Sensation & Perception Final Review Flashcards
1
Q
What form of energy does each sense capture?
Photons, air pressure, chemicals, etc.
A
Vision: Electromagnetic
Audition (Hearing): Mechanical forces
Vestibular (Sense of balance): Mechanical forces
Proprioception (Sense of limb positions):
Mechanical forces
Touch
2
Q
How can the different forms of energy be described?
Dimensions? Units? Etc.
A
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3
Q
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A
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4
Q
The quantitative relationship between physical energy and sensation:
A
psychophysics
5
Q
How does physical energy in the environment become transduced into neural signals within the brain?
A
sensory physiology
6
Q
How are patterns of sensory stimulation interpreted as meaningful events?
A
psychophysiology
7
Q
psychophysics
A
The quantitative relationship between physical energy and sensation
8
Q
sensory physiology
A
How does physical energy in the environment become transduced into neural signals within the brain?
9
Q
psychophysiology
A
How are patterns of sensory stimulation interpreted as meaningful events?
10
Q
Dualism:
A
The idea that the mind has an existence separate from the material world.
Mental phenomena are non-physical.
Rene Descartes (1596-1650):
- Defender of dualism
- Father of modern philosophy -Invented Cartesian coordinate system (X, Y, & Z)
- “I think therefore, I am”
11
Q
Rene Descartes
A
(1596-1650)
- Defender of dualism
- Father of modern philosophy -Invented Cartesian coordinate system (X, Y, & Z)
- “I think therefore, I am”
12
Q
Materialism
A
The idea that only matter and energy exist.
The mind is not separate from the body.
Most psychologists are modern materialists.
13
Q
Gustav Fechner
A
1801–1887
Invented “psychophysics” and is often considered to be the true founder of experimental psychology.
Struggled with the mind/body problem
Worked to exhastion. Went blind looking at the sun. Depression. Spent 3 years alone. Experience a “miracle” when vision returned. Struggled with mind body Also had some crazy ideas.
Believed in panpsychism: The idea that all matter has consciousness.
Wrote Nanna, or Concerning the Mental life of plants
14
Q
Ernst Weber
A
(1795–1878) discovered that the smallest change in a stimulus, such as the weight of an object, that can be detected is a constant proportion of the stimulus level. (i.e. a linear relationship).
These proportions were called Weber Fractions.
Example: Object 1 must weigh 1/40th more/less than Object 2 for the difference to be noticeable or a JND (just-noticeable difference)
15
Q
Fechner’s law
A
A principle describing the relationship between stimulus magnitude and resulting sensation magnitude such that the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity
S = k log R
Example:
100 candles is twice as bright as 10.
10 candles is twice as bright as 1.
16
Q
Stevens’ power law
A
(1962) is a proposed relationship between stimulus energy and perceived intensity is a power function.
Sensation = a * Intensity b
17
Q
Chronological Summary of Laws
A
Weber’s Law:
As stimulus level increases or decreases, the magnitude of change must increase proportionately (linearly) to remain noticeable.
Fechner’s Law:
The magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity.
Steven’s Power Law:
Stimulus energy and perceived intensity is a power function.
18
Q
Doctrine of specific nerve energies:
A
A doctrine formulated by Johannes Müller (1801–1858) stating that the nature of a sensation depends on which sensory fibers are stimulated, not on how the fibers are stimulated
19
Q
Photic Sneeze Reflex
A
Doctrine of specific nerve energies example
Also known as Sun Sneezing
18-35% of the population
2/3 of sun sneezers are female
Correlated with having a nasal septum deviation
Amazingly the cause is still unknown
Caused by light intensity, not spectral composition
Most believe it is caused by “crossed wires”
20
Q
Icy Hot
A
Doctrine of specific nerve energies example
Dulls the pain
2 Active Ingredients:
Capsaicin:
Found in chili peppers
Activates warmth fibers
Menthol:
Activates cold fibers
Sensation of both hot and cold
No actual heat transfer
21
Q
Hermann von Helmholtz (1821–1894)
A
First person to measure the speed of neural impulses
Demonstrated that neurons follow the laws of physics, Against what Müller believed
Invented the ophthalmoscope
Wrote On the Sensations of Tone, one of the first studies of auditory perception
A ton of other stuff for which there is not room to list.
22
Q
Speed of neural transmission
A
~50-100 meters/second
23
Q
Santiago Ramón y Cajal
A
(1852–1934)
Discovered the direction of travel of nerve impulses.
Only ~1% of neurons absorb stain
Improved upon a previous (Golgi’s) staining method and drawings.
Created incredibly detailed drawings of neurons and neural structure.
Ramón & Golgi were 1906 Nobel prize co-winners.
Cajal discovered the direction of travel of nerve impulses in the brain and spinal cord.
He was the first to note that information travels one way from the dendrites to the axon and not the reverse.
**He postulated that neurons are discrete entities.
He was unhappy that, because Spanish was not used in the scientific community, his work was not read outside Spain.
Many so-called discoveries by English, German, and French scientists were actually rediscoveries of his work, which had been previously published in Spanish journals.
24
Q
neurons
A
The processing of perceptions, thoughts and actions in the brain is accomplished by networks of small cells called neurons.
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Dendrites and Soma
Receive information from other neurons
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Axon
Conducts that information to other parts of the brain, sometimes over great distances
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Axon terminals
Transmit information to other neurons
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The Electric Neuron
Molecules in neural tissue are “charged”: + or -
Charged Ions are potassium K+, sodium Na+, chlorine Cl-, and
Proteins-
In the absence of stimulation, the inside of the neuron is slightly more negative than the outside (-70 mv).
This is called the resting potential.
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resting potential.
In the absence of stimulation, the inside of the neuron is slightly more negative than the outside (-70 mv).
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Ion channels pump ions across the membrane.
The Na+–K+ pump preserves the cell’s resting potential by maintaining a larger concentration of K+ inside the cell and Na+ outside the cell. The pump uses ATP as energy.
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synapse
The concentration of ions inside and outside of a neuron can be affected by neurotransmitters that are released into the synapse
Within the axon terminals, the relative charge between the inside and outside of a neuron is called the presynaptic potential.
Within the dendrites and soma, the relative charge between the inside and outside of a neuron is called the postsynaptic potential.
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postsynaptic potential.
Within the dendrites and soma, the relative charge between the inside and outside of a neuron.
When neurotransmitters are absorbed by the dendrites or soma, they can alter the postsynaptic potential.
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presynaptic potential
Within the axon terminals, the relative charge between the inside and outside of a neuron
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Neural Transmission
When neurotransmitters are absorbed by the dendrites or soma, they can alter the postsynaptic potential.
Excitatory neurotransmitters cause the charge to become depolarized (more positive). This change is called an excitatory postsynaptic potential (EPSP).
Inhibitory neurotransmitters cause the charge to become hyperpolarized (more negative). This change is called an inhibitory postsynaptic potential (IPSP).
Both EPSPs and IPSPs are produced by altering the relative concentration of ions between the inside and outside of the cell membrane.
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excitatory postsynaptic potential (EPSP).
Excitatory neurotransmitters cause the charge to become depolarized (more positive).
