PSY 2012 0004 Katie s Notes Chapter 4 Sensation and Perception Sensation the conversion of energy from the environment into a pattern of response by the nervous system Perception the interpretation of that information 4 1 Vision Vision is the detection of light Sensation in general is the detection of stimuli o Stimuli energies from the world around us that affect us in some way Our eyes ears and other sensory organs are packed with receptors o Receptors specialized cells that convert environmental energies into signals for the nervous system What we call light is part of the electromagnetic spectrum o Electromagnetic spectrum the continuum of all the frequencies of radiated energy o Visible light is a small part of the electromagnetic spectrum We see these wavelengths because we have receptors that respond to them o The human eye detects energy wavelengths from 400 700 nm Structures of the Eye Pupil an adjustable opening in the eye that widens and narrows to control the amount of light 012entering the eye Iris the colored structure on the surface of the eye surrounding the pupil Retina light that passes through the pupil travels through the vitreous humor a clear jellylike substance to strike the retina the layer of visual receptors covering the back surface of the eyeball Cornea a rigid transparent structure on the outer surface of the eyeball focuses light coming into the eye Lens a flexible structure that can vary in thickness enabling the eye to accommodate that is to adjust its focus for objects at different distances our eye muscles relax and the lens becomes thinner and flatter When we look at a close object our eye muscles tighten and the lens thickens and rounds Fovea the central area of the human retina that is adapted for highly detailed vision when we look at a distant object 1 PSY 2012 0004 Katie s Notes Table 4 1 pg 100 Common Disorders of Vision Presbyopia Myopia Hyperopia Glaucoma Cataract Impaired ability to focus on nearby objects because of decreased flexibility of the lens Nearsightedness impaired ability to focus on distance objects Farsightedness impaired ability to focus on close objects Condition characterized by increased pressure within the eyeball which can sometimes impair vision A disorder in which the lens becomes cloudy Visual Receptors The two types of visual receptors cones and rods differ in function and appearance Cones adapted for color vision daytime vision and detailed vision Rods adapted for vision in dim light Shape Rods Nearly cylindrical Tapered at one end Prevalence in human retina 90 95 Abundant in All vertebrate species Species active during the day birds monkeys apes humans Area of the retina Toward the periphery Toward the fovea Important for color vision Important for detail Important for dim light Number of types No No Yes Just one Cones 5 10 Yes Yes No Three Dark Adaptation When you walk into a dark room it s hard to see any objects around you But the longer you wait the more detail you can see This is because of our dark adaptation Dark adaptation gradual improvement in the ability to see in dim light Exposure to light chemically alters molecules called retinaldehydes stimulating the visual receptors o The receptors regenerate rebuild the molecules as fast as the light alters them In daylight the molecules are depleted and regenerated at the same rate The amount in the retina is balanced and visual sensitivity level is constant In darkness the receptors regenerate molecules with no depletion The increase in retinaldehydes enhances dark adaptation 2 PSY 2012 0004 Katie s Notes Cones and rods adapt at different rates o Cones regenerate their retinaldehydes more quickly in all conditions but are constantly regenerating o Rods use daytime to fully regenerate their retinaldehydes The large number and undisturbed regeneration process of rods causes their much higher sensitivity to faint light These graphs show dark adaptation to a a light you stare at directly using only cones and b a light in your peripheral vision which you see with both cones and rods Concept Check You may have heard people say that cats can see in total darkness Is that possible Like people cats adapt to dim light No animal however can see in complete darkness There must be some light present for the rods to function After you have thoroughly adapted to extremely dim light will you see more objects in your fovea or in the periphery of your eye You will see more objects in the periphery The fovea contains only cones which cannot become as sensitive as the rods in the periphery Color Vision There are three different stages in which our visual system converts different wavelengths into perception of color 1 Trichromatic theory Young Helmholtz theory states that our receptors respond to three primary colors color vision depends on the response rates of three types of cones a One type is most sensitive to short wavelengths BLUE b One type is most sensitive to medium wavelengths GREEN c One type is most sensitive to long wavelengths RED d Helmholtz found that observes could mix various amounts of three wavelengths of light match all other colors Blue cones are less numerous than the other types of cones Consequently a tiny blue point may look black 3 PSY 2012 0004 Katie s Notes 2 Opponent Process theory states that we perceive color in terms of paired opposites a RED versus GREEN b YELLOW versus BLUE c WHITE versus BLACK d Negative afterimages experiences of one color after the removal of another Stare at one of Daffy s pupils for a minute or more under a bright light without moving your eyes Then look at a plain white or gray background You will see a negative afterimage Negative after images result from alternating stimulation and inhibition of visual system neurons A bipolar neuron that responds to yellow is inhibited by blue After staring at a yellow object your fatigued bipolar cell behaves as if it s been inhibited yielding blue Concept Check A bipolar cell is stimulated by red wavelengths You stare at a red object What will happen when you stop staring You will see a negative after image in green Why do negative after images that you see seem to move around Because the image is in your eye not from any object at which you are gazing 4 PSY 2012 0004 Katie s Notes 3 Retinex theory the tendency of an object to appear nearly the same color under a variety of lighting conditions color constancy a We perceive color when the cerebral cortex compares various retinal