Lec 5/ TB Ch 5&8a

Question 1

• Color constancy
• Newton’s set up
  
  • light → prism = ?
  • can monochromatic light be further refracted?
  • all colors → 2nd prism
• Does light have color?

Answer 1

Colors and social media

• Dress: either gold and white, OR blue and black
• Color is perceived based on the context it is in
• Color constancy: regardless of the lighting conditions, we will see the same color (ex. red)
  
  • It happens most of the time

Newton’s discovery

• Newton: the “white” light we perceive from the sun can be broken down into the colors of the rainbow
  
  • # 1: light from hole -> lens (focus the light) -> prism -> screen
    • prisms break up (refract) white light into spectral components (rainbow).
    • Each color “bends” differenty on the screen
• Any single component could not be refracted into a different color (monochromatic colors = only 1 single wavelength).
  
  • So Newton punched another whole in the screen (ex. where the color red is on the screen), the light enters a 2^nd prism -> red
    
    • Red -> prism -> red
    • • Another case: he puts the 2nd prism behind the 1^st prism
  • The 2^nd prism combines all the colours, and re-create white light.
  • IOW: refraction does not “destroy” the light, you can reform that
• Newton’s conclusion: We perceive the continuum of wavelengths as qualitatively different phenomena.
  
  • Colour (perception) is created in our mind
  • Ex. green and red
    
    • Based on the rainbow spectrum, it does not suggest that red and green are opposite colors
    • But we perceive the as opposite colors (this is related to how we process color info in our brains)
• How many color mechanisms are there in your eyes: we have 3 types of cones
• How many colors can you see?
  
  • At least a million (a lot)

Question 2

The problem of univariance

• Problem of univariance
• 2 things that determine PR response
• graph

Answer 2

Responses of Single Photo Receptors: The Problem of Univariance

• When you download a color image, it has 3 main colors (red, green, bluw)
• You can split the file out into their R,G,B components
• This is similar to how the cones work in our retinas (they are similar to filters)
• Green filter: you can see the nose and stars (and some parts of the hair)
  
  • They appear ~equally bright but actually are very different in colour.

Basic principles of color perception

• If you only have a single type of filter (ex “green” cones), you will only see 1 type of color in different luminance (i.e. a series of greys) -> problem of univariance
• 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 colour discriminations based on wavelength b/c there are 2 things that determines the photoreceptor’s response
    
    • #1: PR responds differently depending on wavelength
    • #2: PR responds differently depending on light energy
  • IOW: If you want to know the wavelength in the image
  • but photoreceptor gives info about wavelength and light
  • -> you can’t tell what wavelengths are there
    
    • Ex. (IOW: Even though the clown has green hair, blue star, orange nose -> same response in a single photoreceptor)
  • The output from a single photoreceptors only varies along 1 dimension
  • It tells you how much it could get stimulated but it can be stimulated by diff things
• Solid curve = different kinds of light has the same amount of energy (the # of photons)
• You can reduce the # of photons (i.e. luminance) and the curve shifts down
• • In this shifted curve, the green light has less luminance -> emits the same receptor response compared to the blue and orange light
• (ex. clown: we can see the orange nose, blue start and green hair stimulates the same photoreceptor by the same amount)
• • (grey line): Only the response of a receptor will tell us something about what light we are looking at.
• However, the output of one cone is completely ambiguous (it responds based on the wavelength and the luminance/energy -> grey line)
• That’s why we don’t call them red/green/blue cones!
  
  • The cones can’t tell the color you are actually seeing
• This is the problem of univariance -> we need several types of cones

Question 3

trichromacy

• scotopic
• rhodopsin
• sensitive wavelength range
• issue of rods
• Which type of cone also has this range?

Answer 3

Trichromacy

• Scotopic: Referring to dim light levels at or below the level of bright moonlight (we see shades of grey and black)
  
  • Cones are not sensitive to scoptic vision; only rods
  • Rods are sensitive to scotopic light levels
  • All rods contain same type of photopigment molecule: Rhodopsin
  • All rods have same sensitivity to wavelength (around 500 nm, cyan or green), making it impossible to discriminate light of different wavelengths.
    
