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Chapter 4: Sensation and
Perception
Sensation and Perception: The Distinction
• Sensation: stimulation of sense organs: for
example absorbing energy from light by the
eyes.
• Perception: selection, organization, and
interpretation of sensory input
• Psychophysics = the study of how physical
stimuli are translated into psychological
experience
Figure 4.1 The distinction between sensation and perception
Psychophysics: Basic Concepts
• Sensation begins with a detectable stimulus
• Fechner: the concept of the threshold, what
is the weakest detectable stimulus
– Absolute threshold: absolute threshold is
the minimal amount of stimulation that an
organism can detect stimulus 50% of the
time
– Just noticeable difference (JND):
smallest difference detectable
• Weber’s law: size of JND proportional to
size of initial stimulus
Figure 4.2 The absolute threshold
Psychophysics: Concepts and Issues
• Signal-Detection Theory: Signal detection
theory holds that the detection of sensory
information is influenced by two things…1)
noise in the system (irrelevant stimuli in the
environment that elicit neural activity), and 2)
decision making processes
• Subliminal Perception: Many researchers,
using very different methods, have
demonstrated that perception can occur
without awareness
• Sensory Adaptation: Decline in sensitivity
Figure 4.3 Signal-detection theory
Vision: The Stimulus
• Light = electromagnetic radiation, that travels
as a wave…the wave travels quickly…the
speed of light.
– Amplitude: perception of brightness
– Wavelength: perception of color
– Purity: mix of wavelengths
• perception of saturation, or richness of
colors.
Figure 4.5 Light, the physical stimulus for vision
The Eye:
Converting Light into Neural Impulses
• The eye:The eye has two main purposes,
providing a “house” for the neural tissue that
receives light, the retina, and channeling light
toward the retina.
• Components:
– Cornea: where light enters the eye
– Lens: focuses the light rays on the retina
– Iris: colored ring of muscle, constricts or
dilates via amount of light
– Pupil: regulates amount of light
Figure 4.7 The human eye
The Retina: An Extension of the CNS
• Retina: piece of neural tissue that lines the
back of the eye, absorbs light, processes
images
• Optic disk: Axons from the retina to the brain
converge at the optic disk, a hole in the retina
where the optic nerve leaves the eye optic
nerve connection/blind spot
• Receptor cells:
– Rods: black and white/low light vision
– Cones: color and daylight vision
• Adaptation: becoming more or less
sensitive to light as needed
The Retina: An Extension of the CNS
• Information processing:
– Receptive fields: the collection of rod and
cone receptors that funnel signals to a
particular visual cell in the retina
– Lateral antagonism: or lateral inhibition,
occurs when neural activity in a cell
opposes activity in surrounding cells.
Figure 4.8 Nearsightedness and farsightedness
Figure 4.9 The retina
Figure 4.10 The process of dark adaptation
The Retina and the Brain:
Visual Information Processing
• Optic Chiasm: the point where the axons
cross to different hemispheres
• Light  rods and cones  neural signals 
bipolar cells  ganglion cells  optic nerve
 optic chiasm  opposite half brain
• Main pathway: lateral geniculate nucleus
(thalamus)  primary visual cortex (occipital
lobe) (90% of axons go to LGN)
– magnocellular: where (ex brightness)
– parvocellular: what (ex color)
The Retina and the Brain:
Visual Information Processing
• Second pathway: superior colliculus 
thalamus  primary visual cortex
– Helps with the perception of motion and
coordination of visual input to other senses
• These 2 channels use parallel processing
– They both extract different info from the
same stimulus
Figure 4.13 Visual pathways through the brain
Figure 4.15 The what and where pathways from the primary visual cortex
Hubel and Wiesel:
Feature Detectors and the Nobel Prize
• Early 1960’s: Hubel and Wiesel
– Microelectrode recording of axons in
primary visual cortex of animals
– Discovered feature detectors: neurons that
respond selectively to lines, edges, etc.
