Psychology 1000 Lecture Notes - Parallelogram, Power Law, Cark
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CHAPTER 5: Sensation and Perception
Psychophysics ~ Subliminal Perception ~ Visual Systems
What do Weber fractions tell you about sensation?
Can we perceive subliminal messages?
How does the visual system work?
Getting the information in
How do we perceive the world around us?
Psychophysics and Psychophysiology
Psychophysics: The relation between the physical stimulus and the
Ex) Vision. We can measure the physical levels how light comes in, but
it is very difficult to measure the response.
Fechner: “Father” of Psychophysics
Can determine a just noticeable difference
JND – Just noticeable difference.
This can be measured, we can measure an intensity
- Value of stimulus characteristic required to produce some response
- Absolute: Lower limit
- Difference: Amount of change for JND
Designated as the lowest intensity at which a stimulus can be
detected 50% of the time. Vision is the most sensitive system
You can see a candle flame on a dark clear night at
You can detect a watch tick at 6m
You can taste a Tsp. of sugar at 8L of water
You can detect 1 drop of perfume in a 6 room apt
Wing of a fly falling on a check from 1m away
-Amount of change for JND
- The relationship between brightness and perceived brightness
- Not a 1:1 relation
- Size of a difference threshold relative to physical intensity of test is
constant. Must be able to calculate this
I = C
I - is the change of intensity
I - is the physical intensity given
C - is the constant number
If I = 50db and a JND is reported at 55db… then the change of the
intensity (I) is 5. (Because the change is 55-50=5) Now we simply
plug this information into the formula
5 = C
can be reduced to 1 = C
1. Now we can predict other JND‟s
eg. What‟s a JND @ 100 db?
To do this we look at the given information. We know I is 100db and C
is 1/10 (just calculated above)
I = 1
From here we simply cross multiply
(10)(I) = (100)
I = 10
- A JND would occur @
new test + 10 = 120 db OR
new test – 10 = 90 db
- Value of JND is NOT CONSTANT
The relative difference is
2. Can compare the sensitivity of different system
The smaller the fraction is, the better the system
The Weber‟s Constant – The amount of change in order for humans to
detect that change
Vision (Brightness) 1/60
Kinesthesis (Weights) 1/50
Pain (Thermal) 1/30
Audition (Mid Pitch; Mod. Loud) 1/10
Pressure (Skin) 1/7
Smell (India Rubber-most sensitive smell) ¼ - change intensity by
25% to notice a difference
Taste (Salt) 1/3
The human vision system is the most sensitive system because the
change is very small (Small fraction – larger number))
The human taste system is the least sensitive because the change has
to be very large (larger fraction –smaller number) in order for a
human to notice
*Fehner‟s Law (Wont have to calculate this. Only Weber constants)
- Sensation increases with the logarithm of intensity.
S = k log I
Compare: delta I = CI
This is more general and cognitively economic
Steven‟s Power Law (Don‟t have to calculate this either)
S = k log I^n
Most predictive across a wide variety of sensation
Even though you are not consciously aware of it, you simply
have to experience it and it will affect your unconscious.
Can we perceive stimuli that are below threshold?
Is our behavior going to be affected by subliminal stimuli?
James Vicary (1957)
Claimed the 50% increase in popcorn sales
o Didn‟t really. Nobody questioned him because they
wanted to believe it.
Concern about the use of subliminal “cuts”
o Embedding subliminal cue „BUY POPCORN DRINK SODA‟
o People became worried because maybe you will make
them do stuff that they don‟t want to do.
Often have hidden sexual images in advertising, saying “SEX”
To be more likely to like a product and want to buy because
you think of sex, and want it more