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