Chapter 6

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Chapters 6
Sensation
Chapter 6 - Sensation
Slide 1
Overview
In this chapter we will be focusing on our “input systems”,
the senses we use to perceive the world around us.
We will focus primarily on vision, but will also discuss
audition, touch and smell.
Much of the information will attempt to explain how these
systems work … though we will also discuss illusions that
show us how our sensory systems can be tricked.
Throughout, we will again emphasize the distinction between
sensation, the information arriving through the sense organs,
and perception, the subjective impression of the world that we
end up with.
Chapter 6 - Sensation
Slide 2
Sensation versus Perception
The textbook actually defines sensation and perception
slightly differently from how I have, though the flavour
is the same. According to the text:
Sensation is the detection of simple stimulus properties
such as brightness, colour, sound frequency, sweetness.
Perception is the detection of
objects, their location, their
movements, their background.
Thus, seeing purple and blue
in the picture is sensation,
whereas seeing beautiful
fireworks is perception
Chapter 6 - Sensation
Slide 3
Transduction
Transduction is the process by which sense organs convert
energy from environmental events into neural activity …
eventually ending up in the brain.
Sense organs differ in terms of the kinds of environmental
energies they are sensitive to, and in the manner they
transduce that energy.
Usually the transduction is accomplished via specialized
receptor cells that release specialized neurotransmitters
that stimulate other neurons.
Steve, show table 6.1 here to show some of the variety
across the sense organs
Chapter 6 - Sensation
Slide 4
Sensory Coding
As we discussed earlier, nerve cells can only send a message
or not. The message does not vary in terms of what it says,
or how it says it.
So how then can the same nerve cells transmit the
fact that bananas are yellow, but carrots are orange?
The answer lies in the use of code … think of Morris Code
for example. In Morris Code complex semantic messages
were transmitted using simple clicks
Chapter 6 - Sensation
Slide 5
How does the Brain Code Information?
The first answer to this question is: We’re not absolutely
sure yet … the puzzle still needs more unraveling.
But, we do think the following two codes are involved.
Anatomical Coding - Sensory organs located in different
parts of the body send their signals to different locations
in the brain … the brain uses this to interpret the signals
correctly. Example, rubbing your eyes & phantom limbs
Temporal Coding - Information can be coded according to
time. The easiest way to do this is with respect to rate of
neural firing. May be the main way to code the intensity
of stimulation
Chapter 6 - Sensation
Slide 6
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 7
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 8
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 9
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 10
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 11
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 12
Psychophysics - Physics of the Mind
The systematic study of the relation between the physical
characteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 13
Just Noticeable Differences - JNDs
Ernst Weber was the first to measure JNDs, the smallest
change in the magnitude of a stimulus that can be detected.
He found that the JND is directly related to the magnitude
of the stimulus.
For example, when examining people’s ability to detect
differences in weights, he found that if the weights were
within 1/40th of each other, no difference was detected.
Weber examined all the senses in this respect. For example,
to detect a difference in brightness, the difference must
exceed 1/60 of the average brightness of the stimuli.
These results are now called Weber Fractions.
Chapter 6 - Sensation
Slide 14
Measuring Sensitivity via Thresholds
The JND is a threshold is a difference threshold. It reflects
how big a difference has to be before it is detected.
There is a more simple thresholds as well, how intense does
a stimulus have to be before the subject claims to see, hear,
smell, taste, or feel it?
These sorts of thresholds have been used to assess the
sensitivity of our sensory apparatus … and have lead to
areas of research such as subliminal perception.
How do we know if a message is subliminal?
> simple threshold approach - establish the “limon”, the
point where it is noticed half the time, and go below
Chapter 6 - Sensation
Slide 15
Vacation Anyone?
Fly with us on Air Joordens!
We’ll take you anywhere you want to go!!
And our serving staff are all REALLY sexy!!
