CLASS NOTES FOR SENSATION AND PERCEPTION

Lecture 1
Lecture 2
Lecture 3
Lecture 4
Lecture 5
Lecture 6

Lecture 1
Chapter 1: Introduction to Sensation and Perception May 9, 2006
Syllabus
Professor: Dr. Bennett Schwartz
office: DM 281A phone: 348-4025 office hours:
email: schwartb@fiu.edu ; web site: www.fiu.edu/~schwartb/sensation.html
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Syllabus
Class sessions: Tuesdays and Thursdays, 9:30 a.m. to 12:15 p.m..
Texts: Sensation & Perception (7th Edition, 2007). Goldstein.

Syllabus
Exams: There are three exams, each worth 1/3 of the final grade. The dates for the exams are as follows: May 23, June 8, and June 22.
NO MAKEUPS

Syllabus
1. If you take all three exams, the best two will count.
2. If you take the first two exams, and miss the third exam, your lowest exam grade on the first three exams will be counted double.
3. If you take two exams, including the third exam, your grade with be the arithmetic mean (average) of those two exams.
4. A grade of “F” will be given to anyone who misses 2 or more exams.
There will be NO Makeups!

Overview of Questions
Why study perception?
How are perceptions determined by unconscious processes?
What is the difference between perception and recognition?
How is perception measured?

Why Study Perception?
Future careers
Graduate school work in perception
Medical applications
Devices to assist people with vision and hearing losses
Understanding how you perceive the world
Language processing
Color vision
Depth perception

The Perceptual Process
Environmental stimuli
All available stimuli for an observer
Attended stimuli
Stimuli that are the point of focus for the observer
Stimulus on the receptors
“Image” of stimulus

The Perceptual Process - continued
Transduction
Change from environmental energy to electrical energy in the nervous system
Neural processing
Interconnected neurons that propagate the electrical signal from receptor cells throughout the brain

The Perceptual Process- continued
Perception
Conscious sensory experience
Recognition
Ability to place objects in categories that provide meaning
Action
Motor activities that occur in reference to the perceived and recognized object

Two Interacting Aspects of Perception
Bottom-up processing
Processing based on incoming stimuli from the environment
Also called data-based processing
Top-down processing
Processing based on the perceiver’s previous knowledge
Also called knowledge-based processing

Approaches to the Study of Perception
Levels of Analysis
Observing perceptual processes at different scales
Psychophysical level - the stimulus-perception relationship
Physiological level - the stimulus-physiology relationship
These levels are interconnected and communicate with one another

Psychophysics - Qualitative Methods
Description
Basic description of what a person perceives
First step in studying perception
Called phenomenological method
Recognition
Categorization of stimuli
Psychophysics - Quantitative Methods
Absolute threshold - smallest amount of energy needed to detect a stimulus
Method of limits
Stimuli of different intensities presented in ascending and descending order
Observer responds to whether she perceived the stimulus
Cross-over point is the threshold

Quantitative Methods - continued
Absolute threshold (cont.)
Method of adjustment
Stimulus intensity is adjusted continuously until observer detects it
Repeated trials averaged for threshold

Quantitative Methods - continued
Absolute threshold (cont.)
Method of constant stimuli
5 to 9 stimuli of different intensities are presented in random order
Multiple trials are presented
Threshold is the intensity that results in detection in 50% of trials

Quantitative Methods - continued
Difference Threshold (DL) - smallest difference between two stimuli a person can detect
Same methods can be used as for absolute threshold
As magnitude of stimulus increases, so does DL
Weber’s Law explains this relationship
DL / S = K

Quantitative Methods - continued
Magnitude estimation
Stimuli are above threshold
Observer is given a standard stimulus and a value for its intensity
Observer compares the standard stimulus to test stimuli by assigning numbers relative to the standard
Quantitative Methods - continued
Magnitude estimation (cont.)
Response compression
As intensity increases, the perceived magnitude increases more slowly than the intensity
Response expansion
As intensity increases, the perceived magnitude increases more quickly than the intensity

Quantitative Methods - continued
Magnitude estimation (cont.)
Relationship between intensity and perceived magnitude is a power function
Steven’s Power Law
P = KSn

Other Measurement Methods
Searching for stimuli
Visual search - observers look for one stimulus in a set of many stimuli
Reaction time (RT) - time from presentation of stimulus to observer’s response is measured

Lecture 2
Light and the anatomy of the human eye May 11, 2006
Overview of Questions

How is light transformed into electricity in the eye?
How is what we see determined by the properties of the receptors in our retinas?
Basic Brain Structure
The brain has modular organization
The sensory modalities have primary receiving areas
Vision - occipital lobe
Audition - temporal lobe
Tactile senses - parietal lobe

Key components of neurons:
Cell body
Dendrites
Axon or nerve fiber
Receptors - specialized neurons that respond to specific kinds of energy

Synaptic Transmission of Neural Impulses
Neurotransmitters are:
Released by the presynaptic neuron from vesicles
Received by the postsynaptic neuron on receptor sites
Matched like a key to a lock into specific receptor sites
Used as triggers for voltage change in the postsynaptic neuron
Light is the Stimulus for Vision
Electromagnetic spectrum
Energy is described by wavelength
Spectrum ranges from short wavelength gamma rays to long wavelength radio waves
Visible spectrum for humans ranges from 400 to 700 nanometers
Most perceived light is reflected light