– Groundbreaking research: Nobel Prize in
1981
• Later research: cells specific to faces in the
temporal lobes of monkeys and humans
Basics of Color Vision
• Wavelength determines color
– Longer = red / shorter = violet
• Amplitude determines brightness
• Purity determines saturation
Figure 4.16 The color solid
Figure 4.17 Additive versus subtractive color mixing
Theories of Color Vision
• Trichromatic theory - Young and Helmholtz
(mid 1800’s)
– This theory holds that the human eye has
three types of receptors with differing
sensitivities to different light
wavelengths…one for red, one for green,
and one for blue
• Opponent Process theory – Hering (1878)
– 3 pairs of antagonistic colors
– red/green, blue/yellow, black/white
• Current perspective: both theories necessary
Figure 4.18 The color circle and complementary colors
Perceiving Forms, Patterns, and Objects
• Reversible figures: drawing that is
compatible with two interpretations that can
shift back and forth
• Perceptual sets: a readiness to perceive a
stimulus in a particular way
• Inattentional blindness: failure to see fully
visible objects or events in a visual display
• Feature detection theory - people detect
specific elements in stimuli and build them up
into recognizable forms, bottom-up
processing
Perceiving Forms, Patterns, and Objects
• Form perception - involves top-down
processing…clearly emphasized by the Gestalt
psychologists, who demonstrated that the whole is
more than the sum of its parts
• Subjective contours: phenomenon whereby
contours are perceived where none actually exist
• Gestalt psychologists: the whole is more than the
sum of its parts
– Reversible figures and perceptual sets
demonstrate that the same visual stimulus can
result in very different perceptions
Figure 4.25 The principle of figure and ground
Figure 4.28 A famous reversible figure
Figure 4.22 Feature analysis in form perception
Figure 4.23 Bottom-up versus top-down processing
Figure 4.24 Subjective contours
Principles of Perception
• Gestalt principles of form perception:
– figure-ground, proximity, similarity,
continuity, closure, and simplicity
– The Gestalt emphasis is still felt in the
study of perception, as they had useful
insights that have stood the test of time,
raised important issues
Principles of Perception
• Recent research:
– Distal (stimuli outside the body) vs.
proximal (stimulus energies impinging on
sensory receptors) stimuli
– Perceptual hypotheses
• Context, your brain makes a decision as
to what an object is
• Objects that are thought to be “out of
place” are more easily remembered
Principles of Perception
• Phi Phenomenon
– The illusion of movement presented by
visual stimuli in rapid succession
• Figure and ground:
– Figure is the object being looked at
– Ground is the background on which it
stands
• Proximity: we group things that are close
together
Principles of Perception
• Closure: your brain fills in the gaps to
complete objects forms
• Similarity: we group objects of similar size or
shape
• Simplicity: we group elements together to
form a simple figure
• Continuity: we follow objects in the direction
we are led
Figure 4.26 Gestalt principles of perceptual organization
Figure 4.27 Distal and proximal stimuli
Figure 4.29 The Necker cube
Figure 4.30 Context effects
Depth and Distance Perception
• Depth perception involves interpretation of
visual cues that indicate how near or far away
something is
• Binocular cues – clues from both eyes
together
– retinal disparity: objects within 25 feet
project images to slightly different locations
on the left and right retinas
– Convergence: feeling the eyes converge
toward each other as they focus on a
target.
Depth and Distance Perception
• Monocular cues – clues from a single eye
– motion parallax : having images of
objects at different distances moving
across the retina at different rates
– Accommodation: change in the shape of
the lens as the eye focuses
– pictorial depth cues: cues about distance
that can be given in a flat picture
Depth and Distance Perception
• Monocular cues (cont.)
– pictorial depth cues: cues about distance that
can be given in a flat picture
• Linear perspective: lines converge in the
distance
• Texture gradients: small details can not be seen
from far away
• Interposition: objects that come b/w you and
another are closer
• Relative size: closer objects are bigger
• Height in plane: distant objects are higher in the
picture
• Light and shadow: gives direction and distance
Stability in the Perceptual World:
Perceptual Constancies
• Perceptual constancies – stable
perceptions amid changing stimuli
– when a person walks toward you, they get
larger perceptually…do you think they are
growing? No
– Size
– Shape
– Brightness
– Hue
– Location in space
Optical Illusions:
The Power of Misleading Cues
• Optical Illusions - discrepancy between
visual appearance and physical reality
• Famous optical illusions: Muller-Lyer
Illusion, Ponzo Illusion, Poggendorf
Illusion, Upside-Down T Illusion, Zollner
Illusion, the Ames Room, and Impossible
Figures
• Cultural differences: Perceptual hypotheses
at work, Muller-Lyer and Poggendorf
demonstrate the importance of perceptual
hypotheses.
Figure 4.41 The Ames room
The Ames Room
Windows
Mac OS X
Figure 4.37 The Muller-Lyer illusion
Figure 4.38 Explaining the Muller-Lyer Illusion
Figure 4.39 Four geometric illusions
Figure 4.42 Three classic impossible figures
Hearing: The Auditory System
• Stimulus = sound waves (vibrations of molecules
traveling in air)
– Amplitude (loudness)
– Wavelength (pitch)
– Purity (timbre)
• Wavelength described in terms of frequency:
measured in cycles per second (Hz)
– Frequency increase = pitch increase
• Amplitude is a description of sound pressure and is
measured in decibels (db). Perceived loudness is
higher with increasing decibel level.