    • So the rods responds in a similar range w/ the M-cone
    • At scotopic levels, we only see shades of grey (not green)
    • This shows that color perception is based on the responses b/w multiple photoreceptors
  • Red-cone = L-cone
  • Green-cone = M-cone
  • Blue-cone = S-cone

Question 4

trichromacy

• Young-Helmholtz(-Maxwell) theory
• Maxwell’s colour-matching technique
  
  • diff b/w LS and RS
• 3 types of cones
  
  • centre of fovea lacks which type of cone?

Answer 4

• Young-Helmholtz(-Maxwell) theory: theory of trichromatic colour vision. Colour vision is based on 3 photoreceptors sensitive to particular ranges of wavelengths
• Maxwell’s colour-matching technique
  
  • A psychophysics technique
  • Left: the “bluish” color is a monochromatic light
  • Right: there’s monochromatic RGB lights respectively
  • Participant are told to adjust the amount of RGB lights to recreate the “bluish” colors
  • Results: you just need 3 types of colors to recreate any color
  • This suggests our visual system has 3 types of cones
• They look the same but they are not physically the same
  
  • L: cannot be broken down if it passed thru a prism
  • R: can be broken down
  • This is a metamer
• X
• Cone photoreceptors: Three varieties
  
  • S-cones: short wavelengths, 420 nm (‘blue’ cones)
  • M-cones: middle wavelengths, 534 nm (‘green’ cones)
  • L-cones: long wavelengths, 565 nm (‘red’ cones)
• A piece of the fovea (birdseye view)
  
  • Color coded based on S,M,L cone
  • Centre in the fovea: there are NO s-cones
    
    • We are colorblind to blue in the centre
    • We still “see” blue b/c there’s filling in from the periphery

Question 5

• what problem does have 3 cone types solve?
• Ex. Clown - Blue stars and orange nose
  
  • M cone/green filter
  • L cone/ red filter
  • S cone/ blue filter

Answer 5

Responses across the Three Types of Cones

• With three cone types we can tell the difference between lights of different wavelengths (aka solve the problem of univariance)
• Ex. clown
  
  • Blue stars and orange nose
  • M-cone: when there’s blue and orange -> produces the same response
  • L-cones: responds more strongly to orange light, responds less to blue
  • S-cones: responds most strongly to blue, not responsive to orange
• Ex. clown
  
  • Green filter: blue stars and orange nose are ~equally bright
  • Blue filter: blue stars are bright, orange nose is dark
  • Red filter: orange nose is bright, blue stars are dark

Question 6

• how is light reflected (Ex. meat)
• graph

Answer 6

Reflected Light from Real-World Objects

• We seldom see one wavelength at a time (ex raindow)
• We usually see a range of wavelengths and color mixes
• How do cones respond to a broad range of wavelengths?
• Graph: how light is reflected in meat
  
  • Red = cooked, blue = raw
  • Cooked: reflects red light a bit more
  • Blue: strong reflection of red light -> see raw meat as red
• We don’t see pure red, we see raw meat as reddish

Question 7

trichromacy cont

• LS vs RS graph
• Metamer
• Additive color mixture
• subtractive color mixture

Answer 7

Metamers

• (Let’s ignore S-cones for now.)
• Red and green light if mixed together in the right proportion will stimulate L- and M-cones the same as yellow light, i.e., it looks like yellow light.
• LHS: Green light -> strong M-cone response, weaker L-cone response
  
  • Red light -> opp
  • When you avg it, we get an equal response from red and green cones
• RHS: Yellow light creates an equivalent response in L and M cones
• Ex. Clown image, hair is yellow
  
  • When we break the image out, we see a patch red in red filter, green in green filtered
• MP: we can create colors that look like monochromatic colors (ex. create Y using RG)
• Metamers: any pair of stimuli that are perceived as identical in spite of physical differences.
  
