Visual perception Flashcards
(235 cards)
1
Q
What are the stages of visual perception?
A
1) Environmental stimulus
2) Light transformation
- 1st transformation and representation
3) Receptor processing
- 2nd transformation and transduction
4) Neural processing
- Retino-geniculo-striate pathway
5) Perception
- Knowledge
6) Recognition
- Knowledge
7) Action
2
Q
What happens during light transformation? (2nd stage of visual perception)
A
1st transformation and representation
- Light waves are converted into retinal object representations:
- Light is focused on lens and reflected onto the retina
- Inverted object representation on retina
- Accommodation
3
Q
Why is accommodation needed?
A
- Need to accommodate eye to perceive near and far objects
- As there is a fixed distance between lens and retina, lens needs to change thickness to accommodate to near and far objects (using ciliary muscles)
- Lens changes shape so that focus point is always the fovea
4
Q
How does anatomy change during accommodation to a far object?
A
- Ciliary muscles are relaxed
- Suspensory ligaments are pulled tight
- Lens is thin
- Little curvature, little focusing power - light is only bent a little
5
Q
How does anatomy change during accommodation to a near object?
A
- Contracted ciliary muscles
- Slack suspensory ligaments
- Thick lens
- Strong curvature, strong focusing power - light is bent a lot
6
Q
Define accommodation
A
The process by which the eye changes optical power to focus on an object as its distance varies
7
Q
What are far and near points in accommodation?
A
Far point - the maximum distance of an object from the eye for which a clear image of the object can be seen
Near point - the minimum distance of an object from the eye for which a clear image of the object can be seen
(Accommodation limit increases with age)
8
Q
What happens during receptor processing? (3rd stage of visual perception)
A
2nd transformation and transduction
- Transduction - retinal object representation is turned into an electrical signal
- Light travels through all layers of eye wall, is then reflected on pigment epithelium
- Light sensitive chemicals within photoreceptors’ outer segments change when hit by light
- The chemical reaction sparks an action potential which travels to bipolar then ganglion cells
9
Q
What types of vision are rods and cones involved in?
A
Rods - scotopic vision
Cones - photopic vision
Both - mesopic vision
10
Q
What are the absolute numbers of rods and cones?
A
Rods = 120 mil
Cones = 6 mil
11
Q
What are the densities and distribution in retina of rods and cones?
A
Rods - none in fovea, mainly in peripheral retina
Cones - most in fovea
12
Q
What are the abilities for dark adaptation of rods and cones?
A
Rods - slowly adapt, but adapt more thoroughly (completely adapt after 20 mins)
Cones - quickly adapt, but plateaus after 8 mins
13
Q
What are the absolute sensitivities of rods and cones?
A
Cones - lower
Rods - higher
14
Q
What is the acuity of rods and cones?
A
Rods - worse as information about different light sources combines - the same ganglion cell receives input from multiple rods
Cones - better - detailed info about different light sources - separate ganglion cells receive input from separate cones
15
Q
What is the spectral sensitivity of rods and cones?
A
Rods - more sensitive to shorter wavelengths - max 500nm
Cones - more sensitive to longer wavelengths - max 560nm
16
Q
What is the colour vision of rods and cones?
A
Rods - dark adapted vision, operates at low luminance, no colour sensation
Cones - light adapted vision, operates at high luminance
S cones - respond to short wavelengths, blue
M cones - respond to medium wavelengths, green
L cones - respond to long wavelengths, red
17
Q
What is the rod-cone break?
A
In adapting to low light - cones stop adapting but rods continue
18
Q
What is neural convergence?
A
Many neurons synapse onto fewer neurons
- Multiple rod cells per ganglion cell
- One cone cell per ganglion cell
- As there are 1 mil ganglion cells, but 6 mil cones and 120 mil rods
- Group of rod cells create a bigger signal than a single cone cell
19
Q
What is the Purkinje shift in spectral sensitivity?
A
Increased sensitivity to short wavelengths in dark-adapted eye
20
Q
What is the diameter of the eye?
A
2.5cm
21
Q
What is the retina?
A
Light sensitive surface on 75% of the inner eye
22
Q
What is the sclera?
A
Outer layer of the eye; tough fibrous coat
23
Q
What are the two key retinal landmarks?
A
1) Fovea - (macula) central vision, highest acuity, most detailed vision (most cones)
2) Optic disc - blind spot, usually unaware because blind spot of one eye corresponds to ‘seeing’ retina in the other eye
24
Q
What parts of the eye are involved in the optical system for focusing?
A
Iris, pupil, cornea and ciliary muscles
25
What is macular degeneration?
Deterioration of the retina in the macula (fovea)
Irreversible blindness, loss of central vision
26
What is the electromagnetic energy of different light wavelengths?
Short wavelength - high electromagnetic energy
Long wavelength - low electromagnetic energy
Spectrum of visible light = 400-700nm
27
What is Myopia? (accommodation problem)
Nearsightedness
- Far objects out of focus, because lens is too thick or eyeball is too long
- Requires concave correction lenses
28
What is Hyperopia? (accommodation problem)
- Near objects are out of focus, usually because the eyeball is too short
- Requires convex correction lenses
29
What is the ganglion cell input during transduction? (Neural convergence)
- Neural convergence - each ganglion cell receives input from 126 photoreceptors (fovea = 1:1 , periphery = 1:100)
30
What is the receptive field?