Focusing Images on the Retina
The cornea, which is fixed, accounts for about 80% of focusing
The lens, which adjusts shape for object distance, accounts for the other 20%
Accommodation results when ciliary muscles are tightened which causes the lens to thicken
Light rays pass through the lens more sharply and focus near objects on retina

Focusing Images on Retina - continued
The near point occurs when the lens can no longer adjust for close objects
Presbyopia - “old eye”
Distance of near point increases
Due to hardening of lens and weakening of ciliary muscles
Corrective lenses are needed for close activities, such as reading

Retinal Processing - Rods and Cones
Differences between rods and cones
Shape
Rods - large and cylindrical
Cones - small and tapered
Distribution on retina
Fovea consists solely of cones
Peripheral retina has both rods and cones
More rods than cones in periphery

Retinal Processing - Rods and Cones - continued
Number
120 million rods
5 million cones
Blind spot - place where optic nerve leaves the eye
We don’t see it because:
One eye covers the blind spot of the other
It is located at edge of the visual field
The brain “fills in” the spot

Diseases that Affect the Retina
Macular degeneration
Fovea and small surrounding area are destroyed
Creates a “blind spot” on retina
Most common in older individuals
Retinitis pigmentosa
Genetic disease
Rods are destroyed first
Foveal cones can also be attacked
Severe cases result in complete blindness

Transduction of Light into Nerve Impulses
Receptors have outer segments, which contain:
Visual pigment molecules, which have two components:
Opsin - a large protein
Retinal - a light sensitive molecule
Visual transduction occurs when the retinal absorbs one photon
Retinal changes it shape, called isomerization
Measuring Dark Adaptation
Three separate experiments are used
Method used in all three experiments:
Observer is light adapted
Light is turned off
Once the observer is dark adapted, she adjusts the intensity of a test light until she can just see it
Measuring Dark Adaptation - continued
Experiment for rods and cones
Observer looks at fixation point but pays attention to a test light to the side
Results show a dark adaptation curve:
Sensitivity increases in two stages
Stage one takes place for 3 to 4 minutes
Then sensitivity levels off for 7 to 10 minutes - the rod-cone break
Stage two shows increased sensitivity for another 20 to 30 minutes

Measuring Dark Adaptation - continued
Experiment for cone adaptation
Test light only stimulates cones
Results show that sensitivity increases for 3 to 4 minutes and then levels off
Experiment for rod adaptation
Must use a rod monochromat
Results show that sensitivity increases for about 25 minutes and then levels off
Spectral Sensitivity of Rods and Cones
Sensitivity of rods and cones to different parts of the visual spectrum
Use monochromatic light to determine threshold at different wavelengths
Threshold for light is lowest in the middle of the spectrum
1/threshold = sensitivity, which produces the spectral sensitivity curve

Spectral Sensitivity of Rods and Cones - continued
Rod spectral sensitivity shows:
More sensitive to short-wavelength light
Most sensitivity at 500 nm
Cone spectral sensitivity shows:
Most sensitivity at 560 nm
Purkinje shift - enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs
Spectral Sensitivity of Rods and Cones - continued
Difference in spectral sensitivity is due to absorption spectra of visual pigments
Rod pigment absorbs best at 500 nm
Cone pigments absorb best at 419nm, 532nm, & 558nm
Average of all 3 equals 560nm
These match the spectral sensitivity curves

Lecture 3

Lecture 3: Visual Neuroscience May 16, 2006
Overview of Questions
How can brain damage affect a person’s perception?
Are there separate brain areas that determine our perception of different qualities?
How has the operation of our visual system been shaped by evolution and by our day-to-day experiences?
The optic chiasm
Assume you are looking straight ahead without moving your eyes. Everything to the right of where you are looking is your right visual word. Everything to the left is your left visual world. The center is the point you are looking at. Information from your left visual world enters the retina of both your left and right eyes.
The optic chiasm and crossing over
Crossing over
Nasal side of retina -- closest to nose
Nasal side of retina crosses over to contralateral side of brain.
Temporal side of retina -- closest to forehead.
Temporal side of retina stays on same side - to ipsolateral side of brain.
This crossing over in the chiasm preserves the actual spatial world (left right) rather than left/right in the retina.

Pathways from eye to brain
Leaves retina through optic chiasm
First stop: Lateral Geniculate nucleus (LGN) of thalamus
Thalamus: major “relay” point in brain (will see it in other sensory systems as well)
Maps: Representing Spatial Layout
Retinotopic map - each place on the retina corresponds to a place on the LGN
Determining retinotopic maps - record from neurons with an electrode that penetrates the LGN obliquely
LGN has 6 layers
Stimulating receptive fields on the retina shows the location of the corresponding neuron in the LGN
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The Map on the Cortex
Cortex shows retinotopic map too
Electrodes recording from a cat’s visual cortex shows:
Receptive fields on the retina that overlap also overlap in the cortex
This pattern is seen using an oblique penetration of the cortex