This change is called an excitatory postsynaptic potential (EPSP).
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inhibitory postsynaptic potential (IPSP).
Inhibitory neurotransmitters cause the charge to become hyperpolarized (more negative).
This change is called an inhibitory postsynaptic potential (IPSP).
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Gated Ion Channels, EPSPs
Causes depolarization of Post-synaptic neuron
Na+ transduction
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Gated Ion Channels, IPSPs
Causes hyperpolarization of Post-synaptic neuron
K+ and Cl- transduction
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IPSPs and EPSPs are Graded Potentials
They vary in magnitude based on the quantity of neurotransmitters with which they are stimulated.
They travel passively and are attenuated with distance.
At the base of the axon (called the axon hillock), a cell’s potential is determined by the sum of all the excitatory and inhibitory inputs in the dendrites and soma.
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action potential.
If the voltage at the axon hillock reaches a threshold of -50 mv, then it will increase rapidly to 50 mv, and then quickly rebound to -70 mv.
This sequence of voltage changes is called an action potential.
During the absolute refractory period it is impossible to generate a new action potential
During the relative refractory period a new action potential can be generated, but it requires a stimulus that is much stronger than usual.
The Toilet Metaphor
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The action potential is non-decremental.
| That is to say, its magnitude does not change as it travels along the axon.
This is quite different from graded potentials (EPSPs & IPSPs).
Their magnitudes become diminished as they travel within the dendrites & soma, and are therefore referred to as decremental.
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When an action potential reaches an axon terminal, it causes neurotransmitters to be released into the synapse.
The stimulation of a neuron modifies the frequency at which it generates action potentials
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Excitation and Inhibition
Any neuron can excite or inhibit another neuron based on the type of neurotransmitter it releases from its axon terminals.
The stimulation of a neuron modifies the frequency at which it generates action potentials
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What is a wave?
Lights and sounds are composed of waves.
A wave is a type of internal motion of a medium, in which the displaced portion returns to equilibrium.
This disturbance propagates in space as well.
Wave: disturbance propagates in x
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Transverse Waves
This is what occurs in the vibrations of stringed instruments
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Longitudinal Waves
This is what occurs in the vibrations of wind instruments
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Sound Waves
Sound waves involve the longitudinal oscillations of air molecules.
The sound vibrations in a long, narrow tube, such as a trombone, flute or trumpet, propagates in one direction.
In open air, a sound wave propagates radially in all directions
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Light Waves
Light waves involve transverse oscillations in electric and magnetic fields
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Light
A wave; a stream of photons, tiny particles that each consist of one quantum of energy.
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Diagram of a wave
All waves are caused by vibrations
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Velocity, Frequency and Wavelength
Within a given medium, light & sound waves travel at a constant velocity.
Thus, long wavelengths oscillate at low frequencies, and short wavelengths oscillate at high frequencies.
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Two waves with the same velocity and different wavelengths will have different frequencies.
Two waves with the same frequency and different wavelengths will have different velocities.
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Interference
The addition and subtraction of waves.
When two or more waves come together, the individual displacements at each point in space are added together to produce a composite wave.
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Absorption
The conversion of energy to a different form, such as heat, when a wave hits an obstacle
When a wave hits a boundary between two media, some of its energy may be converted to heat.
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Reflection
The reversal of direction when a wave hits an obstacle
When a wave hits a boundary between two media, some of its energy rebounds in a different direction
Some animals, such as bats, are able to use reflections of sounds to determine the distance of objects
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Refraction
The bending of a wave as it crosses the boundary between two media
What happens to a wave when it changes speed
When a wave travels from a less dense to a more dense medium at an angle off the line of normal, it slows down and bends towards the line of normal.
When a wave travels from a more dense to a less dense medium at an angle off the line of normal, it speeds up and bends away from the line of normal.
Refraction is what causes a pencil to appear bent when it is partially submerged in water.
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Diffraction
The bending of a wave around an obstacle.
Waves can sometimes bend around obstacles.
Long wavelengths diffract more than short wavelengths
Diffraction through a slit.
Interference patterns can occur when a wave interacts with its own reflection
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Wave Interactions
Interference – The addition and subtraction of waves
Absorption – The conversion of energy to a different form, such as heat, when a wave hits an obstacle
Reflection – The reversal of direction when a wave hits an obstacle
Refraction – The bending of a wave as it crosses the boundary between two media
Diffraction – The bending of a wave around an obstacle
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Scattered
Energy that is dispersed in an irregular fashion.
When light enters the atmosphere, much of it is absorbed or scattered and never reaches the perceiver.
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Transmitted
Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface)
When a wave hits a boundary between two media, some of its energy may be transmitted across the boundary
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Short wave lengths refract more than long wavelengths.
Rainbows are caused by light that is refracted through particles of water.
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Rule to remember
Short things have higher natural frequencies than long things
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Natural Frequencies
Most objects have a specific frequency at which they vibrate most readily. This is called the natural or fundamental frequency.
For a vibrating string, the natural frequency increases with the tension of a string, and decreases with increasing length or mass.
For a vibrating tube, the natural frequency decreases with increasing tube length, and is higher for open tubes than for closed tubes.
The manipulation of these parameters is what allows musicians to play musical instruments.
Rule to remember: Short things have higher natural frequencies than long things
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Tacoma Narrows bridge disaster
Shows what can happen when waves hit an object at its natural frequency.
This is also the reason why singers are able to break a glass by singing at the appropriate pitch.
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Methods of representing waves
Waveform plots
Spectral plots
Spectrograms
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sine waves
Sound is a change in the pressure of the air.
The waveform of any sound shows how the pressure changes over time.
The eardrum moves in response to changes in pressure.
Any waveform shape can be produced by adding together sine waves of appropriate frequencies, amplitudes and phases.
The amplitudes of the sine waves give the amplitude spectrum of the sound.
The amplitude spectrum of a sine wave is a single point at the frequency of the sine wave.
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Spectrum
Amplitude against frequency
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Waveform
Amplitude against time
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Amplitude
Amplitude is a measure of the pressure change of a sound and is related to how loud the sound is.
Amplitude squared is proportional to the energy or intensity (I) of a sound.
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adding sine waves
A sound which has more than one (sine-wave) frequency component is a complex sound.
A periodic sound is one which repeats itself at regular intervals.
A sine wave is a simple periodic sound.
Musical instruments or the voice produce complex periodic sounds.
They have a spectrum consisting of a series of harmonics.
The lowest frequency (of which all the others a re multiples) is called the fundamental frequency.
Each harmonic is a sine wave that has a frequency that is an integer multiple of the fundamental frequency.
The left column shows individual harmonics; the right shows their sum; the yellow panels show the amplitude spectrum of the sound.
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Fourier Synthesis
Here is 1/20th of a second of the waveform and also the spectrum of a complex periodic sound consisting of the first four harmonics of a fundamental of 100 Hz.
All the frequency components are integer multiples of 100 Hz.
A periodic sound consists of a section of waveform that repeats itself.
The period of the complex wave is the duration of this section.
In this case it is 1/100s or 0.01s, or 10 ms.
