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Chapter 7
Sensation
Sensation
The raw experience of a sensory stimulus,
such as a light or sound
Perception: The interpretation of sensory
information according to expectations and
prior learning
The Senses as
Evolved Adaptations
Sensing Tastes and Smells
sensitivity to chemicals important for feeding
and reproduction
chemical receptors became more
sophisticated
Smell vs. Taste receptors evolved
The Senses as
Evolved Adaptations
Sensing Light
responsiveness to the sun’s energy
provides “remote guidance” for sensing
things at a distance
eyes allow us to process form, color,
movement and visual acuity
The Senses as
Evolved Adaptations
Sensing Sounds
sensing sound increases range of sensation
beyond that of smell
allows localization and identification
sound can be used as a form of
communication
The Senses as
Evolved Adaptations
Sensing Touch, Warmth and Pain
skin senses allow location of nearby objects
touch enables skilled movements
pain motivates behavior
Psychophysics
The study of how humans and animals
respond to sensory stimuli
The mathematical relationship of sensory
intensity to the magnitude of a physical
stimulus
Just Noticeable Difference (JND)
The minimal amount of sensory change in
a stimulus that can be detected
e.g. how much more weight do you need to
perceive a difference in weights?
Just Noticeable Difference
Weber’s Law:
jnd = kI
Just Noticeable Difference = Constant x Intensity
The size of the just noticeable difference is
equal to some some proportion of the standard
Constant varies depending on sensory modality
Just Noticeable Difference
Fechner: JND is a measure of the
“psyche”
similar to inches on a ruler
The Absolute Threshold
Minimum amount of stimulation that can be detected
on half the trials
Count up number of “yes” responses (Frequency of
“seeing”)
100
Threshold =
P(yes)
50
50% response
point
0
Stimulus Intensity
Psychophysical Methods:
How to Measure Thresholds
Method of Limits
Start with a low intensity stimulus, gradually
increase until observer reports a sensation
(ascending)
Start with a high intensity, gradually decrease
until observer no longer reports a sensation
(descending)
Problems:
• observer may not pay attention on low intensity trials
• observer may anticipate stimulus on descending series
Psychophysical Methods:
How to Measure Thresholds
Method of Constant Stimuli
Present stimuli in a random order
observer cannot predict whether stimulus is
above or below threshold
Method of Magnitude Estimation
Stevens:
Observers use numbers to describe the
perceived intensity of a stimulus
Relationship between stimulus intensity
and magnitude estimates follows a power
function
Signal Detection Theory
The detection of a stimulus involves
decision processes as well as sensory
processes
Observers responses will change with
motivation
e.g. paid $1 for each detection of stimulus results
in a greater number of detections
Signal Detection Matrix
Judgment
“Yes”
Present
Hit
Absent
False
Alarm
“No”
Miss
Stimulus
Correct
Rejection
Signal Detection Matrix
Pay $1 for
each
detection
Judgment
“Yes”
Present
Hit
Absent
False
Alarm
“No”
Miss
Stimulus
Correct
Rejection
Signal Detection Matrix
Judgment
“Yes”
Present
Hit
Absent
False
Alarm
“No”
Miss
Stimulus
Correct
Rejection
Deduct $2 for
each False
Alarm
Two-Point Limen
A measure of tactile
sensitivity
Sensitivity differs for
different body areas
Sensitivity
corresponds with
Sensory Homunculus
Subliminal Perception
Perception of stimuli below the absolute
threshold
e.g. very briefly flashing messages
no evidence for effectiveness in advertising
However, flashed words can “prime”
awareness of other stimuli
e.g. “bread” - “butter”
A Five-Stage Model
of Sensory Systems
Each sensory system must have:
1. An adequate stimulus
2. Receptors adapted to the stimulus
3. Nerve pathways
4. Destination points in the brain
5. The psychological experience
Seeing
The Stimulus: The Visible Spectrum
The portion of the electromagnetic spectrum
between 400 to 700 nanometers
The Eye
The eye focuses light on the retina
Retina: multilayered structure on the inner
surface of the eye
Transduction
The conversion of energy from one type to
another
The eye transduces light energy into neural
energy at the retina
Transduction occurs at the
photoreceptors:
Rods: dim-light receptors
Cones: bright-light receptors
The Retina
Photoreceptors receive light
Neural signal sent to Bipolar Cells.
Signal then sent to Retinal Ganglion Cells
Ganglion cells send signal out the eye to
the brain
exit point is a “blind spot
The Retina
The Retina
Cones:
Located in the center of the retina
Often see a single cone connecting to a
single ganglion cell
Rods:
Located in the periphery of the retina
Often see many rods connecting to a single
ganglion cell
Visual Nerve Pathways
Axons of ganglion cells for the optic nerve
pathway
Optic nerve sends signals to the lateral
geniculate nucleus (LGN) of the thalamus
Signals are then sent to the primary visual
cortex in the occipital lobe
primary visual cortex = striate cortex
Conscious vs. Non-conscious
Visual Pathways
Retina - LGN - Striate cortex: “conscious visual
pathway”
“Non-conscious pathways”:
Retina - Superior Colliculus: Responsible for
perception of peripheral movement
Retina - Pretectum: Responsible for changing pupil
size when presented with bright light.
