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Chapter 4


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Carolyn Ensley

CHAPTER 4: Sensation and Perception Sensation- the stimulation of sense organs (involves the absorption of energy by sensory organs  usually light or sound, by the ears or eyes) Perception- is the selection, organization and interpretation of sensory input (involves organizing and translating sensory input into something meaningful) Psychophysics- the study of how physical stimuli are translated into psychological experience - Gustav Fechner: Wanted to know, that for any given stimulus, what was the weakest detectable stimulus… o Threshold- a dividing point between energy levels that do and don not have a detectable effect (central concept to psychophysics) o Absolute Threshold- for a specific type of sensory input is the minimum amount of stimulation that an organism can detect  defines the boundaries for organism’s sensory capabilities (the stimulus intensity detected 50% of the time) Just noticeable difference (JND): is the smallest difference in the amount of stimulation that a specific sense can detect - an absolute threshold is simply the just noticeable difference from nothing Weber’s Law: states that the size of a JND is a constant proportion of the size of the initial stimulus - constant proportion = Weber fraction - this law is applied to all the senses - as stimuli increase in magnitude, the JND becomes larger Fechner’s Law: This states that the magnitude of sensory experience is proportional to the number of JND’s that the stimulus causing the experience is above the absolute threshold - constant increments in stimulus produce smaller and smaller increases in the perceived magnitude of sensation - perceptions cant be measured on absolute scales 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 besides stimulus intensity - your responses will depend in part on the criterion you set for hoe sure you must feel before you react - depends on your expectations and on the consequences Subliminal Perception- the registration of sensory input without conscious awareness (limen = threshold, sub = below  below threshold) - messages are likely to be persuasive because people supposedly are defenseless against appeals operating below their threshold of awareness - worthy of experimental investigation Sensory Adaptation- is a gradual decline in sensitivity to prolonged stimulation (factor influencing registration of sensory input) - an automatic, built-in process that keeps people tuned in to the changes rather than the constants in their sensory input - behavioural adaptation that has been sculpted by natural selection THE VISUAL SYSTEM People heavily rely on their sense of sight, and they virtually equate it with that is trustworthy (seeing is believing). Vision- is a filter that permits people to sense but a fraction of the real world. It serves two functions: - to create an internal representation or model of the external world - concerned with the process related to perceiving, which deals with controlling your actions that are directed at those objects Stimulus = light Light- is a form of electromagnetic radiation that travels as a wave - amplitude = heights  mainly effects the perception of brightness - wavelength = distance between the peaks  mainly effects the perception of colour - purity = how varied the mix is  influences the perception of the saturation …incoming visual input must be converted into neural impulses that are sent to the brain: The Eye: serves 2 main purposes… - channel light to the neural tissue that receives it (retina) - house that tissue The path of light… Light  Cornea  Lens  Retina  Brain (visual cortex) Retina- the neural tissue that absorbs light, processes images and sends visual information to the brain - a piece of the CNS  brains envoy to the eye o Receptor Cells- located in the inner most layers that are sensitive to light. The 2 types of receptor cells are… • Cones  play a key role in day-light vision and colour vision (provide sharpness and precise detail) • Rods  play a key role in night vision and peripheral vision - the information processing of the retina involves the stimulation of receptor fields, which then send signals to the brain and laterally, to nearby visual cells o Receptive Field- the centre which receive the signals of the collection of rods and cones o Lateral Antagonism- allows the retina to compare the lights falling in a specific area against general lighting Nearsightedness- close objects are seen clearly but distant objects appear blurry because focus of light from distant objects falls a little short of the retina (eyeball is too long) Farsightedness- distant objects are seen clearly but close objects appear blurry because the focus of light from close objects falls behind the retina (eyeball is too short) Colour: adds richness to information about our perception of the world  may have evolved. Colour is a psychological interpretation. Perceived colour- is primarily a function of the dominant wavelength in these mixtures - longest wavelength = red, shortest wavelength = violet 3 Properties of Light  Hue (wavelength) Brightness (amplitude) Saturation (purity) Subtractive colour mixing- works by removing some wavelengths of light, leaving less light then was originally there Additive colour mixing- works by superimposing lights, putting more light in the mixture than exists in any one light by itself 2 Theories of Colour Vision… 1. Trichromatic Theory: holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths (Hermann von Helmholtz) - the eye contains specialized receptors sensitive to the specific wavelengths associated with red, green and blue - the eye does its own “colour mixing” 2. Opponent Process Theory: holds that colour perception depends on receptors that make antagonistic responses to three pairs of colours - red + green, yellow + blue, black + white - Complimentary colours- are pairs of colours that produce grey tones when mixed together (not explained in previous theory) * It takes both theories to exp
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