Thresholds
&
Sensory Adaption
Module 16
Principles of Sensation
• All senses receive stimuli on
receptor cells then transform
it to action potential then
deliver the info to the brain.
• Transduction— process
where physical energy is
converted into neural signals
the brain understands
Psychophysics
• This is how we experience our physical
world.
• Classroom demo judging weight of pill
bottles.
• Which one do you think weighs the most?
• The more compact weight “felt” heavier to
your brain when in reality it weighed the
same as the large container.
Types of Threshold
• A Threshold is an edge or a boundary
• Psychologists are interested in our
boundaries/thresholds when it comes
to our senses.
• What’s the bare minimum we can
sense?
Absolute Threshold
•The minimum stimulation necessary for a
person to detect a particular stimulus 50% of the
time. (see table for examples)
Difference Threshold
• The minimum difference that a person can detect
between two stimuli 50% of the time.
• Also called just noticeable difference
Absolute Threshold vs. Difference Threshold
• If you put your hand on a burner and turned it on, the first
time you felt something would be Absolute Threshold and
the second you noticed it getting hotter would be
Difference Threshold.
Weber’s Law
•
•
•
Weber’s Law – detection of a stimulus depends on the
original stimulus itself.
LARGE Stimuli require BIGGER Changes for us to
notice them
Example: If a 300 lb. person loses 20 lbs and a 120 lb. Person
loses 20 lbs, which one would you notice lost the weight first?
(120 lbs because 20 lbs is a larger % of their body weight)
Which one would you notice the
$1 price drop more?
Which one would you notice a
2 inch haircut on?
Thresholds:
Signal Detection Theory
• Set of formulas and principles that predict when we will
detect the presence of a faint stimulus (signal) amid
background stimulation (noise)
• Developed out of the Cold War & need to detect nukes
• Seeks to understand:
1. Why people respond differently to the same stimuli ?
2. Why the same person’s reactions would vary as circumstances
change?
• Example: Tired parents of newborns will awaken to a
whimper but sleep through their alarm.
Signal Detection Theory
•
•
•
This theory replaced Weber’s Law
Explains why people respond differently to same stimuli
Whether or not you detect a stimulus is dependent upon 3 kinds of
variables:
1. Stimulus variables (how weak or strong the stimulus is)
2. Environmental variables (what’s going on around you)
3. Organismic variables (Your own experience, expectations,
motivation and alertness)
Detect “real” hamburger amongst soy
burger
Where’s Waldo?
Detect poisonous snake amongst
harmless ones
Signal Detection Theory
This can be used for jobs that depend on one’s ability to accurately detect
a threat/stimulus. (Cold War Nuke Detection, Airport Security, Cancer
Doctor)
Signal
Present
Absent
Says “No”
Miss:
Person did not
detect signal
Responder
Says “Yes”
Hit:
False Alarm:
Person correctly No signal but
detected signal
person said yes
Correct Rejection:
Person correctly
noticed there was
no signal
Signal Detection Theory
Signal
Present
Responder
Hit
Signal
Absent
False Alarm
Present
Hit
Absent
False Alarm
Says “Yes”
30
Miss
30
Corr. Rej.
20
Miss
20
Corr. Rej.
Says “No”
20
20
Liberal:
Person more likely to correctly detect
A signal but also more likely to falsely
detect a signal.
30
30
Conservative:
Person less likely
to detect a signal but more
likely to correctly notice
when stimulus is not there.
Do subliminal messages work?
• Subliminal messages make two assumptions:
1. We can unconsciously sense the subliminal (below
threshold) message. - TRUE
2. Without our awareness, the stimuli have suggestive
powers. - FALSE
• Subliminal messages can PRIME how we “feel”
about something for a short period of time but
CANNOT persuade us to actually go out and do
something. (pg. 158 in text)
• Remember, weak stimuli have weak responses and
strong stimuli have strong responses.
• Extra credit to anyone who can make a coherent song
with lyrics that when played backwards says, “I love
psychology!” 
I Love Psych!!!
Did it work?
• Do you really love psychology now?
• It was worth a try!
• This is an example of priming someone to
feel or think a certain way for a very short
period of time.
Sensory Adaptation
Sensory Adaptation
Diminished sensitivity to an unchanging stimululs
If a stimulus is constant, eventually a person may fail to
respond to it
– Move your watch up your wrist an inch or put your ring on a
different finger. You will feel it at first but later you won’t
notice it. Your senses have adapted to it.
– This is why the lake water is cold at first but you “get used to
it.”
– Also why you don’t feel your glasses on your nose or your
clothes on your body.
– Remember the displacement goggles experiment from class.
This showed motor adaptation to a change in vision.
Come on in,
the water’s
fine