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Chapter 4

PSY100 Chapter 4 Textbook Notes.doc

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University of Toronto Mississauga
Dax Urbszat

Nov/1/2003 CHANAPS Notes From Reading C HAPTER 4: SENSATION AND PERCEPTION I. Psychophysics: Basic Concepts and Issues A. Introduction 1. Virtual Agnosia – inability to recognize objects through sight. 2. Sensation – the stimulation of sensory organs. a. Involves the absorption of energy (i.e. light or sound waves) by sensory organs (i.e. eyes, ears) 3. Perception – the selection, organization, and interpretation of the sensory input. a. Organizing and translating sensory input into something meaningful. 4. Psychophysics – the study of how physical stimuli are translated into psychological experience. a. Important psychologist – Fechner (Germany). B. Thresholds – Looking for Limits 1. Stimulus – any detectable input from the environment. 2. Threshold – a dividing point between energy levels that do and do not have a detectable effect. 3. Absolute Threshold – minimum amount of stimulus that an organism can detect. Defines the boundaries of an organism’s sensory capabilities. a. As Stimulus intensity increases, probability of responding to a stimuli gradually increases. b. As such, absolute threshold is the stimulus intensity that is detected 50% of the time. C. Weighing the Differences: The JND 1. Just Noticeable Difference (JND) – smallest difference in the amount of stimulation that a specific sense can detect. 2. JND is greater for heavier objects than lighter ones. 3. Smallest detectable difference is a constant proportion of the weight of the original object. 4. Webber’s law – the size of a just noticeable difference is the constants proportion of the size of the initial stimulus, known as the “Webber Fraction”. a. Applies not only to weight but to all senses. 5. As the stimulus increase in magnitude, the JND becomes larger. D. Psychophysical Scaling 1. Fechner’s Law – the magnitude of a sensory experience is proportional to the number of JND’s that the stimulus causing the experience is above absolute threshold. a. Constant increments in stimulus intensity produce smaller and smaller increases in the perceived magnitude of sensation (i.e. Light bulbs in a room). 2. Questioned by Stevens, who came up with idea of Magnitude estimation – asking subjects to assign numbers to stimuli on the basis of how intense they appeared to be. a. Magnitude estimation is the best way to map the relations between stimulus intensity and sensory experience. E. Signal Detection Theory 1/7 Nov/1/2003 CHANAPS Notes From Reading C HAPTER 4: SENSATION AND PERCEPTION 1. Signal detection theory proposes that the detection of stimuli involves decision processes as well as sensory processes which are both influenced by a variety of factors beside stimulus intensity. a. Your Performance will depend on the criterion you set for how sure you must feel before you react, the level of noise in the system. 2. Detectability is measured in probability and depends on decision making processes as well as sensory processes. F. Perception without Awareness 1. Subliminal Perception – the registration of sensory input without conscious awareness. 2. Subliminal stimulation generates weak effects. G. Sensory Adaptation 1. Sensory adaptation is a gradual decline in sensitivity to prolonged stimulation. 2. Automatic, built in process that keeps people tuned in to the changes rather than the constants – ignore the obvious, and watch for threats. II. Our Sense of Sight: The Visual System A. The Stimulus: Light 1. Light is a form of electromagnetic radiation that travels as a wave. 2. Vary in amplitude (height), wavelength, purity. a. Amplitude – perception of brightness b. Wavelengths – perception of color c. Purity – perception of richness/saturation B. The Eye: A Living Optical Instrument 1. Two Purposes: a. Channel light to the neural tissue that receives it (retina) b. House the retina 2. Light enters the eye through the cornea (transparent window). Cornea and Lens form an upside down image of the object on the retina. 3. Lens – transparent eyes structure that focuses the light rays falling on the retina. a. Made up of relatively soft tissue, capable of adjustments and accommodation b. Accommodation – when the curve of the lens adjusts to alter visual focus. 