The part of the visual field in which a stimulus can modify the neuron's firing rate
- Input in receptive field either excites or inhibits a ganglion cell - depending on where in a ganglion's receptive field the stimulus is located
31
What is the structure and function of on-center ganglion cells?
On area in the center - if stimulus falls here = excitation
Off area in the surround - if stimulus falls here = inhibition
32
What is the structure and function of off-center ganglion cells?
Off area in center - if stimulus falls here = inhibition
On area in surround - if stimulus falls here = excitation
33
How do excitatory and inhibitory areas affect neuron firing rates?
Excitatory area (+) - on area - increases the firing rate of a neuron when stimulated
Inhibitory area (-) - off area - decreases the firing rate of a neuron when stimulated
34
How does uniform illumination affect an on/off cell?
Both excitation and inhibition in center and surround = activation cancels out
35
What is the function of on/off cells?
To specialise our vision to edges of objects, since a luminance discontinuity (edge of object) causes activation
36
What are the three main types of ganglion cells?
1) Magnocellular (M)
2) Parvocellular (P)
3) Koniocellular (K)
37
Magnocellular ganglion cells - input and colour specificity?
- Receive input from mostly rods
- Not colour specific
38
Parvocellular ganglion cells - input and colour specificity?
- Input from M or L cones
- Colour specific (green or red on/off)
39
Koniocellular ganglion cells - input and colour specificity?
- Excitatory input from S cones
- Inhibitory input from M and L cones
- Blue = on
40
Where do the axons of M,P and K cells go to?
The optic nerve
41
What areas are involved in the retino-geniculo-striate pathway?
Retina
Lateral geniculate nucleus
Striate visual cortex
42
How is lateralisation in the optic chiasm structured?
- Nasal axons cross over to the other side of the brain
- Temporal axons stay on the same side
- Visual fields now represented in contralateral hemispheres (LVF in right hem, RVF in left hem)
Temporal representation of RVF in left eye, stays in left hemisphere, LVF in right eye, stays in right hemisphere
Nasal representation of RVF in right eye, crosses to left hemisphere at optic chiasm, LVF in left eye, crosses to right hemisphere at optic chiasm
43
Axons of which neurons are in the six layers of the lateral geniculate nucleus?
Layers 1 and 2: Magnocellular neurons
Layers 3, 4, 5 and 6: Parvocellular neurons
Koniocellular neurons interlayered
44
Through which structures does visual information travel during neural processing?
Starts at retina
Optic nerve
Optic chiasm (nasal representations cross, temporal representations stay)
Optic tract
Lateral geniculate nucleus/ lateral geniculate body (thalamus) (at the midbrain)
Optic radiations
Visual cortex
45
What are the striate and extrastriate visual areas?
Striate visual cortex = V1 (visual area one) = Primary visual cortex (stripy appearance)
Extrastriate visual areas:
V2, V3, V4, V5 (MT = middle temporal area), IT = inferotemporal cortex
46
What happens to neural convergence and neuron receptive fields as visual information travels through the cortex?
- As visual information travels through the cortex, there is ongoing neural convergence:
- The neuron's receptive fields increase
Visual info gets more complex with ongoing integration
47
Some V1 neurons are orientation selective - what does this mean?
They have elongated receptive fields to capture edges in a particular orientation
They respond when a stimulus in their receptive field matches their preferred orientation (Hubel and Wiesel, 1959)
The less the stimulus matches the preferred orientation, the lower the neuron's firing rate
- Receptive field not stimulated, no response
- Receptive field stimulated with preferred orientation, response
- Receptive field stimulated with non-preferred orientation, no response
48
What did Hubel and Wiesel (1959) find in their experiments with a sleeping cat?
V1 single cell recording
Inserted electrode into single neuron in V1 - wrong orientation - no neuronal response
Right orientation - neuronal response
49
What else are V1 neurons selective for other than orientation?
Other V1 neurons are motion direction selective - they respond when a stimulus in their receptive field matches their preferred motion direction
Other V1 neurons are selective for colour and brightness (circular receptive fields)
50
How big are V2 receptive fields compared to V1?
Twice as big
51
What stimulus features do V2 neurons respond to?
Same as V1 = orientation, motion direction, brightness
More complex features = length, angles, arcs, shapes, texture
52
Which areas are involved in the temporal (ventral) stream?
V1, V2, V4, IT (inferotemporal lobe)
53
Which areas are involved in the parietal (dorsal) stream?
V1, V2, V3, MT (middle temporal area)
54
What do the parietal and temporal streams differentially process?
Parietal (dorsal) stream processes object locations = where
Temporal (ventral) stream processes object identities = what
55
How did Ungerleider and Mishkin (1982) find evidence for two separate streams? (Monkeys)
Monkeys with parietal (‘where’) lesions could distinguish a cube from a triangle (object discrimination), while monkeys with temporal (‘what’) lesions could not.
Monkeys with temporal (‘what’) lesions could learn the position of an object (location discrimination), while monkeys with parietal (‘where’) lesions could not.
Perfect double dissociation - damage of brain areas leads to opposite pattern of impairments - shows those brain areas have separable functions.
56
What are the receptive fields and functions of V3 and MT in the parietal/dorsal (where) stream?
Motion perception:
- Receptive fields are 5 times larger in V3 and 8 times larger in MT than in V1
- Integrated information over a large area of the retina
57
What are the receptive fields and functions of V4 in the temporal/ventral (what) stream?
