Human
Sense
Vision
Absolute Threshold is Equivalent to:
Hearing
The tick of a watch under quiet conditions at 20
feet
1 teaspoon of sugar in 2 gallons of water
Taste
A candle flame seen at 30 miles on a clear night
Smell
1 drop of perfume diffused into the entire volume
of air in a 6 room apartment
Touch
The wing of a fly falling on your cheek from a
distance of 1 centimeter
Subliminal Stimuli: fall below absolute threshold and thus are not usually
detected
Supraliminal Stimuli: are usually detected because they are above absolute
threshold

A mathematical model that can explain why a
person does or does not detect a stimulus. This is
attributed to the following:
Sensitivity: the ability to detect a stimulus
Example: In the chapter on sensation, you learned how
age can affect the ability of the three bones in the
inner ear to conduct sound. A person with a
decrease in bone conductivity would be less
sensitive to external auditory stimuli.
1.
REMEMBER: Sensitivity reflects physical changes in
the nervous system, sensory system, or stimulus not
the motivation of the person
2. Response Criterion: A person’s
willingness or reluctance to respond to a
stimulus. A bias for either is created by
motivation and expectancy
Example: Dr. Charles, a cancer specialist, sees
cancer patients who have been referred to him
by other physicians. Therefore, when he looks
at patients' x-rays for the first time, he expects
to perceive cancerous shadows. Therefore, he is
likely to perceive even faint shadows as cancer
when, in reality, none may exist.
•The smallest difference between stimuli that we can
detect –How small is the difference??
•Determined by two factors:
1. How much stimuli there is to begin with: the weaker
the stimuli are, the easier it is to detect small
differences between them. Example: if you are comparing
the weight of two envelopes, you will be able to detect a
difference of as little as a fraction of one ounce.
2. Which sense is being stimulated
*** the relationship between the two  Weber’s Law
States: that the smallest detectable difference in
stimulus energy is a constant fraction of the
intensity of the stimulus.
•Weber’s Constant or Weber’s Fraction is given the
symbol KK is different for each sense
•Weber’s Law says: that JND=KI(amount of
stimulus)
•Example: the value of K for weight is .02. If an object
weighs 25 lbs(I), the JND is (.02x25)==.o5 or half a
pound—so in order to feel a difference you would have
to add or remove half a pound.
Weber’s Fraction (K) for
Different Stimuli
Stimulus
K
Pitch
.003
Brightness
.017
Weight
.02
Odor
.05
Loudness
.10
Pressure on Skin
.14
Saltiness of taste
.20
Gustav Fechner used Weber’s Law to
study the relationship between the
physical magnitude of a stimulus and
its perceived magnitude.
He reasoned that the JND gets larger
as the stimulus magnitude increases.
Example: when bench pressing 50lbs you
are more likely to notice a 5lb increase than
you would if you were benching 100lbs