The period is the reciprocal of the fundamental frequency (in this case 100 Hz).
If you change the period of a complex sound, you change its pitch.
Shorter periods - higher fundamental frequency - higher pitch.
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Harmonics are integer multiples of the fundamental frequency
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A spectrogram is a 3-dimensional plot of frequency and amplitude as a function of time.
Amplitude is represented in a spectrogram by shades of gray
Spectrograms are especially useful for representing complex waveforms like speech that change over time
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Properties of Waves
Amplitude:
Controlled by the magnitude of the forces that started the wave
Frequency f of oscillations:
Controlled by forces starting the wave and by the nature of the material or object that is oscillating
Spectral Composition:
Refers to the mixture of different frequencies in a wave
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The 3 Main Perceptual Attributes of Sound
Loudness (is most related to intensity)
Pitch (is most related to frequency)
Timbre (is most related to spectral composition)
The terms pitch, loudness, and timbre refer not to the physical characteristics of sound, but to the mental experiences that occur in the minds of listeners.
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The 3 Main Perceptual Attributes of Light
Brightness (is most related to amplitude)
Hue (is most related to wave length or frequency)
Saturation (is most related to spectral composition)
The terms Brightness, Hue, and Saturation refer not to the physical characteristics of light, but to the mental experiences that occur in the minds of observers.
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The stimulus for vision is .....
visible electromagnetic radiation, which can be be characterized by its wavelength.
The human visual system is sensitive to wavelengths from 400 to 700 nanometers (10-9 meter)
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The human eye is made up of various parts:
Cornea: The transparent “window” into the eyeball
Aqueous humor: The watery fluid in the anterior chamber
Pupil: The dark circular opening at the center of the iris in the eye, where light enters the eye
Crystalline lens: The lens inside the eye, which enables changing focus. Focus is controlled by ciliary muscle.
Zonules of Zinn: connect the ciliary muscles with the lens
Vitreous humor: The transparent fluid that fills the vitreous chamber in the posterior part of the eye
Retina: A light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve.
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Cornea
The transparent “window” into the eyeball
no blood supply, but has nerves to feel scratches and dryness. highly organized structure to let light through
MOST OF THE REFRACTION HAPPENS HERE!!
Quick regeneration.
Contact lenses: sit on a layer of tears in front of cornea.
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Aqueous humor
The watery fluid in the anterior chamber
nutrient & oxygen delivery
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Pupil
The dark circular opening at the center of the iris in the eye, where light enters the eye
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Crystalline lens
The lens inside the eye, which enables changing focus.
Focus is controlled by ciliary muscle.
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Zonules of Zinn
connect the ciliary muscles with the lens
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Vitreous humor
The transparent fluid that fills the vitreous chamber in the posterior part of the eye
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FLOATERS
bio-debris.
No concern.
Egg-whites!!
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Retina
A light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve.
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Refraction is necessary to focus light rays and this is done by the cornea and the lens
The lens can change its shape, and thus alter the refractive power: Accommodation !!!
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TAPETUM
the colorful, shiny material located behind the retina that reflects light back through the retina to get a second chance at capturing missed photons!
A layer of tissue in the eye of many vertebrate animals.
It lies immediately behind the retina.
It reflects visible light back through the retina, increasing the light available to the photoreceptors.
This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected light
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Cataracts
loss of transparency in lens (solved with silicone implants)
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Presbyopia
"old sight"
Inability to accommodate nearby objects.
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Pigment Epithelium
nourishes receptors, non-reflective in humans & absorbs stray light
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Receptors face away from light.
Photoreceptors are in the LAST LAYER, mostly because of the pigment epithelium (provide vital nutrients to the photoreceptors
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Photoreceptors
Cells in the retina that initially transduce light energy into neural energy.
100 million photoreceptors
Chemical signals.
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Rods
Photoreceptors that are specialized for night vision
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Cones
Photoreceptors that are specialized for daylight vision, fine visual acuity and color
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CHROMOPHORE
the light-catching part of the visual pigments of the retina.
4 different types of pigments.
Cones have three different kinds which respond to Long, medium and short wavelengths.
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Visual process
biochemical cascade of events:
1. Closing of channels to the outersegment.
2. This causes HYPERPOLARIZATION in the cell body.
3. Reduction of neurotransmitter (GLUTAMATE) at the synaptic level.
Bipolar cell knows a photon has been caught.
GRADED POTENTIALS (not action potentials): the more photons, the less neurotransmitter.
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5 million cones
120 million rods
2 million ganglion cells
1 degree of visual angle where no RODS, only cones. Right behind center of pupil.
Therefore, many receptors must send signals to each ganglion cell an this is called convergence.
The extent of convergence on different parts of the retina produces a tradeoff between sensitivity to low light levels and sensitivity to fine spatial detail.
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The retina’s horizontal pathway:
Horizontal and amacrine cells
Horizontal cells: Specialized retinal cells that run perpendicular to the photoreceptors and make contact with photoreceptors and bipolar cells
Responsible for lateral inhibition, which creates the center–surround receptive field structure of retinal ganglion cells
Amacrine cells: These cells synapse horizontally between bipolar cells and retinal ganglion cells
Have been implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time)
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Horizontal cells
Specialized retinal cells that run perpendicular to the photoreceptors and make contact with photoreceptors and bipolar cells
Responsible for lateral inhibition, which creates the center–surround receptive field structure of retinal ganglion cells
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Amacrine cells
These cells synapse horizontally between bipolar cells and retinal ganglion cells
Have been implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time)
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The retina’s vertical pathway:
Photoreceptors, bipolar cells, and ganglion cells
Bipolar cell: A retinal cell that synapses with one or more rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells
Diffuse bipolar cell: A bipolar cell that receives input from multiple photoreceptors
Midget bipolar cell: A small bipolar cell that receives input from a single cone
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P ganglion cells
Connect to the parvocellular pathway
Receive input from midget bipolar cells
Parvocellular (“small cell”) pathway is involved in fine visual acuity, color, and shape processing.
Poor temporal resolution but good spatial resolution.
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M ganglion cells:
Connect to the magnocellular pathway
Receive input from diffuse bipolar cells
Magnocellular (“large cell”) pathway is involved in motion processing.
Excellent temporal resolution but poor spatial resolution
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Ganglion Cells
last stage before information leaves the eye and travels to and through the brain!
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Rod vision more sensitive to light than cone vision.
There is greater convergence of rods than cones onto ganglion cells
Thus, there is greater summation of rod signals,
and less stimulation per rod is required to obtain a response.
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Cone vision can see finer details than rod vision.
Greater convergence of rods than cones onto ganglion cells limits their spatial sensitivity.
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ON-center ganglion cells
Excited by light falling on center, inhibited by light falling surround
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OFF-center ganglion cells
Inhibited by light falling on center, excited by light falling on surround
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Ganglion cells that receive inputs from the fovea have smaller receptive fields than cells that receive inputs from more peripheral regions.
For an on centered ganglion cell, the response rate is greatest when the stimulus just fills the excitatory central region.
When the stimulus covers the entire receptive field, the cell will fire at its background rate.