- Receptive fields are 5 times larger in V4 than in V1
- V4 neurons respond to object-defining features such as colour, orientation, complex shapes, texture
58
What are criticisms of seperate parietal/dorsal and temporal/ventral streams?
- How useful is the separation between two visual processing streams?
- The two streams seem to interact heavily
- e.g. reaching and grasping is more precise towards familiar objects
- Directional movements controlled by dorsal stream are influenced by object recognition in the ventral stream
59
What is sensation?
The un-interpreted sensory impressions created by the detection of a stimulus
60
What is perception?
The psychological and cognitive processes of making sense of sensations
61
What is structuralism? (Late 19th century - father and method?)
- The study of the elements of consciousness: conscious experience (perception) can be broken down into basic elements (sensory elements), which can then be combined again to describe any human experience
- Father of Structuralism: Wilhelm Wundt, 1879, started the first psychological laboratory in Leipzig, Germany
- Method: introspection
- Goal: describe elements of perception (sum of sensory events)
62
What evaluation is there of structuralism?
:) - First 'school of thought' in psychology
:( - Validity - no measures, purely descriptive
:( - Reliability - observations are not consistent and constant
:( - Objectivity - observations depend on observer - biased
63
What is psychophysics? (Gustav Fechner, 1860)
Measuring rather than describing (structuralism) elements of perception
Goal - objective measure of perceptual elements
Basic idea - measuring the absolute perception threshold (just noticeable intensity to detect a stimulus)
64
What is the method of adjustment in psychophysics?
Participants adjust the intensity of a light, until they are just able to perceive the light.
e.g. Dark adaptation experiment - The threshold of detecting light stimulus differs based on how adapted participants have become to dark conditions
65
What is the method of limits in psychophysics?
Participants are presented with trials of increasing/decreasing light intensity and asked if they can perceive the light.
Trials with ascending light intensity - point at which they start to perceive light
Trials with descending light intensity - point at which they stop perceiving light
Both ascending and descending are done in many trials and mean perception threshold is calculated from each absolute perception threshold
Stimulus is either detected 100% or 0%
66
What is the method of constant stimuli in psychophysics?
Like the method of limits (same task), but:
- Many more trials per light intensity
- Randomised light intensity across trials - no stimulus order (eliminates confounding variable)
Percentage of yes answers to perceiving stimulus is plotted as a function of stimulus intensity
Results in psychometric function - a fine grained threshold: probability with which a stimulus of certain intensity is detected
67
What is psychophysics? (Ernst Weber, 1834)
Basic idea - measuring the difference threshold (minimum intensity difference to discriminate two stimuli)
68
What is Weber's law? (psychophysics)
The change in a stimulus to be able to discriminate it from another stimulus, is a constant ratio of that original stimulus
i.e. the brighter light 1 is, the greater the required brightness change for light 2 to be perceived as different
Required brightness change has a constant ratio
69
What are examples of Weber's law in the real world? (Coffee, pub)
Can you tell the difference between coffee with 1 or 9 cubes of sugar and coffee with 8 or 9 cubes of sugar?
When in a pub with many people, you have to shout to be heard but in a library you only need to whisper
70
What is psychophysics? (Stanley Stevens, 1957) (Response expansion and compression)
Basic idea - subjective magnitude estimation (measuring the relation between stimulus intensity (objective measure) and perceived intensity (subjective experience)
- An increase in the perceived stimulus intensity can be larger (response expansion) or smaller (response compression) than the increase in the measured stimulus intensity
71
What is Steven's power function? (Psychophysics)
P = KS (to the power of) n
P = perceived intensity
K = constant (scales values to size)
S = stimulus intensity
n = exponent
72
What is the subjective magnitude estimation of brightness and pain?
Brightness - big difference but we don't perceive it to be that different
Pain from electric shock - small change produces much bigger subjective pain response
73
What evaluation is there of psychophysics?
:) - Valid, reliable, and objective measures (hard data)
:( - No theoretical account of perception (unclear how we perceive)
74
What is James Gibson's (1966) Ecological theory of perception?
Perception is direct (perception = sensation)
'The real world is rich in sensory information and provides sufficient context for the visual systems to directly perceive what is there'
- Perception must be investigated in a natural environment as perception is directly offered by the world (sensation)
- Goal - explain how we attach meaning to sensory input
- Basic idea - perception takes place in the optic array (light in the environment) and is directly based on invariant information in the visual field, which is extracted by the observer's movement
- i.e. as an observer moves, the optic array becomes ambient (the light surrounding the observer changes), but some information remains constant (invariant)
75
What is optic flow pattern? (Invariant visual information) (Driving example)
The focus point of a driver remains motionless while the rest of the visual field appears to move away from this point
76
What is texture gradient? (Invariant visual information)
The elements of a textured ground are denser in the distance
77
What is horizon ratio? (Invariant visual information)
The proportion of the object above and below the horizon is constant for objects of the same size standing on the same ground
78
What evaluation is there of Gibson's ecological theory of perception?
:) - Theoretical account - tries to explain how perception and object recognition work
:( - Assumes that perception, offered by the real world, is fast, readily available and effortless, and always accurate
79
What type of accounts of perception are structuralism, psychophysics and ecological theory?
Bottom-up accounts
80
What is affordance of objects?