The Map on the Cortex - continued
Cortical magnification factor
Fovea has more cortical space than expected
Fovea accounts for .01% of retina
Signals from fovea account for 8% to 10% of the visual cortex
This provides extra processing for high-acuity tasks
Brain Imaging Techniques
Positron emission tomography (PET)
Person is injected with a harmless radioactive tracer
Tracer moves through bloodstream
Monitoring the radioactivity measures blood flow
Changes in blood flow show changes in brain activity
Brain Imaging Techniques - continued
PET - subtraction method
Brain activity is determined by:
Measuring activity in a control state
Measuring activity in a stimulation state
Subtracting the control activity from the stimulation activity

Brain Imaging Techniques - continued
Functional magnetic resonance imaging (fMRI) measures blood flow by:
Hemoglobin carries oxygen and contains a ferrous molecule that is magnetic
Brain activity takes up oxygen, which makes the hemoglobin more magnetic
fMRI determines activity of areas of the brain by detecting changes in magnetic response of hemoglobin
Subtraction technique is used like in PET

Structural MRI 2
Summary of fMRI
Organization in Columns
LGN receives signals for right and left eyes
Layers 2, 3, and 5 receive input from the ipsilateral eye
Layers 1, 4, and 6 receive input from the contralateral eye
Electrodes inserted perpendicular to the surface show that receptive fields along the track are in the same location in the retina
Organization in Columns - continued
Visual cortex shows:
Location columns
Receptive fields at the same location on the retina are within a column
Orientation columns
Neurons within columns fire maximally to the same orientation of stimuli
Adjacent columns change preference in an orderly fashion
1 millimeter across the cortex represents entire range of orientation
Organization in Columns - continued
Visual cortex shows (cont.)
Ocular dominance columns
Neurons in the cortex respond preferentially to one eye
Neurons with the same preference are organized into columns
The columns alternate in a left-right pattern every .25 to .50 mm across the cortex

Lesioning or Ablation Experiments
First, an animal is trained to indicate perceptual capacities
Second, a specific part of the brain is removed or destroyed
Third, the animal is retrained to determine which perceptual abilities remain
The results reveal which portions of the brain are responsible for specific behaviors

What and Where Pathways
Ungerleider and Mishkin experiment
Object discrimination problem
Monkey is shown an object
Then presented with two choice task
Reward given for detecting the target object
Landmark discrimination problem
Monkey is trained to pick the food well next to a cylinder

What and Where Pathways - continued
Ungerleider and Mishkin (cont.)
Using ablation, part of the parietal lobe was removed from half the monkeys and part of the temporal lobe was removed from the other half
Retesting the monkeys showed that:
Removal of temporal lobe tissue resulted in problems with the landmark discrimination task - What pathway
Removal of parietal lobe tissue resulted in problems with the object discrimination task - Where pathway

Path of information in Visual Cortex
After information leaves V1, it goes to a number of different regions in the occipital lobe, including
V2, V3, V4 (collectively called extrastriate areas)
MT -- in parietal lobe
IT -- in temporal lobe

What and Where Pathways - continued
What pathway also called doral pathway
Where pathway also called ventral pathway
Both pathways originate in retina
Ventral pathway begins in small or medium ganglion cells
Called P-cells
Axons synapse in layers 3, 4, 5, & 6 of LGN
Called parvocellular layers
What and Where Pathways - continued
Dorsal pathway begins in large ganglion cells
Called M-cells
Axons synapse in layers 1 & 2 of LGN
Called magnocellular layers
Ablation research with monkeys shows:
Parvo channels send color, texture, shape and depth information
Magno channels send motion information

What and Where Pathways - continued
Where pathway may actually be “How” pathway
Dorsal stream shows function for both location and for action
Evidence from neuropsychology
Single dissociations: two functions involve different mechanisms
Double dissociations: two functions involve different mechanisms and operate independently

Modularity: Structures for Faces, Places, and Bodies
Module - a brain structure that processes information about specific stimuli
Inferotemporal (IT) cortex in monkeys
One part responds best to faces while another responds best to heads
Results have led to proposal that IT cortex is a form perception module
Temporal lobe damage in humans results in prosopagnosia

Modularity: Structures for Faces, Places, and Bodies - continued
Evidence from humans using fMRI and the subtraction technique show:
Fusiform face area (FFA) responds best to faces as well as when context implies a face
Parahippocampal place area (PPA) responds best to spatial layout
Extrastriate body area (EBA) responds best to pictures of full bodies and body parts

Evolution and Plasticity: Neural Specialization
Evolution is partially responsible for shaping sensory responses:
Newborn monkeys respond to direction of movement and depth of objects
Babies prefer looking at pictures of assembled parts of faces
Thus “hardwiring” of neurons plays a part in sensory systems

Evolution and Plasticity: Neural Specialization - continued
Plasticity of neurons also shapes sensory responses
Experience-dependent plasticity in animals
Monkeys trained to recognize specific view of unfamiliar object
Other views of object showed decline in recognition as object rotated from trained view
Neurons in the IT cortex showed maximal response to the trained orientation

Evolution and Plasticity: Neural Specialization - continued
Experience-dependent plasticity in humans
Brain imaging experiments show areas that respond best to letters and words
fMRI experiments show that training results in areas of the FFA responding best to:
Greeble stimuli
Cars and birds for experts in these areas

Sensory Code: Representation of Environment
Sensory code - representation of perceived objects through neural firing
Specificity coding - specific neurons responding to specific stimuli
Leads to the “grandmother cell” hypothesis
Recent research shows cells in the hippocampus that respond to concepts such as Halle Berry

Problems with specificity coding:
Too many different stimuli to assign specific neurons
Most neurons respond to a number of different stimuli
Distributed coding - pattern of firing across many neurons codes specific objects
Large number of stimuli can be coded by a few neurons
Sensory Code: Representation of Environment - continued
Coding can be distributed across many brain areas
Monkeys’ IT cortex shows overlap of activation caused by different stimuli
fMRI experiments with humans show the same type of effect
Thus, although there is specific response within modules, there is also activation across modules for specific stimuli

Neuropsychology
Understanding the behavior of brain-damaged patients by correlating the locus of the brain damage with behavioral deficits.