  • In terms of light: different mixtures of wavelengths that look identical.
• X
• Additive colour mixture: A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of colour, the effects of those two lights add together
  
  • Add lights together
  • Red + green = yellow
  • Yellow + blue = white
  • This is what happens when mixing light with different colours
• But what happens if we mix differently coloured paints? Red + green = ?
• Subtractive colour 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 colour
  • Ex red + green
    
    • Red paint filters out all colors except for red
    • Green paint filters out all colors except for green
    • -> dark
  • LHS: paints filter out
  • RHS: add on

Subtractive color mixture

• Blue pigment absorbs long wavelength color from sunlight, and reflect short wavelength (blue)
• Yellow: opp
• Blue and yellow: absorbs long and short wavelengths -> reflect green
  *

Question 8

trichromacy

• represent color in 3D space
  
  • 2 ways
• non spectral hues

Answer 8

• Color space is 3D b/c we have 3 cones
  
  • # 1: Each pixel on the screen can vary in RGB dimensions
  • # 2: Hue, saturation, brightness
    • Hue: Chromatic aspect of color (RGB)
    • Saturation: Chromatic strength of a hue (saturated to faded red/grey)
    • Brightness: Distance from black in colour space (light and dark red)
  • Saturation = chroma
  • Brightness = lightness
  • Vertical = brightness
  • Horizontal = saturation
  • Column = hue
  • HSB = hue, saturation, brightness
  • RGB
  • If we change HSB, RGB also change automatically; vv
    
    • • Hue is expressed w/ the color wheel (in degrees)
• For both representations, we can see there’s all the colors on the rainbow spectrum (red -> purple)
  
  • The red & green circles = magenta, which is NOT a monochromatic light
  • • Non-spectral hues: hues that don’t exist as pure forms of light but only as mixtures of different wavelengths
• Ex. 450 nm + 625 nm stimulates L- and S-cones but not M-cones -> we see magenta
  
  • For actual wavelengths of lights, 450 nm + 625 nm will give you a color somewhere in b/w orange and blue (ex. green)
  • IOW, this magenta color is not a wavelength on the spectrum

Question 9

opponent processes

• Hering’s idea of “illegal” colours
• Opponent colour theory
  
  • Opp col theory vs RGB vs HSD → which his more accurate?
• middle figure: what continuum
  
  • middle of continuum → color?
  • Implication
• RS fig: what continuum?
  
  • How do we have yellow?
  • middle color = ?
  • Implication?
• LS fig: what cont?
• support for opp col theory

Answer 9

Trichromacy theories

• Hering’s idea of “illegal” colours (e.g., reddish green, or bluish yellow)
  
  • We have cones that are good at detecting reds and greens, but we never see a color that is reddish green
• Opponent colour theory: The theory that perception of colour is based on the output of three mechanisms, each of them on an opponency between two colours; red–green, blue–yellow, and black–white
• Recall: trichromacy theory: RGB = 3 dimensions
  
  • RGB are orthogonal to eo = independent of eo
• Recall: HBS
  
  • This is more accurate of how we see colors
  • It shows red and green are opposites; blue and yellow are opp
  • • Opponent color theory: proposes that red-green, blue-yellow are dependent
  • Middle figure: red-green opponent
    
    • Red-green continuum, middle = grey
    • There’s no reddish green
    • Determine how much red vs green
  • RHS figure: yellow-blue opponent
    
    • We don’t have yellow cone
    • So yellow = red + green cones (+)
    • Red+green (yellow) vs blue
    • In yellow-blue continuum, middle = grey
    • There’s no bluish yellow
  • LHS: black and white
    
    • Add up of RGB = white
    • There’s a black and white continuum (middle = grey)
• Support for opponent theory
  
  • Afterimage: A visual image seen after the stimulus has been removed
  • Ex. look at the -ve image -> you see color on a black and white image

Question 10

• illusion
  
  • # 1 8 ray star
  • → after image of 4-ray vertical star?
  • → after image of 4-ray oblique star?
• What color is the middle of the image
• What do we perceive?
• What is this process called?
• Reason?
• x
• Neurophysiological support for the Opponent Colour Theory:
  
  • LGN cells
    
    • # 1 Red-green opponency
    • 2: Green-red opponency (opp of abv)
    • # 3: Blue-yellow opponency
    • # 4: yellow-blue opponency
• Which area use trichromacy theory?
• Which area use Opponent colour thoery?

