Lecture+13+ +Notes4boobs

10/21/2013

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Objectives for Today

Sensation & Perception

What is the difference between these two?

What process links them?

Perception Research

What is the absolute threshold?

What is the Difference Threshold (JND)?

What is the basic idea of Weber’s Law?

Calculate the expected difference needed, given fraction and

magnitude of the first stimulus

Psychophysical Scaling

Does perceptual intensity match physical intensity?

Examples of ever-increasing/ever-decreasing stimuli


Signal Detection Theory

How is perception a decision-making process?

What kinds of things can affect the decisions?

What are the 4 possible types of responses in a

SDT experiment?

Subliminal Perception

What is it? Examples?

Sensory Adaptation

What is it? How does it help us?


Vision

What are the basic functions of eye structures?

What causes myopia? Hyperopia?

Retinal cells

What is the difference between rods/cones?

Color vision? Acuity? Seeing in Light/Dark?

What is the Fovea Centralis?

why is it important in our vision?

What is Dark Adaptation?

Is it a smooth progression? Why/why not?


Theories of Color Vision

What are the basic ideas of the 2 historical theories?

What is the current “best” theory of color vision?

Color Blindness

What causes color blindness

What is the term for missing one cone?

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Sensation & Perception

“There is no truth. There is only perception” - Gustav Flaubert

Sensation:

Stimulation of the sensory receptors e.g. light eye sound waves ear

Perception:

Organizing sensory input and giving it “meaning”

Recognizing a friend’s face or a melody

Qualitative experience of the sensation

Light of certain wavelength stimulates receptors

in the eye (sensation) “red” (perception)

Is my red is the same as yours? Video: http://www.youtube.com/watch?v=evQsOFQju08

Other differences between sensation & perception:

http://www.youtube.com/watch?v=evQsOFQju08

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Sensation:

Stimulation of the sensory receptors e.g. light eye sound waves ear

Perception:

Organizing sensory input and giving it “meaning”

Qualitative experience of the sensation

Tra

ns

du

ctio

n

Physical stimulus

Nerve impulses

Transduction:

process of ‘translating’ the physical,

environmental stimulus into electrical signals

(nerve impulses)

Visual receptors: light in, electricity out

Auditory receptors: sound waves in, electricity out

Olfactory receptors: scent molecules in, electricity out

GENERAL PRINCIPLES IN

PERCEPTION RESEARCH

Absolute Threshold

Difference Thresholds (Just Noticeable Differences)

Magnitude Estimation (Psychophysical Scaling)


Perception without Awareness

Sensory Adaptation

Psychophysics:

Studies how physical stimuli are translated

into psychological experiences

Absolute threshold

Difference threshold

Magnitude estimation

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Absolute Threshold

Absolute threshold:

Idea: we can’t detect everything, so what’s the

weakest/lowest level we can detect?

Problem: the graph didn’t look like it was supposed to

Gradual increase in ability to detect, not a big jump

Absolute Threshold

Absolute threshold:

Idea: we can’t detect everything, so what’s the

weakest/lowest level we can detect?

Problem: the graph didn’t look like it was supposed to

Gradual increase in ability to detect, not a big jump

Solution: New Definition:

Absolute Threshold: lowest intensity necessary for a

stimulus to be detected 50% of the time

Lower threshold means we are more sensitive

Difference Threshold & JND

Difference Threshold:

a.k.a. the Just-Noticeable Difference (JND)

the smallest difference between two stimuli that

people can detect… 50% of the time

Can you tell the difference between 300g weight

and a 500g weight? What about 300 vs. 301?

What is the minimum difference between 2 tones

before you can tell they are different?

Really good musicians tend to have a more sensitive

JND for pitch


Difference Threshold:

a.k.a. the Just-Noticeable Difference (JND)

Weber’s Law:

The size of the JND is proportional to the magnitude of

the first stimulus

Every stimulus type has a different fraction/proportion

e.g. Weber’s fraction for lifting weights is 1/30

Can you tell the difference between 300g and 310g?

What about 900g and 910g?

As stimuli get more ‘intense’, difference must

increase to be perceived

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Weber fraction Sample Questions Activity

The Weber fraction for brightness is 1/60. If you see a light that is

300 lumens, the comparison light must be _____ lumens (or

more) for you to judge it as brighter.