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Receptive field
The region on the retina in which stimuli influence a neuron’s firing rate.
Ganglion cells are unaffected by average light intensity, and are most responsive to DIFFERENCES in light intensity.
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Hermann Grid
Why do spots appear at the junctions, and why do they disappear when a junction is fixated?
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Hermann Grid
Why do spots appear at the junctions, and why do they disappear when a junction is fixated?
Spots do not appear at the fixated junctions because receptive fields in the fovea are smaller than in more peripheral regions.
Cells with receptive fields at the junction receive more inhibition (two more minus signs), so that the junctions appear darker.
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Acuity
The smallest spatial detail that can be resolved
At some point your perception of the black and white bars turns to grey. This threshold is a result of your visual acuity.
At some point, the lines will be seen as uniform grey…
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Measuring visual acuity:
Eye doctors use distance (e.g., 20/20)
Vision scientists use the smallest visual angle of a cycle of a grating
Cycle is one black + one white stripe.
The smallest part of the pattern.
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The visual system “samples” the grating discretely.
This is just like digital cameras…
Remember the arrangement of cones at the back of the retina…
Well, each cone takes up some room, and a cycle will only be perceived if it as at least the width of 2 cones.
If cycle is the width of one cone, VS won’t be able to encode it.
CONES at the fovea have a separation of 0.008degrees of visual angle. PRETTY SMALL!
Normal Human Visual acuity is 0.017degrees. Twice the separation of cones!!
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Herman Snellen invented method for designating visual acuity in 1862
DUTCH eye doctor.
LETTER is Five times as large as the strokes that form the letter.
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So, what does it mean to be 20/20?
the distance at which you can just identify the letters
the distance at which a person with “normal” vision can just identify the letters
20/20 is not perfect but normal
20/8 is the physiological limit of human vision (based on cone density). As perfect as is possible.
20/200 while wearing corrective lens is legally blind in the US
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So, what does it mean to be 20/20?
the distance at which you can just identify the letters
OVER
the distance at which a person with “normal” vision can just identify the letters
20/20 is not perfect but normal
20/8 is the physiological limit of human vision (based on cone density). As perfect as is possible.
20/200 while wearing corrective lens is legally blind in the US
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Spatial Frequency
The number of cycles of a grating per unit of visual angle (usually specified in degrees)
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Contrast:
The difference in illumination between a figure and its background.
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Why does an oriented grating appear to be gray if you are far enough away?
This striped pattern is a “sine wave grating”
The visual system “samples” the grating discretely
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Sensitivity
A value that defines the easy with which an observer can tell the difference between either
The presence or absence of a stimulus
The difference between stimulus 1 and 2
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Just Noticeable Difference (JND)
AKA a difference THRESHOLD
The smallest detectable difference between two stimuli
The minimum change in a stimulus that enables it to be correctly judged as different
Thresholds are inversely proportional to sensitivity
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Thresholds and sensitivity are inversely related
James Bond:
Low pain sensitivity
High pain threshold
Carleton Banks:
High emotional sensitivity
Low emotional threshold
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contrast sensitivity function (CSF)
The contrast sensitivity function (CSF) is a plot of the threshold contrast to detect the grating (as opposed to seeing a uniform gray) as a function of spatial frequency.
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For photopic (daylight) vision, the CSF peaks around 2 to 4 cycles per degree.
Note how sensitivity is reduced for mesopic (twilight) or scotopic (nighttime) vision.
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Retinal cellslike spots of light.
Ganglion cells respond well to stripes!!
Which stripes?
Depends on their receptive Field!!
The smaller the RF, the higher the Freq they like!
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Not only is the spatial frequency important, but so is the phase
Phase: The phase of a grating refers to its position within a receptive field
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lateral geniculate nuclei (LGN):
Axons of retinal ganglion cells synapse there
6 layered structure. Geniculate means bent.
Bottom 2 layers have larger cells: MAGNOCELLULAR layers.
Fast, large moving objects
Top 4 are PARVOCELLULAR (small in latin).
Details of static objects
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The world is divided at the LGN:
Left side of space goes right.
Right side of space goes left.
Each layer: input from ONE eye.
Each layer = organized map of half of the visual field
TOPOGRAPHICAL MAPPING
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TOPOGRAPHICAL MAPPING
LGN, vision
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LGN is not only a “relay” between eyes and visual cortex, it also receives information from a number of other brain areas, functioning as a gate to the cortex.
PART OF THE THALAMUS.
INHIBITION DURING SLEEP.
Opening your eyes won’t make you SEE at night!
Each LGN cell responds to one eye or the other, but never to both
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Ipsilateral: Referring to the same side of the body (or brain)
Contralateral: Referring to the opposite side of the body (or brain)
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Striate cortex
Also known as primary visual cortex, area 17, or V1
Major transformation of visual information takes place in striate cortex
V1 has about 200 million cells!
Retina Recap:
100 million photoreceptors
1-1.5 million ganglion cells
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Two important features of striate cortex:
Topographical mapping:
The organization of sensory surface matches the organization of the sensory world.
Neighboring ‘stuff’ in the visual field will be processed by neighboring cells
Cortical Magnification:
The dramatic scaling of information from different parts of visual field
1 degree of visual angle at fovea is processed by 15 times more neurons than 1 degree of visual angle just 10 degrees away from fovea. Why?
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Visual crowding
One consequence of cortical magnification is that images in the periphery have much lower resolution than images at fixation
This can lead to visual crowding: the deleterious effect of clutter on peripheral object detection
Stimuli that can be seen in isolation in peripheral vision become hard to discern when other stimuli are nearby
This is a major bottleneck for visual processing
When we can’t see an object due to crowding, we have to move our eyes to look directly at it with our high acuity foveal receptive fields
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David Hubel and Torsten Wiesel
DISCOVERY:
ELONGATED RECEPTIVE FIELDS in the Striate Cortex
(Not circular spots of light).
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Selective Responsiveness
Orientation tuning: tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others
More cells are responsive to Horizontal and Vertical than to Oblique lines.
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How are the circular receptive fields in the LGN transformed into the elongated receptive fields in striate cortex?
Hubel and Wiesel: Very simple scheme to accomplish this transformation
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Simple cells
responds primarily to oriented edges and grating
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Complex cells
respond primarily to oriented edges and gratings, however it have a degree of spatial invariance
sensitive to motion. Responding regardless of location, as long as within RF.
respond to bar, regardless of exact positioning within RF
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Ocular dominance in V1
Each striate cortex cell can respond to input from both eyes with preference for one eye’s input
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End Stopping
Process by which cells in the cortex first increase their firing rate as the bar length increases to fill up its receptive field, and then decrease their firing rate as the bar is lengthened further.
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Column
A vertical arrangement of neurons
Hubel and Wiesel:
Found systematic, progressive change in preferred orientation;
all orientations were encountered in a distance of about 0.5 mm;
same orientation preference in columns perpendicular to the surface of cortex.
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Hypercolumn
A 1x1-mm block of striate cortex containing “all the machinery necessary to look after everything the striate cortex is responsible for, in a certain small part of the visual world”
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Cytochrome oxidase blobs
COLOR PROCESSING…
But big mystery!!