When we combine our physical psychological states with our constantly changing arrays, we are enabled to recognise not only what it is but what it does.
e.g. coffee mug = affords grabbing it (handle), drinking from it (cupped shape), enjoying it (smell)
81
Does retinal image fully determine perception?
No - What we perceive is not always what is represented on our retina
82
What is the Fraser spiral illusion?
James Fraser (1908)
- Looks like spirals, actually circles
83
What is the vertical-horizontal illusion?
Vertical looks longer - bisecting lines are perceived as being longer than bisected (interrupted) lines
84
What is the irradiation illusion?
(Hermann von Helmholtz, 1867)
White inner area of black shapes looks bigger than black inner area of white shapes
- Light areas appear to be larger than dark areas, because light from a white region irradiates adjacent dark regions
84
What is equivocal perception? (Ambiguous figures)
Perceiving different objects from the same retinal image
85
What is figure ambiguity?
- Retinal image does not change, but we perceive different objects
- Focus on foreground or background
e.g. vase or two faces?
86
What is feature ambiguity?
- Same feature can be different parts of different objects
e.g. duck or rabbit?
87
What is depth ambiguity?
- 3D perception changes in the same retinal image
e.g. Necker cube (Louis Albert Necker, 1832) - where is the front and where is the back?
88
How do visual illusions critique Gibson's ecological theory of perception and other bottom up accounts?
Bottom up accounts cannot explain these illusions
Retinal images do not fully determine perceptions
However, in Gibson's view - such illusions and figures are carefully constructed to mislead us and do not exist in the real world therefore they have no ecological validity
However, real world visual illusions exist too
89
What is the moon illusion?
The moon appears larger when it is close to the horizon as compared to when it is high up in the sky
90
What is the waterfall illusion?
Motor aftereffect: after observation of motion in one direction (waterfall for 30-60 seconds) stationary objects (e.g. trees) appear to move in the opposite direction
Neurons adapt to the motion we observe, so we don't find it too disturbing, this effect lasts so neurons are confused with the lack of motion of stationary objects
91
What is the wagon wheel effect?
- Stroboscopic effect: a moving wheel appears to stand still or move in opposite direction to its true rotation
Motion adaptation of neurons
92
Are all retinal images ambiguous and what is the inverse projection problem?
All retinal images are ambiguous
The inverse projection problem - a 3D object is represented on a 2D retinal surface.
The same pattern of light on the retina can be caused by different objects.
The real-world 3D object cannot be derived from the retinal image
- Superimposed objects and untypical angles
93
What does the existence of perceptual illusions show about perception?
- Perception is more than just sensation - sensory information from the retina is insufficient, perception is a matter of interpretation
94
What do top-down accounts of perception propose?
Perception is an interaction between sensation and cognition
i.e. stored object knowledge, context info, personal expectation and motivation, are used to help interpreting sensory stimulation
95
What is Richard Gregory's (1970) Constructive theory of perception?
- Basic idea - perception is indirect (interpreted), and a constructive process of hypothesis testing based on internal factors is used to interpret the sensory input
- For ambiguous figures, two equally plausible hypotheses are established
- As perception is based on individual factors, incorrect hypotheses can be formed, which leads to perceptual errors (as for visual illusions)
- Goal - explain how we attach meaning to sensory input
- Perception = interpretation of sensation
- The sensory input matches the perceptual hypothesis
96
What evaluation is there of Constructive theory of perception?
:) - Theoretical account - tries to explain how we perceive
:) - Is able to explain perceptual failure (visual illusion) and ambiguity
:( - Cannot explain why visual illusions persist even when they are known
:( - Suggests that perception is not effortless (active and continuous hypothesis testing) - does not match our subjective experience
97
What is Gestalt psychology?
Max Wertheimer, Wolfgang Kohler, Kurt Koffka, 1910
'The whole is more than the sum of its parts' (the whole configuration)
e.g. Two light flashes presented in a rapid alternating fashion to create illusory movement:
The sensory experience (two light flashes) is not sufficient to explain the perceptual experience (illusion of movement)
- Basic question - how do we achieve the whole
- Answer - by perceptual organisation
98
What types of perceptual organisation are there and which cells/neurons enable this?
- Grouping (putting together elements that form an object)
- Segmentation (separating different groups of elements)
- Ganglion cells have centre-surround receptive fields that are designed to detect luminance discontinuities
- V4 neurons respond to (colour or texture) discontinuities enabling us to detect that some areas in the visual field have similar properties while others differ from each other
99
What are heuristics in perceptual organisation?
Principles that lead to a perceptual whole
100
What are the 9 elements of perceptual organisation?
1) Proximity
2) Similarity
3) Common fait
4) Good continuation
5) Closure
6) Relative size
7) Surroundedness
8) Orientation
9) Symmetry
101
What is proximity? (perceptual organisation)
Elements that are close together are grouped together
102
What is similarity? (perceptual organisation)
Elements that look similar are grouped together (seems to override proximity)
103
What is common fait? (perceptual organisation)
Elements that appear to move together are grouped together
(This is why camouflage animals can only be seen when they move)
104
What is good continuation? (perceptual organisation)
Elements that continue are grouped together (Static version of common fate)
105
What is closure? (perceptual organisation)
Elements that close a figure are grouped together
106
What are relative size, surroundedness, orientation and symmetry? (perceptual organisation)
Elements that are relatively smaller, in a surrounded area, horizontally/vertically orientated, symmetrical are grouped together
107
What is the Law of Pragnanz? (Good Gestalt)
'Of several geometrically possible organisations, that one will actually occur which possesses the best, simplest and most stable shape'
108
What does Gestalt psychology say perception is?