If area A is damaged, what can’t the person do.

General neuropsychology
Damage to hippocampus -- amnesia (memory deficit)
Damage to parietal lobe -- attention deficit
Damage to Broca’s area -- broken speech
Damage to Wernicke’s area -- inability to understand speech.
Damage to motor cortex -- movement deficits
Damage to orbito-frontal cortex --emotional control deficits/release of inhibition
Visual Neuropsychology
Deficits that result from areas of striate and extrastriate cortex damage
Blindsight
Area of brain damaged: V1
Functional damage: blindness in visual field associated with V1 damage. No visual experience.
Paradox: under some circumstances, patients make visual responses (in absence of reported seeing)
Blindsight
Patient D.B. - healthy normal active man in early 30’s.
Malignant tumor discovered on V1 of left hemisphere.
Tumor removed, DB returned to healthy life, job, family, etc.
Because V1 on right hemisphere was intact, he could see normally in everyday life
Blindsight
In lab, however, when forced to keep eyes still, DB had a SCOTOMA
Scotoma: blind spot due to V1 damage.
When stimuli were presented in his scotoma, DB could not see them.
Blindsight
In one experiment, DB had to judge whether lines presented were horizontal or vertical. 100% correct and reported seen in left visual field.
In right visual field, scotoma, DB claims not to see anything. But when forced to guess, he was 90% correct.
Blindsight
DB and other patients have been able to discriminate
X’s and O’s
Colors
Shapes
Orientation
All in the absence of reported seeing
Blindsight Explanations
1. Spared cells in V1 (islands of preservation theory): Gazzaniga
2. Direct connections from LGN to V2 , also Gazzaniga
3. Non-cortical routes. That is, blindsight is mediated by superior colliculus, which normally controls eye movements and also receives optic nerve input.
Agnosia
Agnosia: deficits in visual perception
Patients can see, but have difficulties with some aspects of perception.

Agnosia
Rare cases:
Damage to MT: motion agnosia
Damage to V4: color agnosia
More common: visual form agnosia -- can see, but don’t know what one is seeing.
Prosopagnosia
A selective deficit in the perception and recognition of faces.
People may not be able to recognize faces of close family members (recognize instantly via voice, but not from vision).
May extend to other stimuli too (familiar animals, etc)
Associative Agnosia
Can describe objects in detail; presumbly perceptually normal.
But do not know functions of these objects.
“a kind of a purse with five outpouchings; might be useful for five different size coins”
(a glove)
Damage: usually IT

Lecture 4

Lecture 4: Object Perception May 18, 2006
Overview of Questions
Why do some perceptual psychologists say “the whole differs from the sum of its parts”?
How do “rules of thumb” help us in arriving at a perception of the environment?
How do we distinguish objects from their background?
Why are even the most sophisticated computers unable to match a person’s ability to perceive objects?
The Challenge of Object Perception
The stimulus on the receptors is ambiguous
Inverse projection problem: an image on the retina can be caused by an infinite number of objects
Objects can be hidden or blurred
Occlusions are common in the environment

The Challenge of Object Perception - continued
Objects look different from different viewpoints
Viewpoint invariance: the ability to recognize an object regardless of the viewpoint
The reasons for changes in lightness and darkness in the environment can be unclear

The Structuralist Approach
Approach established by Wundt (late 1800s)
States that perceptions are created by combining elements called sensations
Structuralism could not explain apparent movement
Stimulated the founding of Gestalt psychology in the 1920s by Wertheimer, Koffka, and Kohler

The Gestalt Approach
The whole differs from the sum of its parts
Perception is not built up from sensations but is a result of perceptual organization
Principles of perceptual organization
Pragnanz - every stimulus is seen as simply as possible
Similarity - similar things are grouped together

Principles of Perceptual Organization - continued
Good continuation - connected points resulting in straight or smooth curves belong together
Lines are seen as following the smoothest path
Proximity - things that are near to each other are grouped together
Common fate - things moving in same direction are grouped together

Principles of Perceptual Organization - continued
Meaningfulness or familiarity - things form groups if they appear familiar or meaningful
Common region - elements in the same region tend to be grouped together
Uniform connectedness - connected region of visual properties are perceived as single unit
Synchrony - elements occurring at the same time are seen as belonging together

The Gestalt Approach - continued
Researchers have found neurons that respond maximally to displays that reflect:
Good continuation
Similarity
Gestalt principles do not make strong enough predictions to qualify as “laws”
They are better understood as heuristics - “best guess rules”
Perceptual Segregation
Figure-ground segregation - determining what part of environment is the figure so that it “stands out” from the background
Properties of figure and ground
The figure is more “thinglike” and more memorable than ground
The figure is seen in front of the ground
The ground is more uniform and extends behind figure
The contour separating figure from ground belongs to the figure

