Thresholds
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)
half an inch.
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.
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)
Signal Detection Theory
•
•
This theory replaced Weber’s Law
Whether or not you detect a stimulus is dependent upon
three 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)
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
• Seeks to understand why people respond differently to
the same stimuli and 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 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.
Sensory Adaptation
Sensory Adaptation
• Diminished sensitivity as a result of constant stimulation
• If a stimulus is constant and unchanging, 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.
Selective Attention
Selective Attention
• Focusing conscious awareness on a
particular stimulus to the exclusion of
others
• The ability to focus on one stimulus at
a time
• Allows a person to function in a
world filled with many stimuli
• People with ADD have trouble doing
this.
Read the following words to yourself
•Did you notice that the word “the” was written twice?
•If not, it shows how your brain was selectively paying
attention to the meaning of what you were reading and
not what was actually written.
Read the following words to yourself
•Did you notice that the word “the” was written twice?
•If not, it shows how your brain was selectively paying
attention to the meaning of what you were reading and
not what was actually written.
Selective Attention: An Example
• View this updated version of Neisser’s Selective Attention
Test basketball video clip below. Count the number of
passes made by the girls in white. (click HERE to start)
Did you notice the lady walk across the room with the
umbrella? No! You were too busy watching & counting the
passes. Try these: http://www.dothetest.co.uk/