Sensation & Perception
Chapter 4
• Transduction: Transformation of one form of
energy into another – especially the transformation
of stimulus information into nerve impulses
• Receptors: Specialized neurons that are activated by
stimulation and transduce (convert) it into a nerve
• Sensory pathway: Bundles of neurons that carry
information from the sense organs to the brain
Sensation and Perception
• Sensation: Neurons in a receptor create an internal
pattern of nerve impulses that represent the
conditions that stimulated it
• Perception: A process that makes sensory patterns
meaningful and more elaborate
• Stimulation  Transduction  Sensation  Perception
Sensory Adaptation
• Sensory adaptation: Loss of responsiveness in
receptor cells after stimulation has remained
unchanged for a while
• Examples???
• Absolute threshold: Amount of stimulation
necessary for a stimulus to be detected (50% of
• Difference threshold: Smallest amount by which a
stimulus can be changed and the difference be
detected (also called just noticeable difference –
Looking at the JND
• Weber’s law: The JND increases with the magnitude of the stimulus.
– The JND is large when the stimulus intensity is high, and
small when the stimulus intensity is low
• TV (volume high – turn down a lot, volume low – turn down
• Groceries (50 pound bag – need to add more than you would to
a 25 pound bag)
• Steven’s power law:
– Used for wider array of stimuli (shock, temperature)
– Fills in gaps left by Weber and Fechner
Signal Detection Theory
• Signal detection theory: Perceptual judgment as
combination of sensation and decision-making processes
– Based on each individual’s sensitivity and response criterion
– Example: holiday weekend on the interstate
False Alarm (brakes and no cop)
Hit (brakes and cop)
Miss (no brakes and cop)
Correct Rejection (no brakes and no cop)
– Lowers response criterion and raises hit rates
• Flawed merchandise off the assembly line
• TSA putting weapons in bags
Subliminal Persuasion
• Studies have found that stimuli flashed subliminally
on a screen can “prime” a person’s later responses.
• No controlled research has ever shown that
subliminal messages delivered to a mass audience
influences buying habits.
– Subtle, fleeting effect on thinking
– No powerful, enduring effect on behavior
The Anatomy of Visual Sensation
Retina: Light-sensitive layer at the back of the
eyeball – transduction occurs here!
• Photoreceptors – Light-sensitive cells in
the retina that convert light energy to
neural impulses
• Rods – Sensitive to dim
light but not colors
• Cones – Sensitive to
colors but not dim light
• (6:32)
Fovea: Area of
sharpest vision in
the retina
The Anatomy of Visual Sensation
• Optic nerve: Bundle of neurons that carries visual
information from the retina to the brain
• Rods/conesbipolar cellsganglion cells
Blind spot: Point where
the optic nerve exits the
eye and where there are
no photoreceptors
(money in the bank
Color Blindness & Afterimages
• Color blindness: Vision
disorder that prevents an
individual from discriminating
certain colors
– Red-green is most common!
• Afterimages: Sensations that
linger after the stimulus is
• Fix your eyes on the center for
a negative afterimage…
How the Visual System Creates Color
• Visual cortex: Part of the brain – the occipital cortex –
where visual sensations are processed
• Brightness: Sensation caused by the intensity of light waves
• Color: Psychological sensation derived from the
wavelength of visible light – color, itself, is not a property
of the external world
• Electromagnetic spectrum: Entire range of electromagnetic
energy, including radio waves, X-rays, microwaves, and
visible light
• Visible spectrum: Tiny part of the electromagnetic
spectrum to which our eyes are sensitive
How the Visual System Creates Brightness
Sensing Colors
• Trichromatic Theory: Idea that colors are sensed by
three different types of cones sensitive to light in the
red, blue and green wavelengths.
– Explains the earliest stage of color sensation.
• Opponent-process Theory: Idea that cells in the
visual system process colors in complimentary pairs,
such as red or green or as yellow blue.
– Explains color sensation from the bipolar cells onward in
the visual system.