Figure-Ground Segregation - continued
Factors that determine which area is figure:
Elements located in the lower part of displays
Units that are symmetrical
Elements that are small
Units that are oriented vertically
Elements that have meaning

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Modern Research on Object Perception
Modern research emphasizes:
Obtaining measurements over descriptions
Determining the mechanisms responsible for object perception
This is in contrast to the Gestalt approach, but builds upon Gestalt principles
Questions Used in Modern Object Perception Research
Why does the visual system respond best to specific types of stimuli?
Must a figure be separated from ground before we can recognize objects?
How do we recognize objects from different viewpoints?
How does the brain process information about objects?
Why Does the Visual System Respond Best to Specific Types of Stimuli?
Regularities in the environment
There is a preponderance of verticals and horizontals
Oblique effect - people are more sensitive to these orientations
Occurs due to biology and experience
Gestalt heuristics are reflected in environmental objects

Must a Figure Be Separated from Ground Before We Can Recognize Objects?
Research has shown that objects may be recognized before or during the separation of figure from ground
Stimuli with a standing woman and a less meaningful shape were used
The meaningful stimulus (the woman) was recognized more often than the other
When the picture of the woman was turned upside down, this effect disappeared

Structural-description models
3-D objects are based on 3-D volumes called volumetric features that are combined for a given shape
Marr’s model proposed a sequence of events using simple geometrical features
The sequence begins with identifying edges and proceeds to recognition of the object

Structural-Description Models - continued
Recognition-by-components theory by Biederman
Volumetric features are called geons
Theory proposes there are 36 geons that combine to make all 3-D objects
Geons include cylinders, rectangular solids, and pyramids

Structural-Description Models - continued
Properties of geons
View-invariant properties - aspects of the object that remain visible from different viewpoints
Accidental property - a property that appears rarely and from certain viewpoints
Discriminability - the ability to distinguish geons from one another
Principle of componential recovery - the ability to recognize an object if we can identify its geons

Image-Description Models
Ability to identify 3-D objects comes from stored 2-D viewpoints from different perspectives
For a familiar object, view invariance occurs
For a novel object, view invariance does not occur
This shows that an observer needs to have the different viewpoints encoded before recognition can occur from all viewpoints

How Does the Brain Process Information About Objects?
Perceiving an object - sunburst or butterfly?
Experiment by Sheinberg & Logothetis
Monkey was trained to pull a lever for a sunburst or a butterfly
Binocular rivalry was used - each picture shown to one eye
Neuron in the IT cortex was monitored
Firing was vigorous for only the butterfly
Identifying an Object: Is That Harrison Ford?
Grill-Spector experiment
Region-of-interest approach: the FFA for each person was determined first by:
Showing participants faces and non-faces
Finding the area that responded preferentially to faces
Grill-Spector Experiment
FFA in each participant was monitored
On each trial, participants were shown either:
A picture of Harrison Ford’s face
A picture of another person’s face
A random texture
All stimuli were shown for 50 ms followed by a random-pattern mask
Participants were to indicate what they saw
60 pictures of each type were presented
Grill-Spector Experiment - continued
For trials that only included Harrison Ford’s face, results showed that FFA activation:
Was greatest when picture was correctly identified as Ford
Was less when picture was identified as other object
Showed little response when there was no identification of a face
Neural processing is associated with both the presentation of the stimulus and with the response to the stimulus

Freedman et al. experiment
Stimuli preparation - images of a cat and dog were morphed in 5 steps
Physiological measurement - neurons in monkeys’ IT and PF cortex were monitored
IT cortex is a module for form perception - what pathway
PF cortex responds when an object is recognized

Freedman et al. Experiment
Behavioral task - delayed-matching-to-sample
Sample period - “sample” stimulus is presented
Delay period - 1 sec period with no stimulus
Test period - “test” stimulus is presented
Task is for monkey to decide whether the test and sample are in the same category (dog or cat)

Figure 5.42 Delayed-matching-to-sample procedure.
Results of Freedman et al. Experiment
IT neuron responded:
Best when category dog presented during sample period
Equally to dog and cat during test period
PF neuron responded:
Equally to dog and cat during sample period
Best when category dog presented during delay and test period

Results of Freedman et al. Experiment - continued
IT neurons respond differently to presentation of cat and dog stimuli
Visual since they respond to perception
PF neurons respond differently to decision about stimuli
Behavioral since they guide the actual response in the task
Perceptual Intelligence
Theory of unconscious inference
Created by Helmholtz (1866/1911) to explain why stimuli can be interpreted in more than one way
Main Principle - perceptions are result of unconscious assumptions about the environment
Likelihood principle - objects are perceived based on what is most likely to have caused the pattern

Modern Ideas on Perceptual Intelligence
Palmer experiment
Observers saw a context scene flashed briefly followed by a target picture
Results showed that:
Targets congruent with the context were identified 80% of the time
Targets that were incongruent were only identified 40% of the time

Modern Ideas about Perceptual Intelligence - continued
Light-from-above-heuristic
Objects are generally perceived with the assumption that illumination comes from above
This is consistent with our experiences from the environment