The Weber fraction for weights is 1/30. If you lift a weight that is

300g, the comparison weight must be _____ g (or more) for you

to judge it as heavier.

The Weber fraction for weights is 1/30. If you lift a weight that is

600g, the comparison weight must be _____ g (or less) for you to

judge it as lighter.


Psychophysical Scaling:

If we have a room with a 60 Watt light, and turn

on another, do we perceive it as twice as bright?

“Does perception match sensation?”

If this level of light is a 10, what level is a 20? Or a 5?

Constant increases in stimulus intensity produce

smaller and smaller (or larger and larger) increases

in the perception of intensity

Different types of stimuli act very differently

Line length vs. brightness vs. electric shock


Psychophysical Scaling:

Line length vs. brightness vs. electric shock


Signal Detection Theory:

Some people always say they have “seen the light”

This can mess with our “absolute threshold” or JND…

SDT: detecting stimuli (or differences) involves not

only perception, but a decision-making process

Each person decides how certain they need to be before

they say “yes, I see it” called the decision criterion

Varies between people

Also varies within people

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Solution: sometimes show people a stimulus,

sometimes nothing people say they saw it or not

4 possible outcomes:



Influences on “threshold” other than stimulus:

Personality of the participant

Fatigue/Attention

Rewards/Cost

“Noise”

Basic idea of SDT is that there is a decision-making

process involved in perception

Perception (Thresholds and JNDs) depend on sensation

AND on cognitive processes

As such, there is no fixed level of sensitivity!

Perception Outside Awareness

Remember D.F.’s “blindsight”

Proves that perception can happen without

conscious awareness

Subliminal perception:

Registering sensory input without conscious

awareness

“Limen” = “threshold”

Extensively studied in advertising

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Lipton Ice experiment

Sensory Adaptation

Sensory Adaptation:

a.k.a. “habituation”

Gradual decline in sensitivity to a stimulus

over prolonged stimulation

e.g. wearing a watch, entering a swimming pool

Occurs at the neuronal level

Happens in all senses (except maybe pain)

Visual saccades prevent adaptation

Helps us concentrate on what is important

VISION

Human Eye

Nearsightedness and Farsightedness)

Cells of the Retina

Visual Transduction

Dark Adaptation


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Basic Anatomy of the Eye

Cornea

“Bump” at the front of the eye

Light first enters here

Pupil

Basically a hole or opening that allows a certain

amount of light into the eye

Our eyes try to keep that amount ~optimal

Dilation or contraction controlled by muscles in the

iris (colored portion of the eye)

Basic Anatomy of the Eye

Lens

Crystalline structure that focuses light onto the

retina (where the receptors are)

Can change shape to alter visual focus

Thinner to focus distant objects

Thicker to focus near objects

Projects a 2D image onto the back of the eye

Image is actually upside-down and backwards

brain must “flip” it back

“Accommodation”

Myopia & Hyperopia

Myopia:

Eyeball is too long

Lens focuses the light in front of the retina

“Nearsighted”

Myopia & Hyperopia

Hyperopia:

Eyeball is too short

Lens focuses the light behind of the retina

“Farsighted”

Occurs frequently as part of aging

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Cells of the Retina

Retina

Multi-layered structure at the back of the eye

Sends visual signals to the brain

Layers are different types of cells

1. Receptor cells (rods/cones)

2. Horizontal cells

3. Bipolar cells

4. Amacrine Cells

5. Ganglion cells

Axons of ganglion cells make up the optic nerve

Back of eye

Front of eye

2 Types of Receptor Cells

1. Rods

120 million per eyeball

Extremely sensitive to light vision in dark

Many rods one ganglion cell

Even more light-sensitive, but poor “acuity”

Don’t really see color

2. Cones

6 million per eyeball

Less sensitive to light vision in bright environment

Required for color vision

3 types, sensitive to different colors

Only cones are in the fovea centralis

Fovea Centralis

Fovea:

Small area (<1mm2) in the center of the retina

used for the vast majority of vision

This is the portion of the eye involved in directed

looking

Has the greatest density of receptors

Only cones, each of which connects to 1 ganglion cell

Great visual acuity in this area

Blind Spot

Ganglion cells are

closer to the inside of

the eye than the

receptors. How do they

get out?