Answer 10

• Illusion
  
  • # 1: see the 8-ray star
  • # 2: You see vertical 4-way star, the vertical star’s afterimage looks pink
  • # 3: you see oblique 4-way star, the oblique star’s afterimage looks blue-green
  • The outline of the star is interacting w/ color perception
    
    • There are diff after images depending on what the shade is
  • 8-star: Middle of the image = grey
  • 4-star: we still the whole star as either pink or green
    
    • Color filling in happens
  • Recall: centre of fovea lacks S-cones, it needs to color fill for it

Opponent processes

• Neurophysiological support for the Opponent Colour Theory:
  
  • LGN has colour-opponent cells: neurons whose output is based on a difference between sets of cones
  • LGN cells have centre-surround organization
    
    • # 1 Red-green opponency
      • Centre: activated by red, deactivated by G & B
      • Surround: activated by G&B, deactivated by R
    • # 2: Green-red opponency (opp of abv)
      • Centre: activated by G&B, deactivated by R
      • Surround: activated by R, deactivated by G&B
    • # 3: Blue-yellow opponency
      • Centre: activated by B, deactivated by R&G
      • Surround: activated by R&G, deactivated by B
    • # 4: yellow-blue opponency
      • Centre: activated by R&G, deactivated by B
      • Surround: activated by B, deactivated by R&G
• Evidence for colour processes after LGN
  
  • e.g., L-M cell: for red/green -> we see: red – bluish green
• Trichromacy and opponent theories are both correct
  
  • Trichromacy: true for photoreceptors
  • Opponent: true for later lv, like LGN

Question 11

• which cellular systems are responsible for color in LGN?
• color system in V1?
• color system in V2?
• CO blobs project to → ?
• CO blobs also project to → ?
• Zeki 1993
  
  • where is the color area?
  • methods
• Damage to v4 → ?
  
  • same effect seen when we lesion which area?
  • What do patients perceive?

Answer 11

Colors in the cortex

• (Recall, color perception is only related to the LGN’s parvocellular and koniocellular systems, not magnocellular)
• Colour system in V1: blobs (CO blobs)
• Colour system in V2: thin stripes (arrows)
• x
• CO blobs project directly to the thin stripes (v2) -> v4 (processes color and shape)
• Other CO parts projects to the pale and thick stripes (v2) -> V5 or MT
• x
• Zeki 1993: found human V4 = ‘colour area’
  
  • Showed ppl of colored squares -> black and white squares
  • Then he subtracted their activation responses -> see where V4
  • x
• If you damage right V4 -> achromatopsia on visual field on the left
• Achromatopsia: An inability to perceive colors that is due to damage to the central nervous system
• Damage to v4 (or only v2 thin stripes, rare)
  
  • But early stages of color processing are still intact
  • Green and red is next to each other w/ similar luminance
  • For achromatic patients, they can’t tell b/w red and green, but you can still see the boundary b/w the colors (in grey)
• If you damage V1 and v2 -> blind

Question 12

Does Everyone See Colours the Same Way?

• Yes: 2 reasons
• No: 2 reasons
• color viision defficiency
  
  • Ishihara test
• 2 types of ppl who are truly color blind
  
  • Cone monochromat
  • Rod monochromat
• 3 types of of colour-anomalous/color deficient ppl
  
  • Define Deuteranope
  • Protanope
  • Tritanope
• Idea of cultural relativism: what does it affect?

Answer 12

Does Everyone See Colours the Same Way?

• Yes
  
  • General agreement on colours
  • Same metameres.
• “No”
  
  • Some variation due to age (lens turns yellow)
  • UV damage -> turns lens yellow
  • X
  • About 8% of male population, 0.5% of female population have some form of colour vision deficiency (commonly called Colour blindness – inaccurate)
  • Ishihara test
  • • 2 types of ppl who are truly color blind
      
      • Cone monochromat: Only one cone type; truly colour-blind
        
        • -> problem of univariance
      • Rod monochromat: No cones of any type; truly colour-blind, badly visually impaired in bright light
        
        • See things as blurred as rods don’t have great spatial resolution
        • • 3 types of of colour-anomalous/color deficient ppl
      • 1. Deuteranope: no M-cones (Colors are faded; more common)
      • 2. Protanope: no L-cones (red-green weakness)
      • 3. Tritanope: no S-cones (yellow-blue weakness)
           * * Maybe
  • Various cultures describe colours differently
  • English: 11 colour terms (strict definition)
  • Other languages have different numbers, e.g. 2/3 names
  • Idea of cultural relativism
    
