Sensation and
Perception
Sensation: What is it?
• The process by which a stimulus in the
environment produces a neural impulse
that the brain interprets as a sound,
image, odor, etc.
Transduction
• 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 impulse
Transduction
• Sensory pathway –
Bundles of neurons that carry information
from the sense organs to the brain
Sensory Adaptation
• Sensory adaptation –
Loss of
responsiveness in
receptor cells after
stimulation has
remained unchanged
for a while
Thresholds
• Absolute 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)
Thresholds
• Weber’s law –
The JND is always large when the
stimulus intensity is high, and small when
the stimulus intensity is low
How Are the Senses Alike?
How Are They Different?
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
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
Sense
Stimulus
Sense Organ
Receptor
Sensation
Vision
Light waves
Eye
Rods and
cones
Color,
brightness,
motion
Hearing
Sound waves
Ear
Hair cells
Pitch,
loudness
Skin
Senses
External
Contact
Skin
Nerve endings Touch,
warmth, cold
Smell
Volatile
Substances
Nose
Hair cells
Odors
Taste
Soluble
Substances
Tongue
Taste buds
Flavors
Pain
Intense or
Extreme
Stimuli
Pain Fibers all Pain receptors Pain
over body
Kinesthetic Body position
&
or balance
Vestibular
Semi-circular
canals; joints,
tendons
Hair cells;
specialized
neurons
Body position
The Anatomy of Visual Sensation
Retina –
Light-sensitive layer at the back of the eyeball
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
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
Blind spot –
Point where the
optic nerve exits
the eye and
where there are
no photoreceptors
Transduction of Light in the Retina
The Anatomy of Visual
Sensation
• Visual cortex –
Part of the brain – the occipital cortex –
where visual sensations are processed
Neural Pathways in the Human
Visual System
How the Visual System Creates
Color
Color –
Psychological sensation derived from the
wavelength of visible light – color, itself, is
not a property of the external world
How the Visual System Creates
Color
• 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
Two Ways of Sensing Color
• Trichromatic theory
– Three different types of cones that sense different
parts of the visible spectrum (i.e., red, green, & blue)
– Explains initial stages of color vision
• Opponent Process Theory
– From bipolar cells onward, visual system processes
color in either-or, complementary fashion (i.e., red vs
green or blue vs yellow)
– Sensations of one color (e.g., red) inhibits sensation
of its complementary color (i.e., green)
– Explains negative afterimages & color blindness
Afterimages
• Afterimages –
Sensations that linger after the stimulus is
removed
• In the following slide, fix your eyes on the
dot in the center of the flag
Color Blindness
Perception
Perception
• Same sensory input can give rise to very
different perceptions
• Perceptual Set
– Readiness to perceive stimuli in specific ways
• Reversible figures – drawing that is
compatible with two different
interpretations
Reversible Figures
Reversible Figure
Perception
• What is Perception?
– Active process in which we organize
and interpret sensory information
• i.e., How we make sense of (or
understand) what we see, hear, feel,
taste, and smell
Object Perception
• Distal Stimulus
– Stimuli that lie in the distance (i.e., in the
outside world)
– Three-dimensional
• Proximal Stimulus
– Stimulus that impinges directly onto your
sensory receptors (i.e., the retina)
– Distorted and two-dimensional
Distal Stimulus
Proximal Stimulus
Images Projected on the Retina
Object Perception
• Feature Analysis
– Analyze individual features and put them
together to form a whole
Bottom-Up Processing
• Start with the elements and progress to
the whole
• Evidence
– Hubel and Wiesel – cells in the cortex
operate as highly specialized feature
detectors
Top-Down Processing
• Start with the whole and work towards the
elements
– What we perceive is influenced by what
we expect to see
• Context and prior experience are
important
Object Perception
• A number of Gestalt psychology principles can
help explain how we organize information in
order to perceive a coherent whole
•
•
•
•
•
•
•
Figure/Ground
Proximity
Similarity
Continuity
Common Fate
Closure
Goodness of Form or Pragnanz
Figure / Ground
Proximity
Similarity
Continuity
Common Fate
Closure
Goodness of Form or Pragnanz
=
Images Projected on the Retina
Proximal Stimulus
Depth Perception
• Binocular Depth Cues
– Binocular disparity – Each eye has a slightly different
view of the world (i.e., the distal stimulus)
• The brain thus has two different BUT overlapping images of
the world
• The difference between these two retinal (i.e., proximal)
images is used to compute distances to nearby objects
• For example:
– Object at 25 feet – image projected to slightly different
locations on the right and left retina
– Closer objects project images on locations that are
further apart on the right and left retina
Depth Perception
• Monocular Depth Cues
– Occlusion
– Relative Size
– Familiar Size
– Linear Perspective
– Texture Gradient
– Position relative to horizon
• Motion cues for Depth
– Motion Paralax
– Optic Flow
Proximal Stimulus
Distal Stimulus
Size Perception
• The size of the retinal image depends on
the distance of the object from the
observer
– Further away = smaller retinal image
– Thus, to determine size, visual system must
know how far away the object is
Linear Perspective
A
B
Which box is bigger, A or B?
Moon Illusion
Final Thoughts
• Visual system is exceptionally good at
analyzing the outside world
– i.e., perception is a faithful representation of
the distal stimulus
• Understanding how the visual system is
tricked (i.e., by studying common illusions)
helps us to understand how it works