Transcript COLOUR - Department of Psychology

Psy280: Perception
Prof. Anderson
Department of Psychology
Vision 6
Colour, depth and size
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Need for colour
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Some tasks are impossible without it
Can you find the word?
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COLOUR: What's it for?
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Identification / discrimination
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Detection (non-detection)
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Detection
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Potential mates, enemies, prey
Camouflage
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What are colours?
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Light varies in both intensity and wavelength
Light of different wavelengths appear as different colours
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COLOUR: ATTRIBUTES
THIS IS
NOT RED!
It is 690nm
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Colours don’t exist – they’re in our heads!
Psychological property
Interaction: physical light - nervous system
There are no color, just wavelengths…
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Newton’s dorm room
experiment
Light through prism
= rainbow
 Why?
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Diff wavelengths have diff
refractory properties
Long (red) bent least, short
(blue) most
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COLOUR: ATTRIBUTES
Isaac Newton (1666): “colour” of light.
 White light (sunlight) = sum of components
 Individual component = different colour exp.
 Colour = wavelengths subtracted from light
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Redux:
Do wavelengths have colour?
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“The Rays to speak properly are not coloured. In
them there is nothing else than a certain Power and
Disposition to stir up a Sensation of this or that
Colour…” Newton
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Different sensory system would result in different
rainbow
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SHORT
400-450nm violet
450-490nm blue
MEDIUM
LONG
500-575nm green 590-620nm orange
575-590nm yellow 620-700 red
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Spectral reflectance curves
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See objects = light reflected from them
Reflectance curve
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Achromatic colour: equal reflectance across wavelengths
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White, black, grey
Chromatic colour: selective reflectance across wavelengths
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Not all light the same
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Different light sources have differing spectral
composition
Sunlight: White
light bulb: Yellow/red
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Additive and subtractive
mixing
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Lights mix additively
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more wavelengths = closer to white (like sunlight)
Pigments mix subtractively
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more wavelengths = closer to black
Subtractive
Additive
B absorbs Y
Y absorbs B
B & Y commonly reflect green
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How many colours can we
perceive?
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~2,000,000= 200 hues x 500 brightness levels x
20 saturations levels
Hue
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Brightness
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wavelength
amplitude of wave = intensity
# of photons
Saturation
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Degree of white
RED vs PINK
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Trichromatic theory of colour
perception
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2 theories from the 1800s based on
psychophysical data
Trichromatic theory of colour vision
Young and von Helmholtz
Colour-matching experiments
 Mix 3 pure lights (420, 560, 640) until
matches another light (500nm)
 Conclusions: able to duplicate colour by
adjusting proportion
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Trichromatic theory of colour
perception
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Trichromatic theory (cont’d)
Colour vision depends on 3 receptor
mechanisms with different spectral
sensitivities
 Particular wavelength stimulates 3
mechanisms to different degrees and pattern
of activity in 3 mech = perception of colour
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Trichromatic theory: Physiology
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Physiology – a century later…
3 cone visual pigments with different
absorption:
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Short: 419nm
Middle 531nm
Long: 558nm
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Colour: Its all in the ratio
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Perception of colour depends upon ratio of excitation across
receptors
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Metamers
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Lights that are physically different can look
identical
How so?
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Ratio of excitation across receptors is =
Same colour despite different wavelengths
Explains colour-matching experiment
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Although both lights have different wavelengths, they
perceptually look the same
Metamers look the same because generate same
activation responses in 3 types of cone receptors
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Principle of univariance
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Do we need 3 receptors?
What about 1?
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NO, not possible due to
principle of univariance
Varying intensity (# of
photons) can allow to have
same # of isomerized
molecules of pigments
This is why we don’t see
colour in dim light,
because rely on one ROD
pigment
What about 2?
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YES but fewer colours
(see text)
More confusion btwn
colours
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Opponent process theory of
colour
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Ewald Hering
Opposing responses generated by blue and
yellow and by red and green.
 Phenomenological observations
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Afterimages
 Simultaneous color contrast
 Can’t picture reddish-green or bluish-yellow
 Colour-blind: red+green; blue-yellow
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Afterimages
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Afterimages and
simultaneous colour contrast
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Colour opposites
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Opponent process:
Colour appearance
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Rating of colour
experience for
different
wavelengths
Little co-occurrence
 Reddish-green
 Bluish-yellow
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Opponent-theory
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3 mechanisms: respond in opposite ways to
intensity and wavelength
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Black (-) | white (+)
Red (+) | Green (-)
Blue (-) | Yellow (+)
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Physiology: Opponent neurons
in retina and LGN
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Signals from cones are transformed
early.