Chapter 6 - Sensation
Slide 16
The Problem With Simple Thresholds
Jastrow’s (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART
Chapter 6 - Sensation
SHIT
Slide 17
The Problem With Simple Thresholds
Jastrow’s (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART
Chapter 6 - Sensation
SHIT
Slide 18
The Problem With Simple Thresholds
Jastrow’s (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART
Chapter 6 - Sensation
SHIT
Slide 19
The Problem With Simple Thresholds
Jastrow’s (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART
Chapter 6 - Sensation
SHIT
Slide 20
The Problem With Simple Thresholds
Jastrow’s (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART
SHIT
Demand characteristics and response bias can prevent
accurate measurements of a threshold
Chapter 6 - Sensation
Slide 21
Separating Bias from Sensitivity
In 1974 (1974!) Green & Swets came up with a way of
measuring sensitivity free of response bias … it is an
approach called signal detection theory.
Signal detection theory involves presenting a stimulus
on some trials, and not on others … then asking subjects
on each trial to state (guess) whether or not a stimulus
was presented.
This leads to four possibilities:
Hit - saying a stimulus is present when it is.
Miss - saying a stimulus is not present when it is.
False Alarm - Saying a stimulus is present when it
is not.
Correct Rejection - Saying a stimulus is not present
when it is not.
Chapter 6 - Sensation
Slide 22
Is there a word?
CLOUD
QUEEN
TRUCK
PAPER
APPLE
Chapter 6 - Sensation
RADIO
Slide 23
Is there a word?
CLOUD
QUEEN
TRUCK
PAPER
APPLE
Chapter 6 - Sensation
RADIO
Slide 24
In Reality, stimulus was
Present
“Present”
Hit
“Absent”
Miss
Absent
False
Alarm
Correct
Negative
So far bias can still have an effect, but it effects both hits
and false alarms.
Chapter 6 - Sensation
Slide 25
How do we get rid of bias?
The trick here is to use some manipulation that will vary
response bias, and test a given subject under a number of
levels of this manipulation .. E.g., payoffs
1
Null Sensitivity
Hits
0.75
0.5
0.25
0
0
0.25
0.5
0.75
1
False Alarms
Chapter 6 - Sensation
Slide 26
How do we get rid of bias?
The trick here is to use some manipulation that will vary
response bias, and test a given subject under a number of
levels of this manipulation .. E.g., payoffs
Hit / FA
1
$1 / .00
0.75
Hits
$1 / -.50
0.5
.50 / -.50
0.25
.50 / -$1
0
.00 / -$1
0
0.25
0.5
0.75
1
False Alarms
Chapter 6 - Sensation
Slide 27
How do we get rid of bias?
The trick here is to use some manipulation that will vary
response bias, and test a given subject under a number of
levels of this manipulation .. E.g., payoffs
1
0.75
Hits
Maximum
Sensitivity
Free of
Bias
0.5
Null
Sensitivity
0.25
0
0
0.25
0.5
0.75
1
False Alarms
Chapter 6 - Sensation
Slide 28
Vision - Near vs. Far
Some of our senses are primarily
concerned with provide information about
stimuli and events in our immediate environment
(e.g. touch, perhaps smell). Others provide information
about stimuli and events that are further away (e.g., vision,
audition).
Clearly, knowledge about things not in our immediate
environment can be critical as actions can than be taken that
either bring the stimulus close if it is desirable, or make
sure it stays far away if it is undesirable.
Of our “early warning” senses, vision is primary. So how
does it work?
Chapter 6 - Sensation
Slide 29
Vision - What you see and What you don’t
First of all, we often believe that we see all there is to see
in the world. That is simply false.
As illustrated in the overhead Steve will soon show (see
Figure 6.7 in the text) the light spectrum ranges from
short wavelength signals like Gamma Rays, up to
long wavelength like AC television and radio waves.
We see only a small part of this called the visible spectrum.
It ranges from light with wavelengths of between 380
nanometers (violet) up to 760 nanometers (red).
This is really a tiny part of the light spectrum,
other beasties can see more.
Chapter 6 - Sensation
Slide 30
Vision - Basic Anatomy of Outer Eye
Steve, put up overhead corresponding to Figure 6.8 in some
way that also allows the students to see these overheads.
Iris - The coloured part of your eye … it is actually a
muscle that controls the size of the pupil.
Pupil - The black part in the middle of the eye … simply is
the opening that allows light into the eyeball.
Sclera - The white part of the eye … a tough membrane
that serves as protection for the eye itself.
Cornea - The fluid filled outer coating of the eye …
provides moisture and nutrients to the above parts.
OK, so now the light is in the eye … what happens next?