    • The # of color names may influence how we perceive colors
    • It won’t change how we see metamers
    • But it changes how we remember colors

Question 13

• Unrelated colour
• Related colour
• x
• Some problems when studying the real world:
  
  • Colour constancy
    
    • Ex. painting in AM vs PM
    • What does this mean? sensation = illumination x reflectance
    • What is the issue?
    • How do we solve this? - 2 ways
      
      • 2 assumptions in physical constraints
    • Exception - roof orientation

Answer 13

Impossible colors

• Unrelated colour: A colour that can be experienced in isolation
  
  • You can see them w/ or w/o a background
  • Ex. red on screen vs red in room = red
• Related colour: A colour, such as brown or grey that is seen only in relation to other colours
  
  • Ex. when there’s a grey spot in a dark rm, you perceive it as dim white
  • Ex. cube (one side orange, one side brown)
    
    • Context matters, helps resolve ambiguity

Some problems when studying the real world:

• # 1: Colour constancy: the tendency of a surface to appear the same colour under a fairly wide range of illumination. (lighting)
  • Illuminants can be quite different.
  • Sun light varies (Monet, Rouen Cathedral)
    
    • Monet painted LHS in the morning
    • Middle = noon
    • LHS = in the evening
  • The same cathedral can look quite different depending on the sunlight
  • Ex Cube - Context matters (in the light vs shadow)
  • Arrows show same color for each image pair.
    
    • RHS “blue” = LHS “red” = grey tile
    • Here, the context of the lighting determines what we perceive
      
      • We want LS top tile (red) = RS top tile (red)
      • But it is not
• An undetermined problem
  
  • Sensation = illuminant (light source) x reflectance (reflecting property of the object, ex cathedral)
    
    • “x” = there’s an interaction (doesn’t mean multiplication here)
  • • Undetermined problem: Ex. 12 = a x b
      
      • A x b has many possibilities
• How do we solve the undetermined problem?
  
  • #1: Perceive color based on the surroundings
  • LHS: illumination = red
  • RHS: illumination = blue
  • This sorta helps but not always (ex. we mistake what is the light source (sun vs LED) -> change perception)
• How do we do it?
  
  • #2: Physical constraints make constancy possible:
    
    • Intelligent guesses about the illuminant
      
      • Assumptions about light sources
        
        • Few (ex 1) light source only
        • Light source is broadband
          
          • Ex sunlight -> contains the whole spectrum of the rainbow
      • Assumptions about surfaces
        
        • Surface reflects fairly broadband (ex red surface reflects red as well as some cooler colors)
        • Mutual reflections
          
          • Ex. when you have a half red, half white card
          • -> you look at the white side
          • -> red reflects from red onto white
          • -> your visual system notices it as a reflection, so it knows the surface is white
          • Ex. if this card is oriented as a roof
          • -> there’s no “reflection explanation”
          • -> visual system perceives it as pink
• Sometimes this fails: poor color constanct w/o context

Question 14

• colors in evolution: gives us 2 types of info
  
  • Dogs: # of PR
  • Chicken: # of PR
    
    • How is it diff from humans in physiology (what is used as its filter)
    • IOW: what is red + green for chickens
• x
• motion perception
• motion define
• Example of Change in position without motion
• Example of Motion without change in position
• x
• 2 types of biological info motion can tell us
• A lack of motion/flicker perception leads to change blindness
  
  • when there’s flicker, what happens?

Answer 14

Animals and color

• Studying animals provide insight into colour perception in humans
• e.g., what’s colour perception good for?
  
  • Information about food (red = ripe = can eat)
  • Colours provide sexual signals
    
    • Colorful male = sexy & healthy

Photopigments

• Color vision in diff species
  
  • Colour vision is accomplished in different ways in different species but follows similar principles:
    
    • Animals have small set of photoreceptor types.
      
      • Dogs: dichromats
      • Chicken: tetrachromats (they don’t have 4 types of cones, only 1 type)
        
        • 1 photopigment covered with different droplet of oil (oil = filter)
        • We see red + green = yellow
        • To chickens, red + green is not yellow
    • Colour constancy in bees and goldfish.