M retinal ganglion cells are
achromatic • dark - light
P retinal ganglion: centre / surround
are sensitive to different wavelengths
of light
• red – green • blue - yellow
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Architecture of opponent
cells
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Dual process theory
L+M–
S+ A- (sum M&L)
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Colour and lightness
constancy
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Pure wavelength information insufficient to
explain colour perception
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Luminance insufficient
to explain lightness
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Wavelengths and colour
perception
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V1
Selective for the wavelength of light
 However, precise wavelength of light often
bears little relationship to the perceived colour
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V4
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Neurons behave as if they are responding to
colours as seen by human observers
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10 minute break
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Depth
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Of feeling? Knowledge?
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Space
3D world —>2D projection on retina—>
3D perception
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Need to “reconstruct” 3D world
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Flatland:
A romance of many
dimensions
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Edwin Abbott (1884)
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A point, a line, a cube
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How do we reconstruct
depth?
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3 sources of information
 Extraretinal oculomotor cues
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Physiological/muscular feedback
Monocular cues
Pictorial
 Can be recovered from one eye
 Lots of them
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Binocular
Disparity
 2 eyes, 2 views of the world
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Oculomotor
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Afferent feedback from body
Vergence
“Convergence”
 Degree of crossing as eyes
fixate
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Near vs far
Accomodation
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Stretching of lens to focus light
on retina
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Monocular depth
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Are 2 eyes better than
1?
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Yes
Are 3 eyes better than
2?
Not many one eyed or
three eyed creatures
Nonetheless, can see
depth with 1 eye
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Monocular cues:
Linear perspective
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Parallel lines converge with distance
Converge at vanishing point (horizon)
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Monocular cues:
familiarity and relative size
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2 objects are of equal
size (familiarity)
Smaller retinal
projection—>further
away
World: Same size
Retina: Different size
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Monocular cues:
Relative height and shadows
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Monocular cues:
Occlusion
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Layers of depth stretching out to horizon
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Monocular cues: Atmospheric
blur and depth of focus
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Blurriness
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Haze
Depth of focus
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In front and
behind of
fixation
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Monocular cues:
Combine to form depth
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Occlusion, relative height, and shadows
Impossible:
Conflicting cues
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Monocular cues:
Dynamics cues
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Motion parallax
Velocity = distance/time
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Km/hour
As observer moves
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Objects closer move faster
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Greater distance across retina
Objects further move slower
E.g. looking out a train window
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Binocular cues: Stereopsis
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Why have two eyes?
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Shared field of view (FOV)
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Not just more = better
2 overlapping but distinct visions of the world
Sacrifice: 360 degree FOV
Gain: depth through horizontal disparity
Predators (overlap) vs prey (larger FOV)
No overlap
Substantial overlap
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Stare at your thumb
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One eye at a time
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Horizontal disparity
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Thumb moves side by
side
2 very different
perspectives on world
Vertical disparity?
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Horopter:
An isodepth sphere
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Horopter
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Uncrossed disparity
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Fixate on an object
An imaginary sphere that
defines corresponding points
on the retinas
Zero disparity
Nasal of fovea
Further in depth
Crossed disparity
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Temporal of fovea
Closer in depth
Uncrossed
Fixation/
zero disparity
Retinas
Crossed
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Remember? LGN retinal layers
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Organization of LGN: Retinotopy
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6 representations of retina in register
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How do we know steropsis
produces depth perception?
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Depth perception may depend solely on
“knowledge”
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Monocular cues
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Occlusion, familiarity etc.
Retinal disparity vs knowledge
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Depth without awareness of form?
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Random dot stereograms
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Stereoscope
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L & R eye shown
separate images
Random dots with
invisible disparities
Disparity alone can
result in depth
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Stereoscope
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Wheatstone
Crossed
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Magic eye: Autostereograms
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3D movies: Anaglyphs
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Color filters project
different images to each
eye
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Disparity representations in
the brain
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Can’t happen at the ganglion cell layer
V1 ocular dominance columns
V1 has neurons tuned to retinal disparities
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Part 2: Perceiving Size
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Not as simple as size of stimulus on retina
Visual angle: retinal projection depends on
distance
Different physical
size
 Same retinal
Projection
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Bigger stimulus further away
= closer smaller stimulus
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Size constancy
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Perception of size remains constant
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Despite different visual angle/retinal size
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Size distance scaling
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Perceived size = retinal image size X
distance from object
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Without depth information
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Perceived size = retinal image size
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Emmert’s law
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Perceived size of an after image depends
on depth perception
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Size-depth illusions
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Moon appears larger on the horizon than the sky
Same retinal size
 Difference in magnitude
estimation
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Horizon provides depth cues
Sky does not
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Appear flattened
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The end
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