Hole in the retina

no receptors here

“Blind Spot” in the eye

Brain fills in gap using

nearby information

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Transduction

Transduction:

process of ‘translating’ the physical stimuli into

electrical signals (nerve impulses)

Rods and cones have different “photo-pigments”

Protein molecules that change shape when they

absorb light

Changing shape sodium channels to open

action potential releases neurotransmitter

Over time, photo-pigments change back to their

original shape

This takes some time

Photo-pigments & Blinding Lights

Takes time for photo-pigments to return to ‘ready’

there are always some ‘ready’ and some not

Can only fire (detect light) when ‘ready’

Constant turn-over

Blinding flash of light will cause all the ‘ready’ photo-

pigments to get used up

But we’re not blinded for too long, because other pigments were

getting close, but still were ‘not ready’

Photo-pigments & Dark Adaptation

Dark adaptation:

Progressive increase in sensitivity to light e.g. “eyes adjusting” to a dark movie theatre…

In bright light, a fair number of photo-pigments

are ‘used up’ at any given time

The likelihood of a photon hitting a ‘ready’ photo-

pigment is not 100%

After a while in the dark, more and more photo-

pigments have regenerated (without immediately

firing again)

The likelihood of a proton hitting a ‘ready’ photo-

pigment is much higher (close to 100%)

Photo-pigments & Dark Adaptation

Dark adaptation:

Not a smooth increase

in sensitivity over time

Due to differences

between rods & cones:

Rods:

Cones:

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Photo-pigments & Light Adaptation

Light adaptation:

Progressive decrease in sensitivity to light e.g. “eyes adjusting” to as you exit the movie…

After a while in the dark, more and more photo-

pigments have regenerated (without immediately

firing again)

The likelihood of a proton hitting a ‘ready’ photo-

pigment is much higher (close to 100%)

When you walk into the lighted hallway, a large

number of receptors fire blinded by the light

Over time, the cells become staggered again

Rods, Cones, and Color

Rods, 3 different types of cones

Each is more sensitive to a certain range of

light waves

Rods, Cones, and Color

Rods, 3 different types of cones

Each is more sensitive to a certain range of

light waves

Rods most sensitive to bluish light, not at all

sensitive to red light

This is why the world seems “blue” when we get up

in the middle of the night

WWII pilots and red sunglasses


Colors of light add differently than colors of paint

Primary colors of light: blue, green, and red

Some combination of these can produce any color in the

visible spectrum “Additive Color Mixture”

This is exactly how color TV works

1. Young-Helmholtz Trichromatic Theory:

We have 3 types of cones, each maximally

stimulated by either blue, green, or red light

Each must send signals to the brain, depending on

how much it is activated by a given wavelength

The visual system must then “add up” the inputs

and figure out the original ‘color’ of the light

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Problems with Trichromatic Theory:

Says “Yellow” is based on red + green input

But, some people with red-green color blindness can still

see yellow!!

Color after-images

2. Opponent Process Theory:

Thought each cone responded to 2 opposite colors

Red/green, blue/yellow, black/white

Opposite colors were assumed to result in 2 different

types of chemical reactions

Explanation of color after-images:

Sensory adaptation for one color

Then, when looking at white (all colors), only the “red”

half of that type of cone was able to respond


Unified Theory of Color Vision

Dual Process Theory:

Currently the best understanding of color vision

Turns out, both Trichromatic and Opponent

Process theories are partially correct

Cones are sensitive to blue, green, red

Opponent processes do occur, just not at the level of

cones

Happens more in ganglion cells, and in visual cortex

Dual Process Theory

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“Color Blindness”

Most people have 3 cones normal color vision

Called “trichromats”

Some people have 2 cones “colorblind”

Referred to as “dichromats”

Not truly blind to color, but can only see some

Missing red cone Red/green colorblind

Missing green cone Red/green colorblind

Missing blue cone (rare) Blue/Yellow colorblind

Monochromats truly colorblind

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Lecture+13+ +Notes4boobs