III: Vision. Objectives:

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1 III: Vision Objectives: Describe the characteristics of visible light, and explain the process by which the eye transforms light energy into neural. Describe how the eye and the brain process visual information. Discuss the theories that help us understand color vision. 1

2 Module 18 The Wavelengths we see What we see as light is only a tiny slice of a wide spectrum of electromagnetic energy, which ranges from gamma rays as short as the diameter of an atom to radio waves over a mile long. The wavelengths visible to the human eye (below) extend 2 from the shorter waves of blue-violet light to the longer waves of red light.

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4 Light Energy Two physical characteristics of light help determine our sensory experience of them. Wavelength The distance from one wave peak to the next determines its perceived hue (the color we experience) Intensity (amplitude) The amount of energy in light waves determined by a wave s amplitude (height) influences brightness 4

5 (a) Waves vary in wavelength (the distance between successive peaks). Frequency, the number of complete wavelengths that can pass a point in a given time, depends on the wavelength. The shorter the wavelength, the higher the frequency. Wavelength determines the perceived color of light (and also the pitch of sound). (b) Waves also vary in amplitude (the height from peak to trough). Wave amplitude determines the brightness of colors (and also the loudness of 5 sounds).

6 Module 18 Retinal Processing Light rays reflected from a candle pass through the cornea, pupil, and lens. The curvature and thickness of the lens change to bring nearby or distant objects into focus on the retina. Rays from the top of the candle strike the bottom of the retina, and those from the left side of the candle strike the right side of the retina. The candle s image on the retina thus appears upside down and reversed. 6

7 Part of EYE Function Cornea Light enters the eye through the cornea. Protects the eye and bends light to provide focus. Pupil Light then passes through the pupil, a small adjustable opening Iris (very distinctive) Lens A ring of muscle tissue that forms the colored portion of the eye around the pupil and constricts in response to light intensity and even inner emotions The transparent structure behind the pupil that changes shape to help focus images on the retina. The lens focuses the rays by changing its curvature in a process called accommodation. Retina The light-sensitive inner surface of the eye, containing the receptor rods and cones plus layers of neurons that begin the processing of visual information 7

8 The Lens: Accommodation (adjustments) The process by which the eye s lens changes shape to help focus near or far objects on the retina. 8

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10 Nearsighted 10

11 Farsighted 11

12 RETINA Function Photoreceptors - Rods and Cones - Retina cells that respond to light Rods - Most sensitive to light and dark changes - Retinal receptors that detect black, white, and gray - Rods are more numerous than cones in periphery of retina Cones - Not sensitive to light; most sensitive to (green, red, blue) - Signals from cones sent to brain which then translates these messages into the perception of color - Only works in bright light - Color vision, fine details - Color blind does not have a particular type of cone in the retina or the cone may be weak 12

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14 Optic Nerve Fovea Blind Spot - The nerve that carries neural impulses from the eye to the brain - Central region of retina = most clear vision - No rods; only cones (packed together closer) - Blood vessels and nerve fibers go around fovea providing a direct path to the photoreceptors - This is where the optic nerves come together and exit the eye on the way to the brain - Part of the retina that does not contain photoreceptors - Any image that fills on this region you will not see 14

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16 Receptors in the Human Eye: Rod-Shaped Rods and Cone Shaped Cones Cones Rods Number 6 million 120 million Location in retina Center Periphery Sensitivity in dim light Low High Color Sensitivity High Low Detail sensitivity High Low When you enter a darkened theater or turn off the light at night, your eyes adapt. Your pupils dilate to allow more light to reach your retina, but it takes 20 minutes or more before your eyes fully adapt. 16

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18 How do the eye and the brain process visual information? after processing by your retina s nearly 130 million receptor rods and cones info travels to bipolar cells then to ganglion cells through axons making up the optic nerve to thalamus where the axons synapse with neurons that run to the visual cortex in the occipital lobe at the back of your brain 18

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20 Feature Detection Nerve cells in the brain that respond to specific features of the stimulus, such as shape, angle, or movement Specialized neurons in the occipital lobe s visual cortex receive information form individual ganglion cells in the retina These cells pass this information to other cortical areas, where team of cells (supercell clusters) respond to more complex patterns 20

21 Nerve cells in the visual cortex respond to specific features, such as edges, angles, and movement. 21

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23 Parallel Processing Our brain computes multiple things at once; a scene is analyzed the brain divides it into subdimensions motion, form, depth, color -and works on each aspect simultaneously Other neural teams integrate the results, comparing them with stored information and enabling perception 23

24 Visual Information Processing Scene Retinal processing: Receptor rods and cones > bipolar cells > ganglion cells Feature Detection: Brain s detector cells respond to specific features edges, lines, and angles Parallel processing: Brain cell teams process combined information about motion, form, depth, color Recognition: Brain interprets the constructed image based on information from stored images 24

25 What theories help us understand color vision? Young-Helmholtz trichromatic theory The theory that the retina contains three different color receptors one most sensitive to red, one to green, one to blue, which when stimulated in combination, can produce the perception of any color Most people with color-deficient vision are not actually color blind. They simply lack function red or green sensitive cones or sometimes both. (monochromatic or dichromatic) 25

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27 Opponent-process theory Can be see when we look at after images! The theory that opposing retinal processes (red-green, yellow-blue, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green If there is a red and green marble competing going through a narrow tube both cannot travel at once (they are opponents) However red and blue travel in separate channels, so we can see a reddish-blue magenta 27

28 Opponent-process theory Cones Retinal Ganglion Cells 28

29 Opponent Process- Afterimage Effect They are caused by fatigued cells in the retina responding to light. 29

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