Regular array of “CO blobs” in systematic columnar arrangement (discovered by using cytochrome oxidase staining technique)
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Method of Adaptation
The diminishing response of a sense organ to a sustained stimulus
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Adaptation that Is Specific to Spatial Frequency
Neurons responding to different enough spatial frequencies are not affected by the fatigue.
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Tilt aftereffect:
Perceptual illusion of tilt, provided by adapting to a pattern of a given orientation
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Selective Adaptation
Evidence that human visual system contains neurons selective for specific stimulus properties (e.g. orientation, frequency, motion, color)
If adaptation to a stimulus occurs, then it must be that a group of neurons was coding that stimulus and became fatigued.
Easy non-invasive method of measuring what the human visual system is sensitive to.
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Why sine waves?
| Many stimuli can be broken down into a series of sine wave components using Fourier analysis
Any sound, including music and speech
Any complex image, including photographs, movies, objects, and scenes
Any movement, including head and limb movements
Our brains seem to analyze stimuli in terms of their sine wave components!
Vision
Audition
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Properties of sine waves
Period or wavelength:
The time or space required for one cycle of a repeating waveform
Phase:
1) In vision, the relative position of a grating
2) In hearing, the relative timing of a sine wave
Amplitude: The height of a sine wave, from peak to trough, indicating the amount of energy in the signal
Even something as complicated and artificial as a square wave can be reproduced by adding the correct sine waves together
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Joseph Fourier developed another useful tool for analyzing signals
Fourier analysis: A mathematical procedure by which any signal can be separated into component sine waves at different frequencies.
Combining these component sine waves will reproduce the original signal
Sine wave:
1. In hearing, a waveform for which variation as a function of time is a sine function.
Also called a “pure tone”
2. In vision, a pattern for which variation in a property, like brightness or color as a function of space, is a sine function.
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Low frequency: broad outlines of image
| high frequency: high detail.
How many people (zebras): low freq.
| stripe pattern? high freq.
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3 steps to color perception:
Detection: Wavelengths of light must be detected in the first place
Discrimination: We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another
Appearance: We want to assign perceived colors to lights and surfaces in the world and have those perceived colors be stable over time, regardless of different lighting conditions
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Color:
Not a physical property but rather a psychophysical property
Most of the light we see is reflected
Typical light sources: Sun, light bulb; emit a broad spectrum of wavelengths 400–700 nm
COLOR of a surface depends on the mix of wavelengths that reach the eye from that surface.
In the electromagnetic spectrum, we perceive light of a wavelength of 700 nm as red.
“There is no red in a 700 nm light, just as there is no pain in the hooves of a kicking horse.”
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Trichromacy
The light coming from an object is composed of a distribution of different wavelengths
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Reflectance Curve:
Proportion of light at different wavelengths that is reflected from a pigment.
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Scotopic
Scotopic vision (with rods only): The moonlit world
Referring to dim light levels at or below the level of bright moonlight
Moonlight and extremely dim indoor lighting are both scotopic lighting conditions
Rods are sensitive to scotopic light levels
All rods contain same type of photopigment molecule: Rhodopsin
All rods have same sensitivity to wavelength, making it impossible to discriminate light
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Photopic
Light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors
Sunlight and bright indoor lighting are both photopic lighting conditions
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Cone photoreceptors:
| 3 varieties
S-cones (420 nm): Cones that are preferentially sensitive to short wavelengths (“blue” cones)
M-cones (535 nm): Cones that are preferentially sensitive to middle wavelengths (“green” cones)
L-cones (565 nm): Cones that are preferentially sensitive to long wavelengths (“red” cones)
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The retina contains 4 types of photoreceptors:
lp[
lp
[
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Problem of univariance:
An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor
One type of photoreceptor cannot make color discriminations based on wavelength.
PLUS, it is actually an INFINITY of possible stimulations giving rise to the same response!!!!
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Trichromacy (Trichromatic Color Theory):
The theory that the color of any light is defined in our visual system by the relationships between a set of 3 numbers, the outputs of 3 receptor types now known to be the 3 cones.
(The Young-Helmholtz theory)
Thomas Young and Hermann von Helmholtz independently discovered the trichromatic nature of color perception
With 3 cone types we can tell the difference between lights of different wavelengths!
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Color space
The 3-dimensional space, established because color perception is based on the outputs of 3 cone types, that describes the set of all colors
A 3-dimensional space that describes all colors.
There are several possible color spaces.
S-response, M-response, L-response
“3 dimensional space”
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RGB color space
Defined by the outputs of long, medium, and short wavelength lights
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HSB color space
Defined by hue, saturation, and brightness
Hue: The chromatic (color) aspect of light
Saturation: The chromatic strength of a hue.
How much hue in light.
Grey = zero saturation.
Brightness: The distance from black in color space.
Physical intensity of light.
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CMYK color Space
Cyan, Magenta, Yellow, and Black.
Used by printers.
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How to get yellow
GREEN: 80 units of response from M cone, 40 units of response from L cone.
Add red:
RED: 80 units of response from L cone, 40 units of response from M cone.
Total activity in L=120, in M=120
YELLOW LIGHT: M=120 L=120.
TOTAL stimulation of M coneis equal to TOTAL stimulation of L cone
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Metamers
Different mixtures of wavelengths that look identical.
More generally, any pair of stimuli that are perceived as identical in spite of physical differences
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Additive color mixture
A mixture of lights.
If light A and light B are both reflected from a surface to the eye, in the perception of color, the effects of those two lights add together.
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Subtractive color mixture
A mixture of pigments.
If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B.
Only the remainder contributes to the perception of color.
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Trichromacy:
| Two warnings:
Mixing wavelengths does not change the physical wavelengths!
ADDING a wavelength of 500 to one of 600 does NOT Create a wavelength of 550!!
Nor the sum 1100.
It produces a change in our PSYCHOPHYSICAL REALITY not in the PHYSICS of the light.
In order for a mixture of a red light and a green light to look perfectly yellow, you have to add just the right amount of red and just the right amount of green
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The visual system begins by picking up light from the environment
- visible light has a wavelength in the hundreds of nanometers
- respond to only a narrow range of light wavelengths
- when light reaches an object part of the light is reflected while part is absorbed
- our perception of brightness is based on the intensity of the reflected light that hits our eye
- ex. Completely white objects reflect all light while black absorb all light
- color of light is called hue
- we are attuned to 3 primary colors of light: red, green, and blue
- the mixing of these 3 colors through additive color mixing can produce any color
- differs from subtractive color mixing such as that with paint or ink
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Filters are subtractive because they absorb light
Different filters absorb different wavelengths
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Pigments
Substances that absorb light at some wave lengths and reflect light at others.
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Reflectance Curve:
Proportion of light at different wavelengths that is reflected from a pigment.
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If we shine “blue” and “yellow” lights on the same patch of paper, the wavelengths will add, producing an additive color mixture.
....
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Color Mixing with Pigments
When a pattern of non-overlapping blue and yellow pigments is blurred, the resultant mixture is additive (gray) as opposed to subtractive (green).
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Pointillism: Additive Color Mixing!
