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Chapter 4

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York University
PSYC 1010

Nov/1/2003 CHANAPS Notes From Reading C HAPTER 4: SENSATION ANDP ERCEPTION 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 organisms 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. Webbers 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. Fechners Law the magnitude of a sensory experience is proportional to the number of JNDs 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 1/7Nov/1/2003 CHANAPS Notes From Reading C HAPTER 4: SENSATION ANDP ERCEPTION 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
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