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Sensation & Sensation & PerceptionPerception
Chapter 5Chapter 5
This multimedia product and its contents are protected under copyright law. The following are prohibited by law: any public performance or display, including transmission of any image over a network; preparation of any derivative work, including the extraction, in whole or part, of any images; any rental, lease, or lending of the program. ISBN: 0-205-37181-7
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Sensation and Perception
Perception –
interpretation, organization of internal or external events (involves awareness and activity)
Sensation –
immediate, basic experience generated by a stimulus
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Sensation & Perception Processes
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Transduction – Transformation of one form of energy into another
Receptors – Specialized neurons that are activated by stimulation and transduce (convert) it into a nerve impulse
Transduction
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ThresholdsAbsolute threshold –
Amount of stimulation necessary for a stimulus to be detected
Difference threshold – Smallest amount by which a stimulus can be changed and the difference be detected (also called just noticeable difference – JND)
Weber’s law – Size of a JND proportional to intensity of stimulus
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Signal Detection TheorySignal detection theory –
Perceptual judgment as combination of sensation and decision-making processes
Stimulus eventStimulus event
Neural activityNeural activity
Comparison with Comparison with personal standardpersonal standard
Action (or no action)Action (or no action)
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Signal-Detection Theory• A psychophysical theory that divides the
detection of a sensory signal into a sensory process and a decision process.
Stimulus is
Present
Stimulus is
AbsentResponse: “Present”
Hit False Alarm
Response: “Absent”
Miss Correct Rejection
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The senses all operate in much the same way, but each extracts different
information and sends it to its own specialized processing
region in the brain
How Are the Senses Alike? How Are the Senses Alike? And How Are They Different?And How Are They Different?
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Sensory Adaptation
Sensory adaptation – Loss of responsiveness in receptor cells after stimulation has remained unchanged for a while
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An Eye on the World• Cornea
– Protects eye and bends light toward lens.
• Lens– Focuses on objects by
changing shape.• Iris
– Controls amount of light that gets into eye.
• Pupil
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The Structures of the Retina
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An Eye on the World
• Retina– Neural tissue lining the back of the
eyeball’s interior
• Two Photoreceptors: Rods: respond to dim light/movement (125 million)
Cones: involved in color vision/detail (7 million)
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Why the Visual System is not a Camera
• Much visual processing is done in the brain.– Some cortical cells respond to lines
orientations (e.g. horizontal).– Other cells in the cortex respond to other
shapes (e.g., bulls-eyes, spirals, faces).
• Feature-detectors
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Hubel & Wiesel’s Experiment
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Constructing the Visual World
• Form perception.
• Gestault Principles– “whole is more than the sum of the parts”
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Form Perception
• Gestalt principles describe the brain’s organization of sensory building blocks into meaningful units and patterns.
Figure – Part of a pattern that commands attention
Ground – Part of a pattern that does not command attention; the background
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Examples “Gestault”
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More “Gestault”• Proximity
– Seeing 3 pair of lines in A.
• Similarity– Seeing columns of
orange and red dots in B.
• Continuity– Seeing lines that
connect 1 to 2 and 3 to 4 in C.
• Closure– Seeing a horse in D.
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Law of Prägnanz
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Depth and Distance Perception
How can you tell what is closer?
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Depth and Distance Perception
• Binocular Cues: – Convergence– Retinal Disparity
• Monocular Cues: – Relative size, overlap, texture gradient,
linear perspective, atmospheric perspective, shadow/shading, motion parralax
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Visual Constancies
• The accurate perception of objects as stable or unchanged despite changes in the sensory patterns they produce.– Shape constancy– Size constancy– Color constancy
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Shape Constancy
• Even though these images cast shadows of different shapes, we still see the quarter as round
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Bottom-Up and Top-Down Processing
Bottom-up processing – Analysis that emphasizes characteristics of the stimulus, rather than internal concepts
Top-down processing – Emphasizes perceiver's expectations, memories, and other cognitive factors
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Context Effects
• The same physical stimulus can be interpreted differently
• We use other cues in the situation to resolve ambiguities
• Is this the letter B or the number 13?
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Visual Illusions
• Illusions are valuable in understanding perception because they are systematic errors.
• In the Muller-Lyer illusion (above)
Visual Illusions
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The Ponzo Illusion
• Linear perspective provides context
• Side lines seem to converge
• Top line seems farther away– But the retinal
images of the red lines are equal!
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How Sound Waves Become Auditory Sensations
Cochlea –Where sound waves are transduced
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Taste Buds• Photograph of
tongue surface (top), magnified 75 times.
• 10,000 taste buds line the tongue and mouth.
• Children have more taste buds than adults.
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Four Tastes
• Four basic tastes– Salty, sour, bitter and sweet.
• Different people have different tastes based on:– Genetics– Culture– Learning– Food attractiveness
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Smell: The Sense of Scents
• Airborne chemical molecules enter the nose and circulate through the nasal cavity..
• Receptors on the roof of the nasal cavity detect these molecules.
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The Skin Senses
• Touch
• Temperature
• Pain
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Perceptual Powers: Origins and Influences
• James J. Gibson : Direct Perception– Invariants and affordances
• Inborn abilities and perceptual lessons
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Psychological and Cultural Influences on Perception
• “needs”, beliefs and emotions can influence perceptions of sensory information.
• Expectations based on our previous experiences influence how we perceive the world.
– Perceptual Set• Readiness to detect a particular stimulus in a given context• We all are influenced by our culture/experience.
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Perceptual Set
• What you see in the center figures depends on the order in which you look at the figures:– If you scan from the left, see an man’s face– If you scan from the right, see a woman’s figure
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Extrasensory Perception
• Extrasensory Perception (ESP):– The ability to perceive something without
ordinary sensory information– This has not been scientifically demonstrated
(issues with reliability/validity)
• Three types of ESP:– Telepathy – Mind-to-mind communication– Clairvoyance – Perception of remote events– Precognition – Ability to see future events