Pointillism:
style of painting developed by the neo-impressionist Georges Seurat in which additive color mixtures are achieved by visually by placing dots of different colors in close proximity to each other, rather than the subtractive mixtures that are obtained when pigments are mixed together in the same location.
Pop culture
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Opponent color theory
The theory that perception of color is based on the output of 3 mechanisms, each of them based on an opponency between 2 colors:
Red–green, blue–yellow, black–white
Some LGN cells are excited by L-cone onset in center, inhibited by M-cone onsets in their surround (and vice-versa).
Red versus green
Other cells are excited by S-cone onset in center, inhibited by (L + M)-cone onsets in their surround (and vice-versa)
Blue versus yellow
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Ewald Hering (1834–1918) noticed that some color combinations are legal while others are illegal
We can have bluish green, reddish yellow (orange), or bluish red (purple)
We cannot have reddish green or bluish yellow
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We can use the hue cancellation paradigm to determine the wavelengths of unique hues
Unique hue: Any of four colors that can be described with only a single color term: Red, yellow, green, blue
For instance, unique blue is a blue that has no red or green tint
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The 3 steps of color perception, revisited
Step 1: Detection.
S, M, and L cones detect light
Step 2: Discrimination.
Cone opponent mechanisms discriminate wavelengths.
[L – M] and [M – L] compute red vs. green.
[L + M] – S and S – [L + M] compute blue vs. yellow.
Step 3: Appearance.
Further recombination of the signals creates final color-opponent appearance
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Color in the Visual Cortex
Some cells in LGN are cone-opponent cells.
These respond to RED-center/GREEN-surround and vice-versa.
In primary visual cortex, double-opponent color cells are found for the first time.
These are more complicated, combining the properties of two color opponent cells from LGN
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Yellow =
| L+M response contrasted against S response
....
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Afterimages
A visual image seen after a stimulus has been removed.
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Negative afterimage
An afterimage whose polarity is the opposite of the original stimulus
Light stimuli produce dark negative afterimages
Colors are complementary.
Red produces green afterimages; blue produces yellow afterimages (and vice-versa)
This is a way to see opponent colors in action
There is no way of explaining these after-images with Trichromacy ALONE.
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Does everyone see colors the same way?
— Mostly Yes
General agreement on colors
Some variation due to age (lens turns yellow)
— No
About 8% of male population, 0.5% of female population has some form of color vision deficiency (Color blindness)
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Protanope
No L-cones
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Deuteranope
No M-cones
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Tritanope
No S-cones
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Color Blindness is sex linked
The genes that produce photopigments are carried on the X chromosome
If some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome
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Achromatopsia
An inability to perceive colors that is due to damage to the CNS
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Tetrachromacy
the condition of possessing 4 different types of cone cells.
Some Human Females have a normal cone gene on one X chromosome and a mutated cone gene on the other X chromosome.
One study suggested that 2–3% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving them a significant increase in color differentiation.
This finding is still debated.
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An unrelated color is experienced in isolation or in front of a neutral background.
like a orange square on top of a totally black back-drop
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A related color is seen only in relation to other colors.
E.g.: brown, gray – perception depends on surrounding colors.
Like two different orange colors paired.
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Illuminant
The light that illuminates a surface
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Color constancy:
The tendency of a surface to appear the same color under a fairly wide range of illuminants
To achieve color constancy, we must discount the illuminant and determine what the true color of a surface is regardless of how it appears
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Perception of color
We often base perception of color on surrounding context.
Physical constraints make constancy possible:
Intelligent guesses about the illuminant
Assumptions about light sources
Assumptions about surfaces
Disney uses pink pavement to make plants look greener!
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Disney uses pink pavement to make plants look greener!
Color in the periphery is an Illusion!
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Receptive Fields
Rods and Cones = Small Spots
Retinal ganglion cells and LGN = Larger Spots
Primary visual cortex = Bars/Edges etc…
Beyond V1 = ????
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Ambiguous figure:
A visual stimulus that gives rise to two or more interpretations of its identity or structure.
In a way, all images are inherently ambiguous.
Any 2D image can have an infinite number of 3D interpretations.
An “ambiguous figure” is usually obviously ambiguous.
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Can we adapt to more complex stimuli?
Like Dalmatians?
no
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Middle vision
A loosely defined stage of visual processing that comes after basic features have been extracted from the image and before object recognition and scene understanding
Involves the perception of edges and surfaces
Determines which regions of an image should be grouped together into objects
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High level vision
Loosely defined stage that comes involves complex image analysis including 3D vision, object recognition, scene understanding and more.
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Extrastriate cortex
Region of cortex bordering the primary visual cortex and containing multiple areas involved in visual processing
V2, V3, V4, etc.
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After extrastriate cortex, processing of object information is split into “what” pathway and “where” pathways of High Level Vision
“Where” pathway is concerned with locations of objects
“What” pathway is concerned with names and functions of objects
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“What” system
object identification
Inferior temporal (IT) cortex
ventral pathway
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“Where/how” system
object localization/manipulation
Parietal cortex
dorsal pathway
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The same visual input occurs in both (a) and (b) and a V1 neuron would respond equally to both. A V2 neuron might respond more to (a) than (b) because the black edge is owned by the square in (a) but not in (b)
The receptive fields for cells in extrastriate areas are more sophisticated than those in striate cortex
They respond to visual properties important for perceiving objects
For instance, “boundary ownership.”
For a given boundary, which side is part of the object and which side is part of the background?
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Structuralism
A school of thought believing that complex objects or perceptions could be understood by the analysis of the components.
Developed by Wilhelm Wundt and mentee Edward Titchener.
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Edges are important, they give us: LINES,
But how do we know which lines go together? Which belong to the same object and which do not?
We see edges that computer algorithms can’t.
Kanisza triangle
other illusory contours
These are “imaginary” edges.
Nothing is there.
But
Sometimes people actually report seeing different shades of white inside and outside of the house
Brain responds differently to both types of white surface!
This image illustrates the point that the visual system knows about physics!! Objects tend to be opaque and obstruct the view of other objects behind them.
Monkeys also see this as a house…
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Gestalt Psychology
“The whole is greater than the sum of its parts”
Gestalt: In German, “form” or “whole”
Gestalt psychology opposed to other schools of thought, such as structuralism.
Gestalt grouping rules:
A set of rules that describe when elements in an image will appear to group together.
Patterns are spontaneously organized by the brain into the simplest possible configurations.
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Good continuation
A Gestalt grouping rule stating that two elements will tend to group together if they lie on the same contour.
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When should we complete edges behind occluders?
When the edges are relatable by an “elbow curve”
S curves = NOT LIKELY THE SAME EDGE
Elbow curve = likely the same edge.
Also, the steeper the curve, the less likely it is the same edge. (PARAMETRIC RULE)
HEURISTIC = mental shortcut (not always right!).
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Parallelism
Parallel contours are likely to belong to the same figure
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Symmetry
Symmetrical regions are more likely to be seen as figure
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Common region
Two features will group if they appear to be part of the same larger region
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Connectedness
Two items will tend to group if they are connected
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Common fate
Elements that move in the same direction tend to group together
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Synchrony
Elements that change at the same time tend to group together
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Camouflage
An organism’s attempt at breaking Gestalt rules so that its features are not perceived as an object on their own, but as parts of a larger object.