Perception = interpretation of sensation in line with the law of Pragnanz (the sensory elements are grouped together in a meaningful way based on the principles of perceptual organisation)
109
What evaluation is there of Gestalt psychology?
:) - Provides a set of useful perceptual heuristics (principles of how sensory input is grouped and segmented) and has neuronal evidence
:( - Aims to be a theoretical account but is mainly descriptive (describes what grouping principles are, not how object perception and recognition work)
110
How does colour enhance top-down control?
Picking out certain colour objects in a group
111
How does colour enhance bottom-up processing?
Something being a bright colour = automatic warning
112
Does colour enhance perceptual organisation?
Yes
113
What is colour? (Wavelengths)
The visible light spectrum
Different wavelengths of the light spectrum are perceived as different colours
114
Does light contain colour?
No
Colour is non-veridical - not actually physically there - cannot match our perception against the stimulus (unlike a stimulus like a shape)
115
What colour are short and medium wavelengths of light?
Blue and green
116
What colour are medium and long wavelengths of light?
Yellow and red
117
What is selective reflection? What does this result in?
Some wavelengths are reflected, others are absorbed
Results in chromatic colours
118
What does equal reflection of light result in?
Achromatic colours
(All wavelengths are reflected)
119
Are grey and black objects the result of equally reflected light?
Yes
For grey objects, all are still equally reflected but just less, and even less for black objects
120
What wavelengths of light would a red object absorb and reflect?
Absorbs short and medium wavelengths (green blue yellow), reflects long wavelengths (red)
(If transparent - transmits not reflects)
121
What wavelengths of light would a white object absorb and reflect?
It would reflect all wavelengths of light - short medium and long
(If transparent - transmitted not reflected)
122
What is selective transmission? What does this result in?
Some wavelengths are transmitted, others are absorbed (transparent objects)
Chromatic colours are the result of selective transmission
123
What does equal transmission result in?
Achromatic colours are the result of equal transmission
i.e. all wavelengths are transmitted
124
How do light waves interact with water vs milk?
Milk - light waves are scattered, water - light waves pass straight through
125
What are the three colour dimensions?
Hue, saturation and brightness
126
What is hue?
Colour value
- Represented on the colour wheel
- Red, green, blue, yellow = pure colours
- Around 200 hues can be discriminated
127
What is saturation?
- The amount of white added to the colour
- (saturated vs non-saturated colour)
128
What is brightness?
- The amount of light reflected
- The intensity of the light (bright vs dim)
129
How many colours can be created along the 3 colour dimensions?
Around 7 million
130
How are colours mixed through mixing light?
e.g. in a display or pixels on a phone
- Additive colour mixture = all wavelengths present alone are also present when other wavelengths are superimposed
All colours added - more wavelengths reflected
131
How are colours mixed through mixing pigments?
e.g. in paints
- Subtractive colour mixture = pigments still absorb the same wavelengths they absorb when they are alone - only the wavelengths reflected by both pigments in common remain
Colours subtracted, only wavelengths all pigments already reflect are left
132
What colour would blue and yellow light make vs blue and yellow paint?
White (reflects short medium and long)
Green (only reflects medium)
133
How did colour-matching experiments support trichromatic theory?
Thomas Young (1802) and Hermann von Helmholtz (1852)
Colour matching experiments - task was to combine three lights so that their colour matches the color shown on the test surface
Results:
1) Any colour (in the test field) could be matched by correctly adjusting the proportions of three different wavelengths
2) Not all colours in the spectrum could be matched with only two different wavelengths
Conclusion:
Full colour vision is based on the ability to combine three different wavelengths
134
What is trichromatic theory of colour vision?
Full colour vision is based on the ability to combine three different wavelengths
135
What physiological evidence supports trichromatic theory?
S cones - respond to short wavelengths - blue
M cones - respond to medium wavelengths - green
L cones - respond to long wavelengths - red
Conclusion:
Colour vision is based on the activity of three different receptor mechanisms (three different cone types) = trichromatism
136
What is monochromatism?
No cones at all (colour blindness)
- No functioning cones (rod vision = scotopic vision)
- Perceive shades of brightness (white, grey black)
137
What is dichromatism?
Only two types of cones (colour deficiency)
138
What is pronatopia?
No L cones - cannot see red and green
139
What is deuteranopia?
No M cones - cannot see red and green
140
What is trinatopia?
No S cones - cannot see blue/yellow
141
What theory does types of colour blindness support?
Trichromatic theory
142
What is opponent process theory? Ewald Hering (1878)
There are three opponent channels which respond in opposite ways to different wavelengths
Based on complementary afterimages
Cannot imagine a reddish green, a yellowish blue, a blackish white
There are selective red-green or blue-yellow colour deficiencies
Red and green, blue and yellow, black and white seem to be paired
143
What are complementary afterimages?
- Adapting to blue creates a yellow afterimage
- Adapting to green creates a red afterimage
- Adapting to black creates a white afterimage
(and vice versa)
144
What physiological evidence supports opponent process theory?