Hearing: How Sound Waves Become Auditory
Pinna  Ear Canal  Tympanic Membrane  Middle Ear
 Oval Window  Cochlea (Basilar Membrane) 
Auditory Nerve
Tympanic membrane:
The eardrum
Hearing: How Sound Waves Become Auditory
Cochlea: Where
sound waves are
Hearing: How Sound Waves Become Auditory
Basilar membrane:
Thin strip of tissue with
hairs sensitive to
Hearing: How Sound Waves Become Auditory
Auditory nerve:
Neural pathway
connecting the ear
and the brain
• Conduction deafness: Results from damage to
structures of the middle or inner ear
• Nerve deafness: Linked to a deficit in the
body’s ability to transmit impulses from the
cochlea to the brain
– Usually involves the auditory nerve or higher
auditory processing centers
Hearing: The Physics of Sound
• Loudness: produced by the amplitude of a sound
- Amplitude: Physical strength of a wave
High Amplitude
Low Amplitude
Hearing: The Physics of Sound
• Pitch: produced by the frequency of a sound wave
– Frequency: Number of cycles completed by a wave in a
given amount of time
Low Frequency
High Frequency
How Sound Waves Become Auditory Sensations
• Pitch:
– Place Theory: different places on the basilar membrane
send neural codes for different pitches
– Frequency Theory: neurons have different firing rates
for different sound wave frequencies
• Timbre: Quality of a sound wave that derives from
the wave’s complexity
• Auditory cortex: Portion of the temporal lobe that
processes sounds
Position and Movement
• Vestibular sense: Sense of body orientation with
respect to gravity
• (7:21)
• Kinesthetic sense: Sense of body position and
movement of body parts relative to each other
• Olfaction: Sense of smell
• Olfactory bulbs: Brain sites of olfactory processing
• Pheromones: Chemical signals released by
organisms to communicate with other members of
the species
• Gustation: The sense of taste
Taste buds: Receptors
for taste (primarily on
the upper side of the
The Skin Senses and Pain
• Gate-control Theory: An explanation for pain
control that proposes we have a neural “gate” that
can, under some circumstances, block incoming pain
• Placebos: Substances that appear to be drugs but
are not
• Placebo effect: A response to a placebo caused by
subjects’ belief that they are taking real drugs
The Machinery of Perceptual Processing
• Percept: Meaningful product of a perception
• Feature detectors: Cells in the cortex that specialize
in extracting certain features of a stimulus
• Binding problem: A major unsolved mystery in
cognitive psychology, concerning the physical
processes used by the brain to combine many
aspects of sensation to a single percept
Bottom-Up and Top-Down Processing
• Bottom-up processing: Analysis that begins with the sense
receptors and works up to the brain’s integration of sensory
• (50)
• Top-down processing: Analysis guided by higher-level
mental processes - emphasizes perceiver's expectations,
memories, and other cognitive factors
• (1:44)
Perceptual Constancies
• Perceptual constancy: Ability to recognize the same
object under different conditions, such as changes in
illumination, distance, or location
• Shape, color, size
Perceptual Illusions
Do you see
Perceptual Ambiguity and Distortion
• Illusions: Distortion of a stimulus pattern,
shared by others in the same perceptual
– More likely when:
• stimulus is unclear
• info is missing
• elements combined in unusual ways
• familiar patterns aren’t apparent
• Ambiguous figures: Images that are
capable of more than one interpretation
The Gestalt Approach
• Gestalt psychology: View that much of perception
is shaped by innate factors built into the brain
– The whole pattern is greater than the sum of its parts.
• Figure: Part of a pattern that commands attention
• Ground: Part of a pattern that does not command
attention; the background
The Gestalt Approach
• Subjective contours: Boundaries that are perceived
but do not appear in the stimulus pattern
• Closure: Tendency to fill in gaps in figures and see
incomplete figures as complete
The Gestalt Laws of Perceptual Grouping
Common Fate
Binocular Cues – two eye depth cues
• Binocular Convergence: lines of vision from each
eye converge at different angles on objects at
different distances
– Can feel eye muscles change as you focus at different
• Retinal Disparity: difference in perspectives of the
2 eyes (greater disparity for nearby objects –
provides us with depth information)
Monocular Cues – one eye depth cues
• Monocular Cues:
– Linear Perspective: parallel lines appear to meet in the
Monocular Cues – one eye depth cues
• Monocular Cues:
– Relative Motion: objects closer to you move faster than
those further away from you
Monocular Cues – one eye depth cues
• Monocular Cues:
– Relative Size: 2 objects the
same size; the one that
appears larger = closer to us
Monocular Cues – one eye depth cues
• Monocular Cues:
– Interposition: hidden objects are more distant than those
objects that hide them
Monocular Cues – one eye depth cues
• Monocular Cues:
– Texture Gradient: as object gets further away from us,
the texture gets smoother
Theoretical Explanations for Perception
• Learning-based inference: View that perception is
primarily shaped by learning, rather than innate factors
(nurture); opposite of Gestalt
• What determines how successful we will be in forming an
accurate percept?
– Context, expectation, perceptual set – each influenced by culture
– Perceptual set: Readiness to detect a particular stimulus in a given
Cultural Influences on Perception
Which box is bigger, A or B?
Muller-Lyer Illusion
• When 2 objects make the same size image on the
retina, and we judge one to be farther away than the
other, we assume that the more distant one is larger.

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