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Figure–ground assignment
The process of determining that some regions of an image belong to a foreground object (figure) and other regions are part of the background (ground)
Gestalt figure–ground assignment principles: surroundedness, size, symmetry, parallelism
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Gestalt figure–ground assignment principles:
Surroundedness: The surrounding region is likely to be ground
Size: The smaller region is likely to be figure
Symmetry: A symmetrical region tends to be seen as figure
Parallelism: Regions with parallel contours tend to be seen as figure
Ambiguous figures are created when these principles compete
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Global superiority effect
“Forest before the trees”
The properties of the whole object take precedence over the properties of parts of the object
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Why do gestalt principles work?
The Gestalt grouping processes capitalize on certain regularities that characterize the physical world.
In other words, the grouping principles have ecological validity.
Awkwardly worded summary provided by book
Bring together what should be brought together
Split asunder what should be split asunder.
Use what you know
Avoid accidents
Seek consensus and avoid ambiguity.
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WHAT do you see in this image?
At the level of the retina, you “see” an array of point-lights bouncing off the page and exciting your rods & cones.
In early visual brain areas, you “see” a collection of oriented lines and a collage of red, green, yellow, and blue color patches.
But your response to this question was almost certainly not “light” or “lines” or “colors”; what we all perceive in this scene are “toys.”
The ability to organize visual sensations into coherent objects and then assign meaningful category labels to these objects is in many ways the ultimate accomplishment of vision.
These organizational and recognition processes are the subject of this chapter.
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Processes in object recognition:
1. Determine features present in image
(“Low-level vision”)
2. Group features into objects
(“Middle vision”)
3. Match perceived to encoded representations
(“High-level vision”)
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Naïve template theory
The proposal that the visual system recognizes objects by matching the neural representation of the image with a stored representation of the same “shape” in the brain.
That is, maintain a memory of many different views for each object we need to recognize.
“Pandemonium” Oliver Selfridge (1959)
Problem: You would need too many templates!
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Structural description theory
A description of an object in terms of the nature of its constituent parts and the relationships between those parts.
I.E., exploit those properties that can distinguish most objects from one another, yet remain relatively stable over changes in view.
When asked to describe a novel object, observers typically do so by identifying different parts.
“Generalized Cones” David Marr (1977)
“Recognition-by-Components” Biederman (1987)
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Object recognition by components
Geons
Objects are defined as configurations of qualitatively distinct parts called Geons.
Geons are defined by configurations of non-accidental properties.
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Geons are distinguished by their non-accidental properties
the number of straight and curved edges
which edges are parallel to one another
the number of vertices of each type
the presence of symmetries
Each type of geon is defined by a particular configuration of non-accidental properties.
Each type of object is defined by a particular configuration of geons.
Deletion of contours in an image should have the greatest effect on recognition performance if it masks non-accidental properties or geons.
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Deletion of contours in an image should have the greatest effect on recognition performance if it masks non-accidental properties or geons.
Task: Subjects are presented with an intact or contour deleted object, and they are asked to name it as quickly as possible.
Recognition performance is more severely impaired by vertex deletion than by midsection deletion.
Task: Subjects are presented with an intact or contour deleted object, and they are asked to name it as quickly as possible.
Recognition performance is more severely impaired by geon deletion than by midsection deletion.
Some evidence suggests that object recognition is only possible for viewpoints that are close to those that were observed during training, the opposite of what recognition by components predicts.
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Problems with structural-description theories
Object recognition is not completely viewpoint-invariant
Geons aren’t always the best descriptions of objects
Observers show some viewpoint effects in object recognition
The farther an object is rotated away from a learned view, the longer it takes to recognize
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The world is 3D and follows the rules of
| Euclidian geometry:
Parallel lines remain parallel as they are extended in space
Objects maintain the same size & shape as they move around in space
Internal angles of a triangle always add up to 180 degrees, etc.
Euclidean geometry is kind of just the fancy term for the geometry you learned in high school
Notice that images projected onto the retina are non-Euclidean!
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Projective geometry
investigates the mathematical relationships between objects in the environment and their optical projections on the retina or on a picture.
Euclidean geometry of the 3-dimensional world turns into something quite different on the curved, 2-dimensional retina
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The optical projections of objects are inherently ambiguous:
For example, all of the black lines shown below would produce exactly the same image on the observer’s retina.
One of the great mysteries of perception is how the visual system is able to resolve this ambiguity to accurately perceive the 3D structure of the environment.
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The Solutions
Depth cue: Information about the third dimension (depth) of visual space.
Monocular depth cue:
A depth cue or perceptual bias that is available even when the world is viewed with one eye alone.
Pictorial depth cue:
A cue to distance or depth used by artists to depict 3-dimensional depth in 2-dimensional pictures. (Basically a monocular cue in a picture.)
Binocular depth cue:
A depth cue that relies on information from both eyes. Primarily stereopsis in humans.
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Depth cue:
Information about the third dimension (depth) of visual space.
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Monocular depth cue
A depth cue or perceptual bias that is available even when the world is viewed with one eye alone.
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Pictorial depth cue
A cue to distance or depth used by artists to depict 3-dimensional depth in 2-dimensional pictures.
Basically a monocular cue in a picture.
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Binocular depth cue
A depth cue that relies on information from both eyes.
Primarily stereopsis in humans.
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The perception of 3D shape from texture (gradients)
In the 60s there was a art style known as
Optical Art emerged that make use of optical illusions.
Here are two pieces by Bridget Riley that depict surfaces with contour textures.
As you can see, these 2D images give rise to 3D perceptions.
The goal of my Master’s research was to model how the visual system interprets these images.
(Cartography, mechanical drawings, & computer graphics)
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on planar cut contours.
Imagine taking an object and slicing it up with a knife.
Those cut lines create planar cut contours.
Now lets take the simplest case where these cut lines are getting farther away from you.
When this is the case we can easily determine the relative distance between any two points by just counting the contours between them.
For example this point is four contours from this point. All we have to do is add a scaling parameter and we know the distance in depth z.
The problem with this simple model is that contour textures are rarely oriented to that we are looking directly at them.
We therefore improved our model to take into account the orientation of the planar cut contours.
It is very important that we take the orientation of the contours into account because it has a dramatic effect on our perceptions.
These two images are of the same surface, the only thing that has changed is the orientation of the planar cut contours.
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Psychophysics:
| The Depth Profile Task
Ok so how to we measure peoples perceptions?
We use a depth profile task where we ask people to recreate the depth they see by moving these dots up and down.
Data with more complex object.
These red lines denote where we asked people to judge depth.
Down here, these black lines are show what the actual shape of the object is.
The red dots are peoples judgments
And the red lines are the model fits.
People judgments are most accurate when the planar cuts are not slanted.
When the planar cuts are slanted to the side, this effects peoples judgments of the horizontal lines
When the planar cuts are slanted down, this effects peoples vertical judgments
And the model is able to capture both of these distortions.