Figure 4.44 Sound, the physical stimulus for hearing
The Ear: Three Divisions
• External ear (pinna): collects sound
• Middle ear: the ossicles (hammer, anvil,
stirrup)
• Inner ear: the cochlea
– a fluid-filled, coiled tunnel
– contains the hair cells, the auditory
receptors
– lined up on the basilar membrane
Figure 4.46 The human ear
Figure 4.47 The basilar membrane
The Auditory Pathway
• Sound waves vibrate bones of the middle ear
• Stirrup hits against the oval window of
cochlea
• Sets the fluid inside in motion
• Hair cells are stimulated with the movement
of the basilar membrane
• Physical stimulation converted into neural
impulses
• Sent through the thalamus to the auditory
cortex (temporal lobes)
Theories of Hearing: Place or Frequency?
• Hermann von Helmholtz (1863)
– Place theory: proposed that perception of
pitch corresponds to the vibration of
different portions, or places, along the
basilar membrane
• Other researchers (Rutherford, 1886)
– Frequency theory: perception of pitch
corresponds to the rate, or frequency, at
which the entire basilar membrane
vibrates, causing the auditory nerve to fire
at different rates for different frequencies
Theories of Hearing: Place or Frequency?
• Like with research in theories of color vision,
researchers argued about these two
competing theories for almost a century. It
turns out that both are valid - in part
• Georg von Bekesy (1947)
– Traveling wave theory: the whole basilar
membrane does move, but the waves peak
at particular places, depending on
frequency
Auditory Localization:
Where Did that Sound Come From?
• Two cues critical:
– Intensity (loudness)
– Timing of sounds arriving at each ear
• Head as “shadow” or partial sound
barrier
• Timing differences as small as 1/100,000 of a
second
Figure 4.48 Cues in auditory localization
The Chemical Senses: Taste
• Taste (gustation)
• Physical stimulus: soluble chemical
substances
– Receptor cells found in taste buds
• Pathway: taste buds -> neural impulse ->
thalamus -> cortex
– Four primary tastes: sweet, sour, bitter,
and salty
– Taste: learned and social processes
The Chemical Senses: Taste
• The ability to taste may depend on the
density of taste buds (genetics)
– 25% Non Tasters
– 25% Supertasters (much more sensitive to
certain tastes: sweet and bitter)
– 50% medium tasters
Figure 4.49 The tongue and taste
The Chemical Senses: Smell
• Smell (Olfaction)
• Physical stimuli: substances carried in the air
– dissolved in fluid, the mucus in the nose
– Olfactory receptors = olfactory cilia
• Pathway: Olfactory cilia -> neural impulse > olfactory nerve -> olfactory bulb (brain)
The Chemical Senses: Smell
• The olfactory receptors synapse directly with
cells in the olfactory bulb at the base of the
brain. Olfaction is the only sense, therefore,
that is not routed through the thalamus
• Odors are not easily classified, and primary
odors have not really been delineated.
• Humans can distinguish among about 10,000
odors, but for some reason have a hard time
attaching names to odors quite frequently
Figure 4.51 The olfactory system
Skin Senses: Touch
• Physical stimuli = mechanical, thermal, and
chemical energy impinging on the skin.
• Pathway: Sensory receptors -> the spinal
column -> brainstem -> cross to opposite
side of brain -> thalamus ->
somatosensory (parietal lobe)
• Temperature: free nerve endings in the skin;
specific for cold and warmth
Skin Senses: Touch
• Pain receptors: also free nerve endings
– Two pain pathways:
• the fast pathway that registers localized
pain and relays it to the brain in a
fraction of a second
• the slow pathway that lags a second or
two behind and carries less localized,
longer-lasting aching or burning pain.
Skin Senses: Touch
• Pain
– Can be influenced by expectations and
belief, mood, personality, and other factors
– You can mentally block pain
– Gate-control theory (Mezlack and Wall
1965): incoming pain signals must pass
through a gate in the spinal cord that can
be closed (to block pain signals)
• No scientific data to support
Skin Senses: Touch
• Pain (cont.)
– Endorphins are our body’s natural pain
killer
– The periaqueductal path (PAG) in the brain
seems to help suppress pain
– Certain types of neural glia cells also help
suppress pain
Figure 4.53 Pathways for pain signals
Other Senses: Kinesthetic and Vestibular
• Kinesthesis - knowing the position of the
various parts of the body
– Receptors in joints/muscles
• Vestibular - equilibrium/balance
– Semicircular canals: the largest part of the
vestibular system; these are fluid filled
canals that contain hair cells similar to
those in the basilar membrane
– When your head moves, the fluid moves,
moving the hair cells, and initiating neural
signals that travel to the brain.