Lecture 5

Lecture 5: Color Perception May 25, 2006
Colors
Color and meaning
Experiencing Color
The phenomenology of color: the personal perceptual experience we get when see colors. Arbitrary. Nothing inherently “blue” about 470 nm.
Color perception terms
Hue: the perceived color (red, green, etc)
Saturation: the relative amount of darkness or brightness in a hue. The less whiteness the more saturated (e.g., red and pink: pink is less saturated)
Overview of Questions
Why do we perceive blue dots when a yellow flash bulb goes off?
What does someone who is “color-blind” see?
What colors does a honeybee perceive?
What Are Some Functions of Color Vision?
Color signals help us classify and identify objects
Color facilitates perceptual organization of elements into objects
Color vision may provide an evolutionary advantage in foraging for food

How Can We Describe Color Experience?
Basic colors are red, yellow, green, and blue
Color circle shows perceptual relationship among colors
Colors can be changed by:
Intensity which changes perceived brightness
Saturation which adds white to a color resulting in less saturated color

Figure 7.3 The color circle. Colors are arranged by placing perceptually similar colors next to each other, so the four basic colors are positioned at 12, 3, 6 and 9 o’clock on the circle. (From Color Vision, by Leo M. Hurvich, 1981. Reprinted by permission of Dr. Leo M. Hurvich.)
What Is the Relationship Between Wavelength and Color Perception?
Color perception is related to the wavelength of light:
400 to 450nm appears violet
450 to 490nm appears blue
500 to 575nm appears green
575 to 590nm appears yellow
590 to 620nm appears orange
620 to 700nm appears red

Colors of Objects
Colors of objects are determined by the wavelengths that are reflected
Reflectance curves - plots of percentage of light reflected for specific wavelengths
Chromatic colors or hues - objects that preferentially reflect some wavelengths
Called selective reflectance
Achromatic colors - contain no hues
White, black, and gray tones

Color of Objects - continued
Selective transmission:
Transparent objects, such as liquids selectively allow wavelengths to pass through
Simultaneous color contrast - background of object can affect color perception

Trichromatic Theory of Color Vision
Proposed by Young and Helmholtz (1800s)
Three different receptor mechanisms are responsible for color vision
Behavioral evidence:
Color-matching experiments
Observers adjusted amounts of three wavelengths to match a comparison field to a test field

Color Matching Experiments
Results showed that:
It is possible to perform the matching task
Observers with normal color vision need at least 3 wavelengths to make the matches
Observers with color deficiencies can match colors by using only 2 wavelengths
Physiological Evidence for the Trichromatic Theory
Researchers measured absorption spectra of visual pigments in receptors (1960s)
They found pigments that responded maximally to:
Short wavelengths (419nm)
Medium wavelengths (551nm)
Long wavelengths (558nm)
Later researchers found genetic differences for coding proteins for the three pigments (1980s)

Response of Cones and Color Perception
Color perception is based on the response of the three different types of cones
Responses vary depending on the wavelengths available
Combinations of the responses across all three cone types lead to perception of all colors
Color matching experiments show that colors that are perceptually similar (metamers) can be caused by different physical wavelengths

Color Mixing
Additive color mixture:
Mixing lights of different wavelengths
All wavelengths are available for the observer to see
Superimposing blue and yellow lights leads to white
Subtractive color mixture:
Mixing paints with different pigments
Additional pigments reflect fewer wavelengths
Mixing blue and yellow leads to green

Are Three Receptor Mechanisms Necessary for Color Perception?
One receptor type cannot lead to color vision because:
Absorption of a photon causes the same effect no matter what the wavelength is - called the principle of univariance
Any two wavelengths can cause the same response by changing the intensity
Two receptor types (dichromats) solves this problem but 3 types (trichromats) allows for perception of more colors

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Color Deficiency
Monochromat - person who needs only one wavelength to match any color
Dichromat - person who needs only two wavelengths to match any color
Anomalous trichromat - needs three wavelengths in different proportions than normal trichromat
Unilateral dichromat - trichromatic vision in one eye and dichromatic in other

Color Experience for Monochromats
Monochromats have:
A very rare hereditary condition
Only rods and no functioning cones
Ability to perceive only in white, gray, and black tones
True color-blindness
Poor visual acuity
Very sensitive eyes to bright light

Color Experience for Dichromats
There are 3 types of dichromatism:
Protanopia affects 1% of males and .02% of females
Individuals see short-wavelengths as blue
Neutral point occurs at 492nm
Above neutral point, they see yellow
They are missing the long-wavelength pigment
Color Experience for Dichromats - continued
Deuteranopia affects 1% of males and .01% of females
Individuals see short-wavelengths as blue
Neutral point occurs at 498nm
Above neutral point, they see yellow
They are missing the medium wavelength pigment

Color Experience for Dichromats - continued
Tritanopia affects .002% of males and .001% of females
Individuals see short wavelengths as blue
Neutral point occurs at 570nm
Above neutral point, they see red
They are most probably missing the short wavelength pigment

Opponent-Process Theory of Color Vision
Proposed by Hering (1800s)
Color vision is caused by opposing responses generated by blue and yellow and by green and red
Behavioral evidence:
Color afterimages and simultaneous color contrast show the opposing pairings
Types of color blindness are red/green and blue/yellow

