Chapters 6

1
Chapters 6

• Sensation

Chapter 6 - Sensation
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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
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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
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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
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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
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How does the Brain Code Information?
The first answer to this question is Were 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
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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
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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
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
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
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
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
13
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
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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 peoples
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
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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? gt simple threshold approach -
establish the limon, the point where it is
noticed half the time, and go below
Chapter 6 - Sensation
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Vacation Anyone?
Fly with us on Air Joordens!
Well take you anywhere you want to go!!
And our serving staff are all REALLY sexy!!
Chapter 6 - Sensation
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The Problem With Simple Thresholds
Jastrows (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART SHIT
Chapter 6 - Sensation
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The Problem With Simple Thresholds
Jastrows (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART SHIT
Chapter 6 - Sensation
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The Problem With Simple Thresholds
Jastrows (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART SHIT
Chapter 6 - Sensation
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The Problem With Simple Thresholds
Jastrows (1897) Subliminal Perception Experiment
H
3
F
1
6
R
B
5
The notion (and evidence) for perceptual defense
CART SHIT
Chapter 6 - Sensation
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The Problem With Simple Thresholds
Jastrows (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
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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
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Is there a word?
CLOUD
QUEEN
TRUCK
RADIO
PAPER
APPLE
Chapter 6 - Sensation
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Is there a word?
CLOUD
QUEEN
TRUCK
RADIO
PAPER
APPLE
Chapter 6 - Sensation
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In Reality, stimulus was
Present Absent
False Alarm
Present Absent
Hit
Correct Negative
Miss
So far bias can still have an effect, but it
effects both hits and false alarms.
Chapter 6 - Sensation
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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
Null Sensitivity
Chapter 6 - Sensation
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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 / .00 1 / -.50 .50 / -.50 .50 /
-1 .00 / -1
Chapter 6 - Sensation
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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
Maximum Sensitivity Free of Bias
Null Sensitivity
Chapter 6 - Sensation
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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
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Vision - What you see and What you dont
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
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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
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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
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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
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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
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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. Essentiall
y, 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
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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
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Demo of Ganglion Rebound
STOP
Chapter 6 - Sensation
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Demo of Ganglion Rebound
Chapter 6 - Sensation
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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, nest pas?
Chapter 6 - Sensation
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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
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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