Introduction to motion perception

• Motion: change in position over time
• Does that mean motion is the same as noticing changes in position?
• Change in position without motion – Sun
  
  • It’s not moving but it looks like it is moving
• Motion without change in position: water fall illusion (Addams, 1834, falls of Foyers; Aristotle)
  
  • It is moving down but it looks like it is moving up at times
• X
• Motion of these moving dots -> biological motion (walking)
• Social interactions (Heider & Simmel 1944)
  
  • Motion of triangles: argument
  • Motion of circle: hiding
• A lack of motion/flicker perception leads to change blindness
  
  • When there’s no motion and flicker ->
  • Present lots of flicker -> visual system is overwhelmed (you can’t see the change in the shadow in the pic)
    *

Question 15

• a Reichardt motion detector
• LHS: 3 neurons - 2 steps
• Middle: big bug - what is happening?
• RH: To perceive motion you need 2 additional neurons → explain
  
  • 2 conditions
• a Reichardt motion detector (part 2)
  
  • system recognizes movement from receptive field A all the way for 5 other receptive fields → process?
  • Implication?
• What happens if the object is jumping, and at the right speed matching the delay neurons?

Answer 15

A Neural Circuit for Detection of Rightward Motion (Part 1)

• a Reichardt motion detector
• LHS: 3 neurons
  
  • Not good enough to perceive motion
  • # 1 Motion of bug: receptive field A -> B
  • # 2: Cells A send AP to stimulate M cell (motion detector); then B sends AP -> M
• Middle: big bug
  
  • Stimulates both receptive fields at the same time (cells A and B)
  • -> M cell is stimulated more strongly b/c it receives APs from cells A and B at the same time
• RHS
  
  • To perceive motion you need 2 additional neurons
  • # 1: Bug goes from receptive field A -> B
  • # 2: cell A receives signals first -> D cell (delay cell) -> X cell
    • B cell sends signal to X cell
    • X cell is stimulated at the same time by cells A and B
    • IOW: X has lots of stimulation
    • X cell – similar to multiplier, it only responds when it receives input from A and B
      
      • If 2 signals are present: 1 x 1 = 1 -> send signals to M
      • If only 1 signal is present: 1 x 0 = 0 (no signal sent to M)
    • This only works when** the receptive fields are next to one another **and the object moves at a specific speed

A Neural Circuit for Detection of Rightward (part 2)

• a Reichardt motion detector
  
  • system recognizes movement from receptive field A all the way for 5 other receptive fields
  • # 1: A cell -> D -> X cell; B1 cell -> X cell
  • # 2: B1 cell -> D -> X cell; B2 cell -> X cell
  • # 3: B2 cell -> D -> X cell; B3 cell -> X cell etc
  • Then, all the X cells synapse to M
• This results in perceiving a continuous motion
  
  • • The receptive fields is as small as a photoreceptor
• If the object is jumping, and at the right speed matching the delay neurons -> we see the jump as a cont motion
• Thus, motion detection is discrete (disconnected) – we can see motion when there isn’t (apparent motion)
  *

Question 16

• Apparent motion
• Ex
• Reichardt detector - Correspondence problem (motion):
• Aperture problem
• Global motion detector: describe

Answer 16

Computation of Visual Motion

• Apparent motion: The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession
  
  • Ex. if you show 2 dots fast enough -> it seems like it is moving from one place to another
• Neural circuit does not need real motion in order to fire (motion detection works w/ apparent motion)
• Motion computation is discrete
• Movies!
  
  • These are discrete photos, but the visual system blends it all into apparent motion
• X

A Neural Circuit for Detection of Rightward (part 2)

• 4 dots in receptive field A -> 4 dots appear in receptive field B
• So which dot corresponds with which dot?
  
  • We know that pink dot 1 corresponds to red dot 2
  • Put there’s an aperture in our receptive field
  • So receptive field A only detects pink dots 2,3,4
  • Receptive field B detects red dots 1,2,3
  • In this case, we cannot tell which pink dot corresponds to which red dot

Computation of Visual Motion

• Correspondence problem (motion): The problem faced by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame 1
• Aperture problem: The fact that when a moving object is viewed through an aperture (receptive field), the direction of motion of a local feature or part of the object may be ambiguous
  
  • We can’t tell the motion direction
  • Aperture: An opening that allows only a partial view of an object
• Ex. red square w/ red dots w/ no aperture -> square looks like it is moving diagonally
• • Ex. red square w/ red dots + aperture -> square looks like it is moving vertically
• -> Apertures can make motion information more ambiguous
• Shortest distance constraint biases toward incorrect percepts.
  
