PSYCH CH. 4.doc

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10 Apr 2012
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Chapter 4: Sensation and Perception
Sensation = the stimulation of sense organs
Perception = the selection, organization, and interpretation of sensory input
- Sensation involves the absorption of energy, such as light or sound waves, by sensory
organs such as ears and eyes
- Perception involves organizing and translating sensory input into something meaningful
Psychophysics
- Psychophysics: the study of how physical stimuli are translated into psychological
experience
- Important contributor: Gustav Fechner
Thresholds
- Sensation begins with a stimulus: any detectable input from environment
- Threshold: the dividing point between energy levels that do and do not have a detectable
effect
- Absolute threshold (for a specific type of sensory input): the minimum amount of stimulation
that an organism can detect
oDefine the boundaries of an organism’s sensory capabilities
oHowever, the absolute threshold is not absolute
i.e. as stimulus intensity increases, subject’s probability or responding to a
stimuli gradually increases
Just Noticeable Difference (JND)
- JND: the smallest difference in the amount of stimulation that a specific sense can detect
oVery similar to absolute threshold
oAbsolute threshold = JND from nothing (no stimulus input)
- JNDs vary by sense
- The smallest detectable difference is a fairly stable proportion of the size of the original
stimulus
- Weber’s law:
othe size of a just noticeable difference is a constant proportion of the size of the initial
stimulus
o‘Constant proportion’ is called the Weber fraction
oThis law applies to all senses
oDifferent fractions apply to different types of sensory input
Psychophysical Scaling
- Fechner’s law: the magnitude of a sensory experience is proportional to the number of JNDs
that the stimulus causing the experience is above the absolute threshold
oConstant increments in stimulus intensity produce smaller and smaller increases in
the perceived magnitude of sensation
oE.g. in a dark room with a single lamp that has three bulbs of the same wattage, you
turn on one light bulb, the difference against the completely dark room is striking, as
you turn on the 2nd bulb, the amount of light is doubled, but the room does not seem
to be twice as bright, by the 3rd bulb, you barely notice the difference
- This means that perceptions can’t be measured on absolute scales
- In terms of sensory experience, virtually everything is relative
Signal-Detection Theory
- Signal-detection theory: the detection of stimuli involves decision processes as well as
sensory processes, which are both influenced by a variety of factors besides stimulus
intensity
oYour responses will depend in part on the criterion you set for how sure you must feel
before you react
oYour performance will also depend on the level of ‘noisein the system
Noise comes from all of the irrelevant stimuli in the environment (the more
noise, the harder to pick up a weak signal)
Perceptions without Awareness
- Subliminal perception: the registration of sensory input without conscious awareness
oSubliminal means below threshold
- There has been research that proved: perception without awareness CAN take place
- Subliminal stimulation generally produces weak effects, thus there is little reason for
concern about the threat of subliminal persuasion
Sensory Adaptation
- Sensory adaptation: a gradual decline in sensitivity due to prolonged stimulation
oWith continued exposure to the stimuli, your sensitivity to it decreases
oIt is an automatic, built-in process
oAllows people to ignore the obvious and focus on changes in their environment that
may signal threats to safety
oSensory adaptation is probably a behavioural adaptation that has been sculpted by
natural selection
The Visual System
The Stimulus Light
- Light = a form of electromagnetic radiation that ravels as a wave, moving at the speed of
light
oLight waves vary In amplitude (height) and in wavelength (distance between peaks)
Amplitude: affects perception of brightness
Wavelength: affects perception of color
oLight also vary in its purity
Purity influences perception of saturation (or richness) of colors
- Vision is a filter that allows people to sense but a fraction of the real world
- For people to see, incoming visual input must be converted into neural impulses that are
sent to the brain
The Eye: Living Optical Instrument
- Eyes have two main purposes:
oChannel light to the neural tissue that receives it, called the retina
oHouse that tissue (retina)
- Light enters the eye through a transparent ‘window’ at the front, the cornea
- The cornea and lens (located behind cornea) then creates an upside-down image of objects
on the retina
- Lens = the transparent eye structure that focuses the light rays falling on the retina
oMade up of relatively soft tissue
oCapable of adjustments that facilitates accommodation
oAccommodation = when the curvature of the lens adjusts to alter visual focus
oWhen you focus on a close object, the lens of your eye gets fatter (rounder) to give
you a clear image
oWhen you focus on distant objects, the lens flattens out to give you a better image of
them
- Near-sightedness: close objects are seen clearly but distant objects appear blurry
oThe focus of light from distant objects falls a little short of the retina