Opponent neurons in V1, V4 and inferotemporal cortex with single-opponent or double-opponent receptive fields (in the ventral stream)
e.g. red green opponent neurons have one part responding to long waves and one part responding to medium waves
e.g. blue yellow opponent neurons have one part responding to medium-long waves and one part responding to short waves
145
What should opponent neurons not be confused with?
Simple on/off centre/surround receptive fields of ganglion cells which pick up luminance
146
What are neurons with single-opponent receptive fields good at?
Detecting uniform colour surfaces
147
Do single opponent neurons fire at colour boundaries?
No
Neuron does not fire, because the two different wavelengths cancel each other out
148
What are neurons with double opponent receptive fields good at?
Detecting colour boundaries
149
Do double opponent neurons fire at colour boundaries?
Yes
Both wavelengths add up
150
Are trichromatic theory and opponent process theory correct?
Yes - both are
151
How would trichromatic and opponent process systems respond to the colour blue?
Blue - reflects short wavelengths, absorbs medium and long ones
Retina: S cones respond, M and L do not
V1, V4, IT: Blue on/ yellow off cells respond
Perception of blue
152
How would trichromatic and opponent process systems respond to the colour green?
Green absorbs small and long wavelengths, reflects medium wavelengths
Retina: M cones respond, S and L do not
V1, V4, IT: Green on/red off cells respond
153
What are depth and size and what do they determine?
Veridical object features
Perception of these determines our movement
154
What is the inverse projection problem?
Despite objects being different sizes in real life, if one is bigger but further away this creates the same retinal image - how do we tell that they are different sizes and that one is further away?
155
What are occulomotor depth cues?
Body related cues
State of the eyes - we feel that an object is close to us or far away through accommodation and convergence - tightened or relaxed cilliary muscles
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What are monocular depth cues?
Stimulus-related cues
Can perceive depth and size cues with one eye
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What are pictorial depth cues?
Extracting 3D information from 2D images
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What does occlusion show about depth?
Occluded (partially hidden) objects are further away than occluding objects
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What does relative height show about depth?
Objects higher in the visual field are further away than objects lower in the visual field. Unless we are on a hill, then objects with their bases closer to the horizon are further away
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What does relative size show about depth?
When two objects are equal in size, the one that is further away will take up less of the visual field
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What does perspective convergence show about depth?
Converging parts of objects are further away
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What does familiar size show about depth? (Epstein experiment, 1965)
Knowledge about the physical size of objects
e.g. Epstein (1965) experiment - one pound coin is judged to be further away, because it is known to be larger than the 5p coin
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What does atmospheric perspective show about depth?
Objects further away are less sharp and have a blue tint
Particles in the atmosphere scatter light (preferentially short wavelengths) which softens and colours further away objects (but not on the moon)
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What does texture gradient show about depth?
Denser textures indicate further away objects
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What do shadows indicate about depth?
Shadow further away - closer object
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What are motion based cues to depth?
We extract 3D information from animated 2D images
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How is motion parallax a cue to depth?
Nearby objects glide away quickly, far away objects glide along slowly
- The image of near objects travels a long distance on the retina of the moving eye
- The image of far objects travels a short distance on the retina in the moving eye
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How are detection and accretion cues to depth?
An object being covered and uncovered is further away (occlusion in motion)
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What is binocular vision
Uses both eyes
Stereoscopic vision
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How is retinal disparity a cue to depth?
Most powerful depth cue we have - the retinal images in the left and right eye are slightly shifted (because our eyes are a few centimetres apart) (the point of fixation is crucial for binocular depth perception)
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How are corresponding retinal points cues to depth perception?
the two foveae are corresponding points (if eyes are overlaid then corresponding points overlap exactly), all objects with the same distance as the fixation point will be represented on corresponding retinal points (same relative position in both eyes)
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What is a horopter? (Retinal points)
Imaginary plane through the fixation point connecting all objects for which retinal images fall on corresponding retinal points (connects all objects that are equidistant from observer)
Semi circle in front of person
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What are non-corresponding retinal points?
Retinal points not on the horopter do not correspond exactly with the part of the fovea they reach
If eyes are overlaid, non-corresponding points do not overlap
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What is retinal disparity?
The distance between corresponding and actual retinal points
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What is stereopsis?
The ability to use retinal disparity as a cue to perceive depth.
Each distance from fixation produces a different amount of retinal disparity, the larger the disparity, the further away the object is from fixation.
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What neuronal evidence supports stereopsis as a cue for depth?
Disparity-selective neurons in V1 and along both ventral and dorsal streams
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Can disparity transform 2D images into 3D perception?
Yes
BUT - stereoscopic photographs can also contain pictorial cues which create depth perception
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What did Bela Julesz, 1971 find from her Random dot stereogram?
Retinal images can transform 2D images into 3D perception
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What is the size of retinal object representations measured in?
Degree of visual angle
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What does the size of retinal object representations depend on?
- Depends on physical object size and physical distance from the observer
- The same object has a larger visual angle when it is closer to the observer and a smaller visual angle when it is further away
- Two differently sized objects can have the same visual angle when the larger one is further away
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What is size constancy?
Perception of an object's size is relatively constant even when viewed from different distances (producing different visual angles)
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How is size constancy calculated?
Size distance scaling equation
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What is the size-distance scaling equation?
S = K(R x D)
S = perceived size
R = size of the retinal image
D = perceived distance
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How would the size distance scaling equation keep the perceived size of a sheep the same as it walks towards you?
