, although objects at different distances are not in focus at the same time.  Distance at which your lens can no longer adjust to bring close objects into focus is called the near point. o Presbyopia: near point increases, as a person gets older. o Myopia/Nearsightedness: inability to see distant objects clearly.  (1) Refractive Myopia: which the cornea and/or lens bends at the light too much (2) Axial Myopia: eyeball is too long.  The distance at which the spot of light becomes focused on the retina is called the far point. o Hyperopia/Farsightedness: can see distant objects clearly but has trouble seeing nearby objects.  Laser-Assisted in Situ Keratomileusis (LASIK) Transforming Light into Electricity Visual Receptors & Transduction  Transduction is carried out by receptors (neurons specialized for receiving environmental energy and transforming this energy into electricity)  Key part for rod transduction is the outer segment, because it is here and the light acts to create electricity. Contains stacks of discs, containing thousands of visual pigment molecules.  Opsin: long strand of protein, which loops back and forth across the disc membrane 7 times. The molecule retinal is attached – crucial for transduction because it is sensitive to light. o When a photon of light hits the retinal, it changes shape, so it is sticking out from the opsin – isomerization. How Does Transduction Occur?  One way to approach the problem is to study the chemistry of visual pigments in a chemistry or physiology lab or to study the relationships.  Another way is to do psychophysical experiments in which we measure relationship PP to provide information about underlying physiology.  Hecht’s Psychophysical Experiment: He concluded that a person can see light if 7 rod receptors are activated, and a rod receptor can be activated by the isomerization of just 1 visual pigment molecule. o Determined a person’s absolute threshold for brief flash of light.  A biological chemical that in small amounts facilitates chemical reactions in this way is called an enzyme; the sequence of reactions triggered by the activated visual pigment molecule is called the enzyme cascade. Pigments and Perception Distribution of the Rods and Cones 1. One small area, fovea contains only cones. Look directly at image – on fovea. 2. Peripheral Retina: includes the entire retina outside of the fovea, contains both rods and cones. 3. Many more rods than cones in peripheral because retina’s receptors are located there 120 million rods: 6 million cones.  Macular Degeneration: which is most common in older people, destroys the con-rich fovea and small area that surrounds it – creates a “blind spot” in central vision so when a person looks at something they lose sight.  Retinitis Pigmentosa: degeneration of the retina that is passed from one generation to the next – first attacks peripheral rod receptors and results in poor vision in peripherals – can result in complete blindness.  Blind Spot: because of the absence of receptors – unaware od because it is located off side of our visual field. Dark Adaptation of the Rods and Cones  When you’re in the dark, the process of dark adaptation occurs which causes the eye to increase its sensitivity to the dark.  Dark Adaptation Curve: a plot of how visual sensitivity changes in the dark, beginning with when the lights are extinguished. Measuring Dark Adaptation  Light adapt observer by exposure to light. While adapting light on, observer indicates their sensitivity by adjusting the intensity of a test light so it can just barely be seen. o Light-Adapted Sensitivity: because it is measured while the eyes are adapted to
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