Opponent-Process Theory of Color Vision - continued
Opponent-process mechanism proposed by Hering
Three mechanisms - red/green, blue/yellow, and white/black
The pairs respond in an opposing fashion, such as positive to red and negatively to green
These responses were believed to be the result of chemical reactions in the retina

Physiology of Opponent-Process
Researchers performing single-cell recordings found opponent neurons (1950s)
Opponent neurons:
Are located in the retina and LGN
Respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other

Trichromatic and Opponent-Process Theories Combined
Each theory describes physiological mechanisms in the visual system
Trichromatic theory explains the responses of the cones in the retina
Opponent-process theory explains neural response for cells connected to the cones further in the brain

Color Processing in the Cortex
There is no single module for color perception
Cortical cells in V1, V2, and V4 respond to some wavelengths or have opponent responses
These cells usually also respond to forms and orientations
Cortical cells that respond to color may also respond to white
But damage to V4 causes loss of color perception and color memory; although patient can still distinguish wavelengths. Called cerebral achromatopsia

Perceiving Colors Under Changing Illumination
Color constancy - perception of colors as relatively constant in spite of changing light sources
Sunlight has approximately equal amounts of energy at all visible wavelengths
Tungsten lighting has more energy in the long-wavelengths
Objects reflect different wavelengths from these two sources

Possible Causes of Color Constancy
Chromatic adaptation - prolonged exposure to chromatic color leads to:
Receptors “adapt” when the stimulus color selectively bleaches a specific cone pigment
Sensitivity to the color decreases
Adaptation occurs to light sources leading to color constancy

Chromatic Adaptation
Experiment by Uchikawa et al.
Observers shown sheets of colored paper in 3 conditions:
Baseline - paper and observer in white light
Observer not adapted - paper illuminated by red light; observer by white
Observer adapted - paper and observer in red light

Experiment by Uchikawa et al.
Results showed that:
Baseline - green paper is seen as green
Observer not adapted - perception of green paper is shifted toward red
Observer adapted - perception of green paper is slightly shifted toward red
Partial color constancy was shown in this condition
Possible Causes of Color Constancy - continued
Effect of surroundings
Color constancy works best when an object is surrounded by many colors
Memory and color
Past knowledge of an object’s color can have an impact on color perception
Memory for color is not exact, so we don’t notice slight changes caused by illumination changes
Lightness Constancy
Achromatic colors are perceived as remaining relatively constant
Perception of lightness:
Is not related to the amount of light reflected by object
Is related to the percentage of light reflected by object

Possible Causes of Lightness Constancy
The ratio principle - two areas that reflect different amounts of light look the same if the ratios of their intensities are the same
This works when objects are evenly illuminated

Possible Causes of Lightness Constancy - continued
Lightness perception under uneven illumination
Perceptual system must distinguish between:
Reflectance edges - edge where the reflectance of two surfaces changes
Illumination edges - edge where illumination of two surfaces changes

Lightness Perception Under Uneven Illumination
Sources of information about illumination:
Information in shadows - system must determine that edge of shadow is an illumination edge
System takes into account the meaningfulness of objects
Penumbra of shadows signals an illumination edge

Sources of Information About Illumination
Orientation of surfaces provides information about illumination and reflectance edges
Perceptual organization of objects in a display affects perception of lightness

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Creating Color Experience
Light waves are not “colored”
Color is a creation of our physiology
Animals with different sensory apparatus, such as honey bees, experience something we cannot
All of our sensory experiences are created by our nervous system

Lecture 6

Lecture 6: Depth perception May 30, 2006
Overview of Questions
How can we see far into the distance based on the flat image of the retina?
Why do we see depth better with two eyes than with one eye?
Why don’t people appear to shrink in size when they walk away?

Cue Approach to Depth Perception
Oculomotor - cues based on sensing the position of the eyes and muscle tension
Convergence - inward movement of the eyes when we focus on nearby objects
Accommodation - change in the shape of the lens when we focus on objects at different distances

Cue Approach to Depth Perception - continued
Monocular - cues that come from one eye
Pictorial cues - sources of depth information that come from 2-D images, such as pictures
Occlusion - when one object partially covers another
Relative height - objects that are higher in the field of vision are more distant
Pictorial Cues
Relative size - when objects are equal size, the closer one will take up more of your visual field
Perspective convergence - parallel lines appear to come together in the distance
Familiar size - distance information based on our knowledge of object size

Occlusion cues
Relative Size (pun intentional)
Familiar Size: we don’t see a large animal on a giant hand.