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Ground Plane Bias
There is a strong bias to interpret scenes such that depth increases with height in the viewing plane. (Floor not ceiling)
Both images are identical, but one has been rotated 180 degrees.
There is no right-wall or left-wall bias so these images are bi-stable.
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Shape from shading:
The visual system can easily determine 3D shape despite variations in illumination and material.
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Convexity Bias:
Convex interpretations are preferred over concave ones.
Note how the concave regions can appear perceptually as convex, even when there is potential information from the cast shadow to specify the correct sign of relief.
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Depth of field
A monocular depth cue that is determined by the amount of accommodation required to focus an image.
When focusing on a distance object, our depth of field is large (or our plane of focus is thick)
When focusing on a close object, our depth of field is small (or our plane of focus is thin)
Special “tilt-shift” camera lens can be used to adjust depth of field to make large distant objects look like small close miniature models.
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Non-accidental Properties
Non-accidental Properties are properties of an image such as co-linearity, co-termination or parallelism that seldom occur by accident within optical projections.
Thus, if lines in an image are parallel (or co-terminate), they will be interpreted perceptually as if they are parallel (or co-terminating) in the 3D environment.
(Also used to define Geons)
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Accidental Alignments –
created when properties of an image occur by accident
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Forced perspective
is a technique that employs optical illusion to make an object appear farther away, closer, larger or smaller than it actually is. It is used primarily in photography, filmmaking and architecture. It manipulates human visual perception through the use of scaled objects and the correlation between them and the vantage point of the spectator or camera.
Forced perspective involves viewing a scene from an accidental viewpoint
These images (kissing sphinx, holding up leaning tower) both contain accidental co-terminations.
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An Impossible Figure
This is a photograph of a real object, but its apparent shape is geometrically impossible.
This illusion is created by having an accidental co-termination of the object’s edges.
In this version of the impossible triangle there is an accidental viewpoint of the sculpture so that the curves are interpreted as straight.
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Forced Perspective
Cinderella’s Castle: as it becomes taller, its proportions get smaller.
For example, using this method, the top spire of the castle is actually close to half of its apparent size.
Major elements of the castle were scaled and angled to give the illusion of distance and height, a method frequently used in Disney theme parks around the world.
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Accidental properties in art
anamorphosis
An anamorphosis is a distorted projection or perspective; especially an image distorted in such a way that it becomes visible only when viewed in a special manner.
"Ana - morphosis" are Greek words meaning "formed again."
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Motion parallax
Images closer to the observer move faster across the visual field than images farther away
The brain uses this information to calculate the distances of objects in the environment
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This photo “pops” with help
| from other monocular cues
This photo looks “flat” due to its lack of monocular cues, motion and stereo vision.
In real life both scenes would “pop” thanks to motion parallax and stereo.
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Accommodation
The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects)
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Convergence
The ability of the two eyes to turn inward, often used to focus on nearer objects
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Divergence
The ability of the two eyes to turn outward, often used to focus on farther objects
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In principle, the distance of an object could be determined by the state of accommodation or convergence, but human observers are not very sensitive to this information
...
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Binocular depth cues
The two 2D retinal images of a 3-dimensional world are not the same!
Binocular depth cue:
A depth cue that relies on information from both eyes.
Primarily stereopsis in humans.
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Binocular summation:
The combination (or “summation”) of signals from each eye in ways that make performance on many tasks better with both eyes than with either eye alone.
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Binocular disparity:
The differences between the two retinal images of the same scene.
The basis for stereopsis.
Stereopsis:
The ability to use binocular disparity as a depth cue.
A vivid perception of the 3-dimensionality of the world that is not available with monocular vision.
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Corresponding retinal points
A geometric concept stating that points on the retina of each eye where the monocular retinal images of a single object are formed are at the same distance from the fovea in each eye
This simple visual scene illustrates how geometric regularities are exploited by the visual system to achieve stereopsis from binocular disparity
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Horopter
The location of objects whose images lie on the corresponding points. The surface of zero disparity
The Vieth–Müller circle and the horopter are technically different, but for our purposes you may consider them the same
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Panum’s fusional area
The region of space, in front of and behind the horopter, within which binocular single vision is possible.
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Binocular Vision and Stereopsis
Objects on the horopter are seen as single images when viewed with both eyes.
Panum’s fusional area: The region of space, in front of and behind the horopter, within which binocular single vision is possible.
Objects closer or farther away from the horopter fall on non-corresponding points in the two eyes and are seen as two images
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Diplopia
Double vision.
If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic
Amount of diplopia of an abject determines the distance from the horopter.
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The bigger the disparity, the farther away from the horopter of the object is.
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Crossed disparity
The sign of disparity created by objects in front of the plane of the horopter
Images in front of the horopter are displaced to the left in the right eye and to the right in the left eye
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Uncrossed disparity
The sign of disparity created by objects behind the plane of the horopter
Images behind the horopter are displaced to the right in the right eye and to the left in the left eye
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How is stereopsis implemented in the human brain?
Input from two eyes must converge onto the same cell
Many binocular neurons respond best when the retinal images are on corresponding points in the two retinas:
Neural basis for the horopter
However, many other binocular neurons respond best when similar images occupy slightly different positions on the retinas of the two eyes (tuned to particular binocular disparity)
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Stereoscope
A device for presenting one image to one eye and another image to the other eye
Stereoscopes were a popular item in the 1900s
Many children in modern days had a ViewMaster, which is also a stereoscope.
Dub's toy
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3D movies were popular in the 1950s and 60s and have made a resurgence in the late 80s and again in recent years
For movies to appear 3D, each eye must receive a slightly different view of the scene (just like in real life)
Early methods for seeing movies in 3D involved “anaglyphic” glasses with a red lens on one eye and a blue lens on the other
Current methods use polarized light and polarizing glasses to ensure that each eye sees a slightly different image among other techniques.
Movie theaters now use polarized glasses to show movies in stereo
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Anaglyph Stereograms
the 3D glasses with a red lens on one eye and a blue lens on the other
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Random dot stereogram (RDS):
A stereogram made of a large number of randomly placed dots
RDSs contain no monocular cues to depth
Stimuli visible stereoscopically in RDSs are cyclopean stimuli
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Cyclopean
Referring to stimuli that are defined by binocular disparity alone
We live in a Cyclopean World. Even though we have two eyes, we only perceive one world that is the combination of two.
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Light waves involve oscillations in electric and magnetic fields
Polarized lenses only pass light whose oscillations are oriented in a particular direction
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Active Shutter 3D Glasses for 3D TVs
| require an infrared transmitter
..
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Free fusion:
The technique of converging (crossing) or diverging (uncrossing) the eyes in order to view a stereogram without a stereoscope
“Magic Eye” pictures rely on free fusion
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Stereoblindness
An inability to make use of binocular disparity as a depth cue
About 5% of the population
Can result from a childhood visual disorder, such as strabismus, in which the two eyes are misaligned
Most people who are stereoblind do not even realize it
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Binocular rivalry
The competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes
If blue vertical bars are shown to one eye while orange horizontal bars are shown to the other, the two stimuli will battle for dominance