Size of sheep's image on your retina (R) gets larger, but your perception of the sheep's distance (D) gets smaller
Perceived size (S) of the sheep remains the same
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What is the Ponzo illusion?
Same sized 2D objects - one is nearer to converging lines so is perceived as larger
Explained by size distance scaling equation
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What is the moon illusion?
Moon perceived to be bigger when it is nearer the horizon, as it is the same retinal size but we think that it is further away than when the moon is high in the sky
Explained by size-distance scaling equation
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What is sensation?
The un-interpreted sensory impressions created by the detection of stimulus
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What is perception?
The psychological processes of making sense of sensations
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What is recognition?
The identification of a stimulus as an object we are familiar with
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What is object representation and what problem does this solve?
Object representation - the way we know an object looks like
If the visual input matches the representation - object recognition
If the visual input does not match the representation - no object recognition
- Problem - an infinite number of retinal images can correspond to a particular object
- Solution - visual input must be matched with internal object representations held in object stores
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What is structural analysis theory? (David Marr, 1982)
Objects may differ in their 2D appearance, but they become equivalent in 3D space (visual input must be analysed at different levels)
- Structural analysis proceeds through 4 sequential levels until a 3D model is achieved, which can be matched with stored object representations
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What are the 4 sequential levels of structural analysis?
Initial representation:
1) Raw primal sketch
2) Full primal sketch
Viewer-centered representation:
3) 2 1/2 D sketch
Object centered representation:
4) 3D model
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What are the stages of initial representation in structural analysis?
1) Raw primal sketch - processing of intensity (brightness) changes across the retina
2) Full primal sketch - geometric organisation of these intensity changes (definition of edges and contours)
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What is the stage of viewer-centered representation in structural analysis?
3) 2 1/2 D sketch (1/2 because it includes depth information) - processing of spatial locations of visible surfaces, includes depth, but only represents the object from the observer's viewpoint (perceived objects cannot be generalised). Edges and contours assembled
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What is the stage of object-centered representation in structural analysis?
4) 3D model - represents the object independently of the observer's position, 3D standard form is achieved (real shapes and their relative positions to each other), can be matched with stored object representations
- Match -> recognition
- Non-match -> re-analysis
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What did Marr's hypothesis inspire?
A cognitive model of object recognition (Ellis and Young, 1988)
- This explains a cognitive mechanism which can be used to make predictions about human behaviour
- Predictions can be tested and thus the model van be verified or falsified - the more evidence we find that confirms the model's predictions, the more reliable and valid the model is
- Most models hierarchical and linear, this one is hierarchical - starts at one point, unfolds and leads to final point
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What is Ellis and Young's (1988) model of object recognition?
Object
Initial representation
Viewer centered representation
Object centered representation
OR
Object recognition units
(Object centered representation then goes to object recognition units)
Semantic system
Name retrieval
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What do the parts of Ellis and Young's (1988) model of object recognition mean?
Each term is a functional processing stage
Arrows show direction of information flow
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What are object recognition units? (Ellis and Young, 1988)
Stored object representations - each object has its own unit (visual object representations stored in memory)
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What is the recognition shortcut? (Ellis and Young, 1988)
For unambiguous viewer-centred representations (arrow between viewer-centred representation and object recognition units) - don't need all info to recognise
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What is the semantic system and name retrieval? (Ellis and Young, 1988)
Semantic system: storage of general, but also personal facts about objects
Name retrieval: accessing these facts generates name retrieval
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What are sensory processing, perceptual processing and recognition stages of Ellis and Young's 1988 model of object recognition?
Sensory processing - initial representation
Perceptual processing - viewer centred representation and object-centered representation
Recognition - object recognition units, semantic system and name retrieval
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How does Mr S support Ellis and Young's 1988 model of object recognition?
Soldier who suffered carbon monoxide poisoning
- He can maintain fixation, name colours and describe sensations, detect small changes in brightness and wavelength, and detect movements of small objects
- But he cannot perceive shape or form (can't copy simple figures, letters, digits or match simple objects
- Also cannot recognise or name objects
- Mr S has no viewer-centered representation (no perception of basic shapes)
- Model predicts behavioural pattern of Mr S - he cannot do any of the subsequent stages
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How do patients with right posterior lesions support Ellis and Young's 1988 model of object recognition? (Warrington, 1982)
Can match objects for size, colour, brightness
Can recognise objects from pictures in usual views.
Cannot recognise objects in unusual views.
No impairments with object recognition in everyday life.
Can explain use of objects.
e.g. “Torch = hand-held light”, “Compass = tools for telling direction you are going”
Therefore, they can do all the stages apart from object centered representation which deals with ambiguous or unusual views of objects
The model predicts the behavioural pattern of these patients
Recognition of objects in usual views, no recognition of objects in unusual views
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How does AB support Ellis and Young's 1988 model of object recognition? (Warrington, 1975)
Civil sergeant with progressive cerebral atrophy (ongoing loss of neurons and connections between them).
- The patient can distinguish basic shapes.
identify colours, numbers and letters, match pictures of different views (usual and unusual)
- Cannot name common objects or famous faces.
e.g., photographs of the faces of contemporary personalities.
- But states that they look “familiar” but he had “forgotten” them.
- Makes substitutions when categorising objects.
e.g., a donkey is a horse, a dog is a cat.
