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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.
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Thresholds &
Sensory AdaptionModule 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
Hit: Person correctly
detected signal
False Alarm:No signal but person said yes
Miss: Person did not
detect signal
Correct Rejection: Person correctly
noticed there was no signal
Signal
Present Absent
Res
pond
er
Says “Yes”
Says “No”
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 Detection Theory
Hit False Alarm
Miss Corr. Rej.
Signal
Present Absent
Hit False Alarm
Miss Corr. Rej.
Signal
Present Absent
Res
pond
er Says “Yes”
Says “No”
30 30
30 3020 20
20 20
Liberal:Person more likely to correctly detect
A signal but also more likely to falsely detect a signal.
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. - TRUE2. 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