Pictorial Cues - continued
Atmospheric perspective - distance objects are fuzzy and have a blue tint
Texture gradient - equally spaced elements are more closely packed as distance increases
Shadows - indicate where objects are located

Motion-Produced Cues
Motion parallax - close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly
Deletion and accretion - objects are covered or uncovered as we move relative to them
Also called occlusion-in-motion

Binocular Depth Information
Binocular disparity - difference in images between the two eyes
Difference can be described by examining corresponding points on the retina that connect to same places in the cortex
The horopter - imaginary circle that passes through the point of focus
Objects on the horopter fall on corresponding points on the retina

Binocular Depth Information - continued
Objects that do not fall on the horopter fall on noncorresponding points
These points made disparate images
The angle between these points is the angle of disparity
Objects located in front the horopter have crossed disparity
Objects located beyond the horopter have uncrossed disparity
Binocular Depth Information - continued
Stereopsis - depth information provided by binocular disparity
Stereoscope uses two pictures from slightly different viewpoints
3-D movies use the same principle and viewers wear glasses to see the effect
Random-dot stereogram has two identical patterns with one shifted to the right

Correspondence Problem
How does the visual system match the parts of images from the two eyes?
Matches may be made by specific features of objects
This may not work for objects like random-dot stereograms
A satisfactory answer has not yet been proposed
Physiology of Depth Perception
Experiment by Tsutsui et al.
Monkeys matched texture gradients that were 2-D pictures and 3-D stereograms
Recordings from a neuron in the parietal lobe showed:
Cell responded to pictorial cues
Cell also responded to binocular disparity

Physiology of Depth Perception - continued
Neurons have been found that respond best to binocular disparity
Called binocular depth cells or disparity selective cells
These cells respond best to a specific degree of disparity between images on the right and left retinas

Size Perception
Distance and size perception are interrelated
Experiment by Holway and Boring
Observer was at the intersection of two hallways
A luminous test circle was in the right hallway placed from 10 to 120 feet away
A luminous comparison circle was in the left hallway at 10 feet away

Experiment by Holway and Boring
On each trial the observer was to adjust the diameter of the test circle to match the comparison
Test stimuli all had same visual angle (angle of object relative to observer’s eye)
Visual angle depends on both the size of the object and the distance from the observer

Experiment by Holway and Boring - continued
Part 1 of the experiment provided observers with depth cues
Judgments of size were based on physical size
Part 2 of the experiment provided no depth information
Judgments of size were based on size of the retinal images

Size Constancy
Perception of an object’s size remains relatively constant
This effect remains even if the size of the object on the retina changes
Size-distance scaling equation
S = K (R X D)
Changes in distance and retinal size balance each other
Size-Distance Scaling
Emmert’s law:
Retinal size of an afterimage remains constant
Perceived size will change depending on distance of projection
This follows the size-distance scaling equation

Monster illusion
Visual Illusions
Nonveridical perception occurs during visual illusions
Müller-Lyer illusion:
Straight lines with inward fins appear shorter than straight lines with outward fins
Lines are actually the same length

Müller-Lyer Illusion
Why does this illusion occur?
Misapplied size-constancy scaling:
Size constancy scaling that works in 3-D is misapplied for 2-D objects
Observers unconsciously perceive the fins as belonging to outside and inside corners
Outside corners would be closer and inside would be further away
Müller-Lyer Illusion - continued
Since the retinal images are the same, the lines must be different sizes
Problems with this explanation:
The “dumbbell” version shows the same perception even though there are no “corners”
The illusion also occurs for some 3-D displays

Müller-Lyer illusion.
Müller-Lyer Illusion - continued
Another possible explanation:
Conflicting cues theory - our perception of line length depends on:
The actual length of the vertical lines
The overall length of the figure
The conflicting cues are integrated into a compromise perception of length

Ponzo Illusion
Horizontal rectangular objects are placed over railroad tracks in a picture
Far rectangle appears larger than closer rectangle but both are the same size
One possible explanation is misapplied size-constancy scaling

The Ames Room
Two people of equal size appear very different in size in this room
The room is constructed so that:
Shape looks like normal room when viewed with one eye
Actual shape has left corner twice as far away as right corner

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The Ames Room - continued
Why does the illusion occur?
One possible explanation:
Observer thinks the room is normal
Women would be at same distance
One has smaller visual angle (R)
Due to the perceived distance (D) being the same
Her perceived size (S) is smaller

The Ames Room - continued
Another possible explanation:
Perception of size depends on relative size
One woman fills the distance between the top and bottom of the room
Other woman only fills part of the distance
Thus, first woman appears taller
Moon Illusion
Moon appears larger on horizon than when it is higher in the sky
One possible explanation:
Apparent-distance theory - horizon moon is surrounded by depth cues while moon higher in the sky has none
Horizon is perceived as further away than the sky - called “flattened heavens”
Moon Illusion - continued
Since the moon in both cases has the same visual angle, it must appear larger at the horizon
Another possible explanation:
Angular size-contrast theory - moon appears smaller when surrounded by larger objects
Thus, the large expanse of the sky makes it appear smaller
Actual explanation may be a combination of a number of cues

Effects of Person’s Ability to Take Action on Distance Perception
Distance perception can also be affected by the perception of ability to take action
Experiment by Proffitt et al.
Participants made distance judgments with or without a backpack
Those with the backpack increased their estimates, even though they did not have to walk the distance

Effects of Person’s Ability to Take Action on Distance Perception - continued
Experiment by Witt et al.
Phase 1:
Participants threw balls to targets 4 to 10 meters away
They used either a light or heavy ball
Distance estimates were larger after throwing the heavy ball

Effects of Person’s Ability to Take Action on Distance Perception - continued
Phase 2:
Participants were divided into two groups:
One group was told they would have to throw the balls while blindfolded
Other group was told that would have to walk to targets while blindfolded
Group that was told they would be throwing balls increased their estimates