  • So the pink dot can correspond to either of the red dots
  • Due to the gestalt rule of proximity, our minds are biased towards the red dot that is closer to the pink dot (but in reality, the pink dot corresponds to the farer red dot)
• What if the square is viewed through more apertures?

Global motion detector

• This is like having multiple neurons
  
  • Grey boxes = 4 receptive fields
  • The 4 receptive fields synapse to the orange neuron (global motion detector)
    
    • Top neuron: says it can be going ↙ ↓ ↘
    • RHS: says it can be going ↗ → ↘
    • Bottom: says it can be going ↙ ↓ ↘
    • LHS: ↗ → ↘
  • The common direction among these 4 arrows is diagonal down motion
  • Global motion detector integrates local motion information
  • • Which structure performs global motion perception?

Question 17

Motion sensitive areas

• Lesions in magnocellular layers of LGN → ?
  
  • Magnocell & color
• Stepping feet motion illusion
  
  • explanation
  • Implication
• Complex cells v1 → what is it sensitive to?
• middle temporal lobe (v5): what is it sensitive to?
• x
• Global motion task
  
  • method
  • Fig A vs B vs C
  • lesion monkey brain ini V5 → ?
  • implication
• Rotatiing ring w/ colo → describe phenom

Answer 17

Computation of visual motion

• Motion sensitive areas:
  
  • Lesions in magnocellular layers of LGN impair perception of large, rapidly moving objects
    
    • Magnocellular layers in LGN
      
      • These cells is color blind
      • Ex you have green dot on red background w/ little luminance contrast but good color contrast -> the motion of the dot seems slower than it is
      • This is b/c the koniocellular system needs to process the motion
    • Stepping feet motion illusion
      
      • When there is a grating, it seems that the yellow and blue rectangles are moving in a stepping motion
      • When the grating is faded, the yellow and blue rectangles are actually moving together
      • Reasons
        
        • # 1: the vertical edges (of the rectangle) tell you about the horizontal motion
        • # 2: sometimes these vertical edges have poor contrast w/ the gratings
          • For yellow, it has poor contrast w/ the white grating
          • Blue has poor contrast w/ the black grating
          • When there is poor contrast, it seems there is a slowdown in motion
        • This shows that the magnocellular cells have little color detection -> it struggles to detect motion when there is little luminance contrast
• Complex cells in V1
  
  • Sensitive to orientation of edges and bars that are orthogonal
• Middle temporal lobe (aka v5): Plays important role in motion perception
  
  • Vast majority of neurons in MT (= V5 = hMT) are selective for motion in particular direction (tuning functions for motion)

Computation of visual motion

• MT/ area V5 -> global motion detection
• Global motion task: (Newsome and Pare, 1988)
  
  • Trained monkey -> monkey indicates the direction
    
    • A: all dots are moving to the right
    • B: 50% of the dots are moving in the same (correlated) direction, 50% are in random (uncorrelated) directions
    • C: 20% of the dots have correlated motion
  • A normal trained monkey can correctly determine the direction with only 2-3% of the arrows.
• When you lesioned a monkey’s V5 area it took 10x more dots for the monkey to determine direction accurately.
• This shows global motion perception in MT
• X
• Why?
  
