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
Vision You can see a candle flame on a dark clear night at
Hearing You can detect a watch tick at 6m
Taste You can taste a Tsp. of sugar at 8L of water
Smell You can detect 1 drop of perfume in a 6 room apt
Touch 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 Subliminal Perception
In general no evidence that subliminal cuts influence
But consider Bruce & Valentine (1986)
o On priming
o If you don‟t know what happened in the subliminal cue
it made you 100 msec faster to register the image.
Fitzsimons et al (2008)
o 30 msce exposure to the apple symbol or the IBM
o Then they were asked to think of creative uses of a
brick. Those people who were shown the apple
generated more creative ideas.
o There must be something that influences a subsequent
task, not necessarily consumer decisions.
1. Accessory Structures (eg. The outer ear)
2. . Transductions -> Receptors
3. Coding (eg. Frequency)
4. Interaction -> Physiological and Psychological
The Visual System
Coloured area on the eye
Sphincter muscle to control the size of the pupil
Controls light level by contracting or expanding
The transparent covering outside of the eye
Protect the eye
Focusing the light
Helps focus information to the back of the eye.
A gel that inflates the eye and keeps the shape inside
Across the back of the eye
Light sensitive receptors are located at the back of the eye
There are three distinct layers in the retina
o Ganglion Cells (Output stage)
Amacrine (Between ganglion and bipolar)
Inhibitory o Bipolar Cells (Between the ganglion and receptor)
Horizontal Cells (Between Bipolar and Receptors)
Exclusively inhibitory. Some sort of inhibition,
responsible for cleaning up the image)
o Receptor Cells (On the very far back on the eye, to
output to the optic nerve)
The most sensitive part of the retina, almost in the middle of
The Optic Nerve
A bundle of axons that escapes from the back of the eye
No visual receptors
axons, blood vessels leave they eye here
The retina ~ Single Cell Recording ~ Lateral Inhibition
Rods and Cones
- The duplex theory
- 120 million rods
- 7 Million cones
- Rods and cones function very differently
operate at low intensity
sensitive to brightness
none in the fovea (low light source – stars- can only be picked
up in peripheral vision because the fovea is center
MONOCHROME – decode in black and white only
Operate at higher intensities
Cones are located in the fovea, more sensitive to images
Full colour But how do rods and cones work?
Visual Pigments are chemicals that are photosensitive – break
down when light hits then
Rods: Rhodopsin – breaks down to create an action potential
and generate a pigment.
Cones: Chlorolabe, Erythrolabe, Cyanolabe
How to demonstrate this
1. Dark Adaptation Curve
2. Spectral Sensitivity
But what about “phantom spots” ? –the weird gray spots you seen in
an optical illusion
Single Cell Recording
Isolate single retinal ganglion cell
Attach microelectrode and record the output.
Project spot of light on screen and move until output
This is retinal area served by the ganglion cell
Movement in ANY direction decreases firing rate
Every ganglion cell has a receptive filed
How a receptive ganglion cell responds.
Centre: Maximum Response area that turns it on
Off-Centre: Minimum response area turns it off.
But not every cell may be on center, some are off center
Doesn‟t have to be a perfect circle
Horizontal cells are inhibitory
Lateral Inhibition ~ Visual Cortex ~ Gestalt Laws
Lateral Inhibition – This is not in the text book but a very important
Caused by inhibitory horizontal cells that run side ways
Phantom spots can also be explained this way (gray smudges
in between black lines)
Off center and on surround which is why we see them at
intersections Lateral inhibition