- Cannot identify meaningful sounds.
e.g., telephone ringing, dog barking
He has intact viewer and object centered representation, but no object recognition units, semantic system or name retrieval
The model predicts the behavioural pattern of AB
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How does the Ellis & Young model (1988) predict the behavioural pattern of patients with two different types of visual agnosia?
Visual agnosia = inability to recognise seen objects.
- Impaired perceptual stages (apperceptive agnosia)
- Impaired semantic stages (associative agnosia)
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Which patients had apperperceptive agnosia and associative agnosia?
Apperceptive agnosia (impaired perception)
e.g., Mr S
Cannot copy or match even simple shapes.
Profound impairment in shape perception.
Associative agnosia (impaired semantics)
e.g., AB
Can copy and match objects.
But intact percept has no meaning.
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Do patients with right posterior lesions have visual agnosia? What is their deficit in object recognition?
They cannot create object-centred representations. (Objects in unusual views)
They have a perceptual categorisation deficit, but don’t actually have visual agnosia (object recognition is intact)
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What conclusions can we make about Ellis and Young's 1988 model of object recognition?
The model is valid.
i.e., it makes accurate predictions about behavioural impairments.
But…
…are single cases valid measures to test models?
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What does motion perception do for us?
- Continuous environmental update
- Grouping and segmentation
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What is real motion vs illusory motion?
Real motion - a stimulus that really moves
Illusory motion - perceived motion that is not real
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What is apparent illusory motion?
Perceived motion in a static stimulus, illusion of motion for successively presented stimuli e.g. film
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What is induced illusory motion?
A moving frame of reference (e.g. clouds) can produce the illusion that a stationary object (e.g. moon) moves in the opposite direction
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What are motion aftereffects of illusory motion?
After adapting to motion in one direction (e.g. waterfall), stationary objects (e.g. trees) appear to move in the opposite direction
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What are visual illusions of illusory motion?
e.g. peripheral drift illusion
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Does illusory motion produce retinal motion?
No
Only real motion produces retinal motion
Retinal motion = successive receptor stimulation by a moving stimulus in the visual field
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What physiological evidence is there for perception of real motion? (Hubel and Wiesel, 1959)
Motion-direction selective neurons in V1 (cats)
When stimulus was oriented in a certain direction, specific neuron firing rate increased
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Where did Hubel and Wiesel (1959) find motion-direction selective neurons?
MT (middle temporal area - V5)
This is in the dorsal (where) stream
Almost all neurons here are direction-selective (to process where an object is)
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What did Newsome et al (1995) find in his monkey experiment with moving dot displays? (MT neurons)
Measured an MT neuron's firing rate (single cell recording) depending on motion-direction coherence
With increasing motion direction coherence, monkeys judged the direction of motion more accurately and the MT neuron fired more rapidly
With 12.8% coherence, monkeys judged motion direction accurately in almost every trial
Conclusion - MT neurons are highly specialised to detect motion direction
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What is a receptive field?
Part of the visual field in which a stimulus can modify a neuron's firing rate
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Do neurons cover the whole visual field?
A single neuron's receptive field only covers a part of the visual field
No neuron responds to the whole visual field - this is a problem for motion perception
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What is the aperture problem?
The direction of a moving stimulus through an aperture (such as a receptive field) is ambiguous
Motion direction is unclear when stimulus is larger than the receptive field
The response of an individual direction-selective neuron does not provide sufficient information to indicate the direction of the movement.
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What are solutions to the aperture problem?
1) Pooling responses of multiple neurons
2) End-stopped cells signal the end of a stimulus (where the direction is unambiguous)
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What does the superior temporal sulcus do? (STS)
Receives projections from MT and IT, combines input about object form and motion direction
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What number V is the middle temporal area?
V5
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What is the dorsal stream?
V1 -> V2 -> V3 -> MT -> STS
Where stream - parietal
Perceives motion
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What is the ventral stream?
V1 -> V2 -> V4 -> IT -> STS
What stream - temporal
Perceives form
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Does the dorsal or ventral stream go to the IT or MT?
Dorsal - MT
Ventral - IT
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What is biological motion vs retinal motion?
Biological motion = self produced motion of a biological being (real motion that produces retinal motion), we are experts in perceiving and recognising biological motion e.g. point light walkers
Retinal motion = successive receptor stimulation
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What is corollary discharge theory? What are the three components?
Motion perception depends on retinal motion AND eye movements
1) Image displacement signal
2) Motor signal
3) Corollary discharge signal
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What is image displacement signal? (Corollary discharge theory)
Object moves along retina (one thing moves one thing stays still)
An image moves across the retina and successively stimulates the receptors (retinal motion)
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What is motor signal? (Corollary discharge theory)
Command to move the eye from motor area of brain to eye muscles (whether eye muscles are static or if eye moves)
Motion command from brain to eye muscles
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What is corollary discharge signal? (Corollary discharge theory)
Copy of this signal to eye muscles is sent to comparator (comparator finds out difference between signals)
Copy of motor signal 'cc' to comparator
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What is neuronal evidence supporting Corollary discharge theory? (Galletti and Fattori, 2003) (Monkeys)
Real motion neurons (dorsa stream) respond to moving stimuli, but not to static stimuli in the moving eye
Neuron fires when stimulus moves through receptive field but not when eyes move over the stimulus
Successfully explains why a moving stimulus in a static or moving eye generates motion perception, whereas a static stimulus in a moving eye does not, despite the fact that there is retinal motion