  • 20% panel is a global motion task b/c we can’t tell coherent motion is horizontal
  • We can see the purple dots are moving horizontally
  • But we need a cell w/ a bigger receptive field to tell us the purple dots are moving right
  • Neurons in MT have large receptor fields
• Global motion detection can suppress info
  
  • When this ring is rotating, the colored dots are not flickering (but actually are)
  • When the ring is not rotating, we can see the dots are changing in hue and luminance
  • So the rotating/global motion suppress the hue and luminance change

Question 18

• Waterfall effect/ Motion aftereffect
• Interocular transfer
• Why does this effect happen after the LGN?
• Why is there MT activity (seen in fMRI) even though there’s no motion?
• x
• First-order motion
• Second order motion
• How are they independent?
• Where are they combined?
• x
• Optic flow
• Focus of expansion
• Focus of constriction
• Biological motion

Answer 18

Computation of visual motion

Waterfall effect/ Motion aftereffect, MAE: The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object

• Existence of this effect implies an opponent process system, like that of colour vision
• Ex. Motion adaptation to downward motion -> stare at blank wall -> perceive things going up
• Interocular transfer: The transfer of an effect (e.g., adaptation) from one eye to the other
  
  • If you look? at motion w/ one eye and adapt to it -> switch eyes, you will see MAE
• This effect happens after LGN
  
  • you receive input from both eyes -> each eye project to diff layers in LGN
  • There is no interaction -> no adaptation for one eye coming from another eye
• fMRI: MAE -> imbalance of neural activity in MT (there’s MT activity even when there is no physical motion, only perception of motion)
• x
• First-order motion: Motion of an object that is defined by changes in luminance
• Second order motion: Motion of an object that is defined by changes in contrast or texture, but not by luminance

Second order motion

• In the pink column
  
  • Frame 1 (black) -> frame 2 (white = inverted the column)
  • So there’s no change of luminance, only changed the contrast
  • This cause flicker from F1 -> F2
• In blue column
  
  • Frame 1 (black) -> frame 2 (white = inverted the column)
  • This cause flicker from F2 -> F3
• This creates apparent motion
• This happens in the real world (ex. animals who camouflage)
• • First and second order motion are independent
    
    • Supported by Neuropsychology: double dissociation
    • Interocular transfer of 2nd order MAE more complete than 1st order MAE
    • 1^st and 2^nd order motion are combined in MT

How do we use motion information to navigate?

• Optic flow: Changing angular position of points in perspective image that you experience as you move through the world (ex driving the car)
  
  • Helps us w/ navigation
• Focus of expansion: centre point where the arrows are leaving from
  
  • Ex. if you see “artificial motion” on the screen, you might move
  • • Focus of constriction: reverse driving (so arrows are point in the opposite direction)
• X
• Biological motion: The pattern of movement of living things (i.e., humans, animals)
  *

Question 19

• Akinetopsia
• Motion-induced blindness
• Motion binding
• Tusi illusion
• Curveball:
• Reverse phi
• Akiyoshi Kitaoki’s Rollers Illusion
• Pinna illusion (same principle)

Answer 19

Motion perception

• The woman who couldn’t see motion
• Akinetopsia: neuropsychological disorder (MT lesions) in which the affected individual has no perception of motion
• Reports to perceive sequences of frozen pictures
• No other visual problems
• • x
• Motion-induced blindness
  
  • When you stare at the centre dot while the “+” are rotating, the yellow spots disappear once in a while
• Motion binding
  
  • When you only see the blue lines, the lines seem to be moving in pairs
  • When the green boxes are added, it seems like the blue lines = a diamond, and the diamond is rotating behind the green boxes
  • Related to gestalt law of grouping/ common fate
• Gestalt law of proximity – Tusi illusion
  
  • Technically, each color dot is moving along a straight line
  • But we see a wheel rotating instead b/c the dots are close together
• • Curveball: integrating two motion directions
    
    • When you look at the grating, it seems like it is moving sideways while moving down
    • However, it is just moving down (no horizontal motion)
• Reverse phi
  
  • We see 4 frames
    
    • First 2 frames: consecutive motion (+ve images)
    • Next 2 frame: same images, but -ve image
  • Physically, after the 2^nd frame, the snowboard goes backward
  • But we seem to see it moving forward b/c there is a switch b/w +ve -> -ve image
  • The moving back becomes difficult to see

Akiyoshi Kitaoki’s Rollers Illusion

• When you make eye movements
• Your retina has to readapt to this image
• Ellipses: black and white outlines -> makes it look like there’s a change in centre of mass -> illusion that the rollers are rolling

Pinna illusion (same principle)

• When you move forward to see this image
• Your retina has to readapt to this image
• Ellipses: black and white outlines -> makes it look like there’s a change in centre of mass -> illusion that the rollers are rolling