4. Nearsightedness – close objects are seen clearly, but distant objects blurry. a. Due to the focus of light from distant objects falling short of the retina. 5. Farsightedness – farther objects are seen clearly but close seem blurry. a. Due to the focus of light from near objects falling behind the retina 6. Iris – colored ring surrounding pupil (black center of the eye) 7. Pupil – the opening in the center of the iris that helps regulate the amount of light passing into the rear chamber of the eye. C. The Retina: The Brain’s Envoy in the Eye 1. Retina- the neural tissue lining the inside back surface of the eye. Absorbs light, processes images, and sends visual info to the brain. 2. Optic Disk - a hole in the retina where the optic nerve fibers exit the eye. 3. Visual Receptors – Rods and Cones 2/7 Nov/1/2003 CHANAPS Notes From Reading C HAPTER 4: SENSATION AND P ERCEPTION a. Cones – specialized visual receptors in the retina that play a key role in daylight vision and color vision. i. Do not respond well to dim light ii. Cones provide visual acuity (sharpness and detail) iii. Concentrated in center of retina iv. Fovea – tiny spot in center of the retina which only contains cones, greatest visual acuity. b. Rods – specialized visual receptors in the retina that play a key role in night vision and peripheral vision. i. More sensitive than cones to dim light ii. Handle peripheral vision iii. Density greatest outside the fovea, and gradually decreases 4. Dark and Light Adaptation a. Dark Adaptation – the process in which the eyes become more sensitive to light in low illumination. b. Light Adaptation – the process where eyes become less sensitive to light in high illumination. 5. Info Processing in the Retina a. Light hitting cones and rods triggers neural signals that pass into the intricate network of cells in the retina. b. Signals move from receptors to bipolar cells to ganglion cells, sending impulses to optic nerve (axons that connect the eye to brain). c. Axons carry visual info encoded as a stream of neural impulses to the brain. d. The Receptive field of a visual cell – retinal area that when stimulated effects the firing of that cell. D. Visual and The Brain 1. Visual Pathways to the Brain a. Axons from the eye form the optic nerve, travel to the optic chiasm b. Optic Chiasm - The point at which the optic nerves from the inside half of each eye cross over and then project to the opposite half of the brain. Ensures that each half of the brain gets info from both eyes. c. After reaching the optic chiasm, the optic nerve splits between 2 pathways – main and secondary. i. Main – hypothalamus ii. Secondary pathway function – coordination of visual info with other sensory info. d. Main visual pathway is divided in 2 more parts – magnocellular (where) and parvocellular (what) i. Engage in parallel processing – simultaneously extracting different kinds of info from the same input. E. Info Processing in the Visual Cortex 1. Cells in the primary cortex don’t respond to little spots, instead they respond to lines, edges, and other stimuli. 2. Feature Detectors – neurons that respond selectively to very specific features of complex stimuli. 3/7 Nov/1/2003 CHANAPS Notes From Reading C HAPTER 4: SENSATION AND PERCEPTION 3. Visual Agnosia – an inability to recognize objects. May be due to result somewhere along the visual pathway. 4. Prosopagnosia – inability to recognize familiar faces. F. Viewing the World in Color 1. The Stimulus for Color a. Wavelengths hold greatest influence. Most closely related to hue, amplitude to brightness, and purity to saturation. b. Subtractive Color Mixing – removing some wavelengths of light leaving less light than way originally there (i.e. Crayons). c. Additive Color Mixing – superimposing lights, putting more light in the mixture than exists in any one light by itself. 2. Trichromatic Theory of Color Vision a. Created by Young, modified by Von Helmotz b. The Theory – human eye has three types of receptors with differing sensitivities to different light wavelengths. (Red, Blue, Green) c. Color Blindness encompasses a variety of deficiencies in the ability to distinguish among colors. Most of these people are dichromats. 3. Opponent Process Theory of Color Vision a. Complementary Colors - pairs of colors that produce gray tones when mixed together. b. Afterimage – visual image that persists after a stimulus is removed. Trichromatic T
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