Slide 1
Chapters 6
Sensation
Chapter 6 - Sensation
Slide 2
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 discussaudition, touch and smell.
Much of the information will attempt to explain how thesesystems work … though we will also discuss illusions thatshow us how our sensory systems can be tricked.
Throughout, we will again emphasize the distinction betweensensation, the information arriving through the sense organs,and perception, the subjective impression of the world that weend up with.
Chapter 6 - Sensation
Slide 3
Sensation versus Perception
The textbook actually defines sensation and perceptionslightly differently from how I have, though the flavouris the same. According to the text:
Sensation is the detection of simple stimulus propertiessuch as brightness, colour, sound frequency, sweetness.
Perception is the detection ofobjects, their location, their movements, their background.
Thus, seeing purple and blue in the picture is sensation,whereas seeing beautifulfireworks is perception
Chapter 6 - Sensation
Slide 4
Transduction
Transduction is the process by which sense organs convertenergy from environmental events into neural activity …eventually ending up in the brain.
Sense organs differ in terms of the kinds of environmentalenergies they are sensitive to, and in the manner theytransduce that energy.
Usually the transduction is accomplished via specializedreceptor cells that release specialized neurotransmittersthat stimulate other neurons.
Steve, show table 6.1 here to show some of the varietyacross the sense organs
Chapter 6 - Sensation
Slide 5
Sensory Coding
As we discussed earlier, nerve cells can only send a messageor 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 Codefor example. In Morris Code complex semantic messageswere transmitted using simple clicks
Chapter 6 - Sensation
Slide 6
How does the Brain Code Information?
The first answer to this question is: We’re not absolutelysure 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 7
Psychophysics - Physics of the Mind
The systematic study of the relation between the physicalcharacteristics 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 physicalcharacteristics 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 physicalcharacteristics 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 physicalcharacteristics 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 physicalcharacteristics 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 physicalcharacteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 13
Psychophysics - Physics of the Mind
The systematic study of the relation between the physicalcharacteristics of stimuli, and the sensations they produce.
Example - JNDs
Chapter 6 - Sensation
Slide 14
Just Noticeable Differences - JNDs
Ernst Weber was the first to measure JNDs, the smallestchange in the magnitude of a stimulus that can be detected.
He found that the JND is directly related to the magnitudeof the stimulus.
For example, when examining people’s ability to detectdifferences in weights, he found that if the weights werewithin 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 15
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 doesa 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 16
Vacation Anyone?
Fly with us on Air Joordens!
And our serving staff are all REALLY sexy!!We’ll take you anywhere you want to go!!
Chapter 6 - Sensation
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 SHIT
Chapter 6 - Sensation
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 SHIT
Chapter 6 - Sensation
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 SHIT
Chapter 6 - Sensation
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
Chapter 6 - Sensation
Slide 21
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 preventaccurate measurements of a threshold
Chapter 6 - Sensation
Slide 22
Separating Bias from Sensitivity
In 1974 (1974!) Green & Swets came up with a way ofmeasuring sensitivity free of response bias … it is anapproach called signal detection theory.
Signal detection theory involves presenting a stimuluson some trials, and not on others … then asking subjectson 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 23
Is there a word?
CLOUD
QUEEN
PAPER
TRUCK
APPLE
RADIO
Chapter 6 - Sensation
Slide 24
Is there a word?
CLOUD
QUEEN
PAPER
TRUCK
APPLE
RADIO
Chapter 6 - Sensation
Slide 25
Present Absent
In Reality, stimulus was
“Present”
“Absent”
Hit False Alarm
Miss CorrectNegative
So far bias can still have an effect, but it effects both hitsand false alarms.
Chapter 6 - Sensation
Slide 26
The trick here is to use some manipulation that will varyresponse bias, and test a given subject under a number oflevels of this manipulation .. E.g., payoffs
0
0.25
0.5
0.75
1
0 0.25 0.5 0.75 1
False Alarms
Hit
sNull Sensitivity
How do we get rid of bias?
Chapter 6 - Sensation
Slide 27
How do we get rid of bias?
The trick here is to use some manipulation that will varyresponse bias, and test a given subject under a number oflevels of this manipulation .. E.g., payoffs
0
0.25
0.5
0.75
1
0 0.25 0.5 0.75 1
False Alarms
Hit
s
Hit / FA
$1 / .00
$1 / -.50
.50 / -.50
.50 / -$1
.00 / -$1
Chapter 6 - Sensation
Slide 28
How do we get rid of bias?
The trick here is to use some manipulation that will varyresponse bias, and test a given subject under a number oflevels of this manipulation .. E.g., payoffs
0
0.25
0.5
0.75
1
0 0.25 0.5 0.75 1
False Alarms
Hit
s
Null Sensitivity
MaximumSensitivity Free of Bias
Chapter 6 - Sensation
Slide 29
Vision - Near vs. Far
Some of our senses are primarilyconcerned with provide information aboutstimuli and events in our immediate environment(e.g. touch, perhaps smell). Others provide informationabout stimuli and events that are further away (e.g., vision,audition).
Clearly, knowledge about things not in our immediateenvironment can be critical as actions can than be taken thateither 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 30
Vision - What you see and What you don’t
First of all, we often believe that we see all there is to seein the world. That is simply false.
As illustrated in the overhead Steve will soon show (seeFigure 6.7 in the text) the light spectrum ranges fromshort 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 380nanometers (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 31
Vision - Basic Anatomy of Outer Eye
Steve, put up overhead corresponding to Figure 6.8 in someway 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 32
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 33
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 34
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 thecolour red, another to green, and a third to blue (approximately)
The photoreceptors of the cones are sensitive to colour in theinverse way that a TV (or computer monitor) displays colour.
HueLumSat
Chapter 6 - Sensation
Slide 35
Vision - Bipolar Cells
Step 2 - Bipolar Cells
The signal from the photoreceptors is then passed on to thebipolar cells which reprocess the signal in a way that tendsto emphasize edges and contours.
Essentially, when the photoreceptors associated with spatiallyclose 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 36
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 37
Demo of Ganglion Rebound
STOP
Chapter 6 - Sensation
Slide 38
Demo of Ganglion Rebound
Chapter 6 - Sensation
Slide 39
Vision Conclusion
The visual system is able to transform light waves intoelectro-chemical nerve impulses through the use ofphotoreceptors.
The image is significantly pre-processed prior to leavingthe eye in ways that allow us to see a rich palette of coloursand that accentuate contours.
This pre-processing is done via different types of nervecells that perform slightly different operations on thesignal prior to passing it along.
Surely such an intricate system requires a creator, n’est pas?
Chapter 6 - Sensation
Slide 40
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 whichis passed along to the brain.
Also like the eyes, the ears pre-process the signal fairlyextensively before passing it along.
However, there differences in the way that auditory and visual information is pre-processed, and these differenceslead to different benefits
example: The synthesis of light versus the analysis of sounds Using Doppler effects to sense relative motion
Chapter 6 - Sensation
Slide 41
The Other Senses Continued
The text goes on to describe how some of our othersenses work including touch, taste, and internal bodysenses.
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
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