Chapter 6 - Sensation
Slide 31
Vision - Basic Anatomy of Inner Eye
Lens - sits behind the pupil and focuses the incoming light
onto the retina. This lens is flexible and slight
alterations in it can alter the focus of it, a process
called accommodation (as an example, squint).
Aqueous Humor - The now focussed light passes through
the eyeball proper which is filled with a liquid called
aqueous humor (watery fluid). This fluid nourishes the
front of the eye in a way that blood vessels normally
would.
Retina - If the eye is properly shaped, a nicely focussed
image lands on the inner coating of the back of the eye.
This inner coating is the retina, and it is the part that
transmits the light signal (focussed or not) into a neural
signal.
Chapter 6 - Sensation
Slide 32
Vision - Transduction
The process of transducing a light signal into a neural
impulse comes about through several interim steps as
illustrated in the overhead similar to Figure 6.12.
Step 1 - Photoreceptors
Light strikes the back of the eye stimulating photoreceptor
cells which can be either rods or cones (show overhead).
Rods are not responsive to colour, but they are very
responsive to dim light … great for low light situations.
Cones are sensitive to colour and provide a much more
detailed image … great for high light, detailed imaging.
The transduction is done via a bleaching process in which
the photopigments are split, causing an action potential.
Chapter 6 - Sensation
Slide 33
Vision - Photoreceptors
Step 1 - Photoreceptors (Continued)
There are actually three kinds of photoreceptors in the retina.
Rods have one type, a type that is only sensitive to brightness
(i.e., shades of grey)
Cones come in three varieties, one roughly sensitive to the
colour red, another to green, and a third to blue (approximately)
The photoreceptors of the cones are sensitive to colour in the
inverse way that a TV (or computer monitor) displays colour.
Hue
Lum
Sat
Chapter 6 - Sensation
Slide 34
Vision - Bipolar Cells
Step 2 - Bipolar Cells
The signal from the photoreceptors is then passed on to the
bipolar cells which reprocess the signal in a way that tends
to emphasize edges and contours.
Essentially, when the photoreceptors associated with spatially
close parts of the retina are sending very different signals,
the bipolar cells accentuate these spots aiding us in our
ability to perceive edges.
Chapter 6 - Sensation
Slide 35
Vision - Ganglion Cells
Step 3 - Ganglion Cells
The third and final step in the retina pre-processing of
visual information is the ganglion cells.
Ganglion cells come in two types, red/green and blue/yellow
Each cell represents an opponent process
system. For example in red/green cells,
the resting behaviour of the cell is to
produce some mid-level rate of responding.
This rate increases when red is present, and
decreases when green is present.
The yellow/blue increases when both red and
green are present (yellow?) but decreases when
blue is present
Chapter 6 - Sensation
Slide 36
Demo of Ganglion Rebound
STOP
Chapter 6 - Sensation
Slide 37
Demo of Ganglion Rebound
Chapter 6 - Sensation
Slide 38
Vision Conclusion
The visual system is able to transform light waves into
electro-chemical nerve impulses through the use of
photoreceptors.
The image is significantly pre-processed prior to leaving
the eye in ways that allow us to see a rich palette of colours
and that accentuate contours.
This pre-processing is done via different types of nerve
cells that perform slightly different operations on the
signal prior to passing it along.
Surely such an intricate system requires a creator, n’est pas?
Chapter 6 - Sensation
Slide 39
The Other Senses
Audition
Like the eye, the ear is able to take a physical stimulus
(sound waves) and convert it to a nerve impulse which
is passed along to the brain.
Also like the eyes, the ears pre-process the signal fairly
extensively before passing it along.
However, there differences in the way that auditory
and visual information is pre-processed, and these differences
lead to different benefits
example: The synthesis of light versus the analysis of sounds
Using Doppler effects to sense relative motion
Chapter 6 - Sensation
Slide 40
The Other Senses Continued
The text goes on to describe how some of our other
senses work including touch, taste, and internal body
senses.
We simply do not have enough time to discuss all of
these in class, so I leave it to you to read and learn.
Given this, let me once again recite our new class motto:
“We, the students of PsyA01, are responsible for all of
the information in the lectures AND all of the information
in the text”
Chapter 6 - Sensation
Slide 41

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