Dark Adaptation
An increase in visual sensitivity as a result
of time spent in the dark
Sensitivity appears to plateau at 10 minutes,
but then starts to increase again at 15
minutes
Rod-Cone Break
Dark Adaptation
Color Vision: Trichromatic Theory
(Young-Helmholtz)
Color vision results from the activity of
three cone pigments, each maximally
sensitive to on of three wavelengths
Trichromatic Theory explains additive color
mixing - the mixing of colored lights to create
other colors
Dichromatism: color blindness resulting from
missing one of three color receptors
Color Vision: Opponent
Process Theory (Hering)
Colors are sensed by “opponent pairs”
Red-Green
Blue-Yellow
White-Black
Can be used to explain negative
afterimages
Color-Opponent
Cells
Ganglion cells are connected to
photoreceptors such that they respond in
an opponent process fashion to color
e.g. inhibited by green and excited by red
Hearing
The Stimulus: Sound Waves
a wave of compressed air resulting from
vibration
Sound Waves
Waves of air
that can vary
in amplitude
and frequency
Sound as a Wave
Amplitude (intensity): related to
psychological dimension of loudness
Frequency: related to psychological
dimension of pitch
Complexity: related to psychological
dimension of timbre
Amplitude
 Determined by size of wave
 Measured in decibels (dB)
Frequency
 Determined by number of waves per second
 Measured in Hertz (Hz)
The Ear
Three Parts:
The Outer Ear
The Middle Ear
The Inner Ear
The Ear
The Outer Ear
Consists of
Pinna
Auditory Canal
Tympanic Membrane (Eardrum)
Main Function:
Gather sounds to send to middle and inner
ear
The Middle Ear
The Middle Ear
Ossicles: Transfer and amplify sound to inner
ear
Malleus (Hammer)
Incus (Anvil)
Stapes (Stirrup)
Oval Window
To inner ear
Inner Ear (Cochlea)
Sound vibrations enter at oval window
Travel through fluid, vibrating basilar
membrane
Organ of Corti
Where sound is transduced into a neural
signal
Sound is transduced by Hair Cells
Cilia on hair cells contact tectorial
membrane
As basilar membrane vibrates, hair cells
are pulled and neural signal is generated
Flowchart of the Ear and
Other Things
Airborne
Vibrations
Bending
(Cilia)
Mechanical
Vibrations
(Eardrum &
ossicles amplify)
Electrical
Charges
(Hair cells)
Pressure
Waves
Ripples
(Cochlear Fluid)
(Basilar
Membrane)
Neurotransmitter
Brain
(Auditory Nerve
Fibers)
Place Theory:
How we perceive pitch
Sound waves generate vibration in
cochlear fluid and basilar membrane travelling wave
Frequency of sound is encoded by the
stimulation of specific place on basilar
membrane
High frequencies cause vibrations at thin
part of basilar membrane near oval window
Low frequencies cause vibrations at thicker
part
Place Theory:
How we perceive pitch
Loudness Perception
Increased amplitude of sound wave leads
to greater displacement of basilar
membrane
Increase displacement of basilar
membrane leads to increased activity of
hair cells
Increased activity of hair cells leads to
greater number of EPSPs
Conductive Hearing Loss
Hearing loss due to reduced functioning of
outer or middle ear
e.g. damage to ear drum or damage to
ossicles
otitis media: middle ear infection
reduces movement of ossicles
Sensorineural Hearing
Loss
Hearing loss due to damage to the
cochlea
Central Hearing Loss
Hearing loss due to damage to brain
areas
e.g. Wernicke’s aphasia - an inability to
attach meaning to language
Central Auditory
Processes
Taste
The Stimulus:
Chemicals in solution
Four basic tastes:
sweet
salt
sour
bitter
Taste is also a product of what we smell
How we Taste
Taste receptors
are found in
taste buds on
the tongue
Membranes of
receptor cells
bathed in
solution of
chemicals in
saliva
How we Taste
Receptor cells
generate action
potentials in taste
nerves
Smell
The Stimulus:
Airborne chemicals (olfactants)
How we Smell
(just terrible)
Olfactants are dissolved in olfactory
mucosa at top of nasal passageway
EPSPs are generated in olfactory neurons
Signals sent to the olfactory bulb then to
brain
How we Smell
Touch
The Stimulus:
Mechanical Pressure
The receptor:
Receptors found in skin
Touch Receptors
Free Nerve Endings
Process touch, temperature and pain
Pacinian Corpuscles:
Process “deep pressure”
Meissner Corpuscles and Organ of Ruffini
Process gradual changes in skin pressure
Pain
The Stimulus:
Typically, damaging stimuli - mechanical,
heat, chemical
Pain can be influenced by non-sensory
factors
e.g. rubbing a hurt area
Phantom Limb Pain
Pain associated in a “limb” even though it
has been amputated