Sensation & Perception 3 Flashcards
What are the figural cues?
regions of the image are more likely to be seen if:
- they are in the front of the image
- at the bottom of the image
- convex
- recognisable
What is the gist of a scene?
- getting an overall impression of what the scene is about
How long do you need for gist perception?
about 40 ms???
Function of motion perception
- help break camouflage
- help attract attention
- help segregate objects from background
- help us interpret events
- help us determine structure of objects
- help us determine what actions people are performing
What happens when camouflaged animal moves?
attention is attracted to it & its camouflage is broken – MOTION ATTRACTS ATTENTION & HELPS US SEGREGATE OBJECTS FROM BACKGROUND
How does motion allow us to interpret events?
by seeing how objects interact, you can infer causality relationships & even social relationships
What is kinetic depth effect?
determining shape of moving object
What is kinetic depth effect?
determining shape of moving object
How does motion allow us to interpret actions?
- static poses are often ambiguous
- when someone moves, their actions & intentions are often made clear
- usually demonstrated using point-light walkers
What are point-light walkers?
- created by placing lights on a person’s joints and having them perform an action
- videoed so only the lights can be see
What is akinetopsia?
inability to perceive motion due to brain damage
E.g. of akinetopsia patient
- L.M. had difficulties pouring a cup of tea, crossing the street, following speech
- could see things had moved but couldn’t see them moving
When do we perceive motion?
- REAL MOTION (actual movement)
- ILLUSORY MOTION (nothing actually moving e.g. rotating snake illusion)
- MOTION AFTEREFFECTS
- INDUCED MOTION – moving background (or moving object) causes a stationary object to appear to move
When does apparent motion occur?
when a series of stationary images are presented in succession to give the impression of motion e.g. 2 separate dots on slide
only works if dots are close together + alternation rate is at a good speed
as separation increases alternation rate needs to decrease –> KORTE’S THIRD LAW OF APPARENT MOTION
What is apparent motion mostly insensitive to?
colour changes
What can colour be used to do?
to disambiguate ambiguous apparent motion
What is induced motion?
- a nearby object or background (usually a large one) either affects the perceived motion of a second object (usually a small one) or causes a second object to appear to move
What is motion aftereffect?
When you’ve been driving down a long, straight road, and you suddenly come to a stop, does the road seem to rush away from you? When you look at a waterfall, then glance away, does it seem as though the scenery is flowing upwards? It’s called motion aftereffect, and it’s a kind of visual adaptation.
EXPOSURE TO MOTION IN OPPOSITE DIRECTION
Motion induced change blindness
- can make it harder to notice changes
- normally colour changes attract attention bc of the transient signals associated with the change (e.g. flicker)
- however when things are moving there are transient signals associated with all objects not just the ones that are changing
- thus attention is no longer drawn preferentially to the changing objects so the changes are not noticed
Motion illusions can inform us of. . .
the processes underlying motion perception
e.g. the footsteps illusion shows us that contrast affects motion perception
How does motion illusion show that contrast affects motion perception?
- higher contrast objects appear to move faster & lower contrast objects appear to move more slowly
- could explain why people often underestimate the speed of cars moving through fog..
CONTRAST MAKES THINGS SPEED UP
What is the aperture problem?
- If you can’t see the ends of a line, the movement of a line is ambiguous.
- Consequently, the motion of the line viewed through an aperture is ambiguous and is “captured” by the movement of the terminators (the points where the line joins the aperture)
What is the Barber Pole illusion?
- lines curve around a cylinder creating virtual terminators that move vertically
- lines appear to move vertically
Function of colour perception
- help search for things
- make judgements
- used to attract mates
- determine if fruit is ripe
opaque object
an object that light cannot pass through
reflects light e.g. if it does not absorb red light it will reflect red light
transparent object
determined by the colour it transmits (e.g. water)
Munsell colour system categorizes according to:
- value (lightness)
- hue (colour)
- chroma (saturation)
Trichromatic theory of vision
- in the retina, there are photoreceptors known as rods & cones
- rods cannot distinguish b/w colours & are active ONLY AT LOW LIGHT LEVELS
- normal light conditions: are NOT active
- (only cones are active in normal light)
LOOK AT RELATIVE EXCITATIONS (for M & L cones since they are similar)
Types of cones
- S cones - 419 nm (blue)
- M cones - 531 m (green)
- L cones - 558 nm (red)
If L cones are most active, the light is primarily. ..
red
if the S cones are the most active, the
light in primarily. . .
blue
What will green light activate? (At 500 nm)
strongly activate M & L
weakly activate S cones
If a patch of light were to activate the S, M, & L cones to the exact same extent, it would look identical to. . .
the test field
metamers
physically different stimuli that appear the same
2 types of colour deficiency
monochromatism
dichromatism
Monochromatism
- usually have no functioning cones ONLY rods
- are truly colour blind & see world in shades of grey
- very sensitive to light & must wear dark glasses
- affects about 1 in 100,000 people
Dichromatism
- lack one of the 3 types of cones
- 3 types of dichromats: protanopes, deuteranopes, tritanopes
Protanopes lack. . .
L cones
- cant distinguish red & green
Deuteranopia lack. . .
M cones
- cant distinguish b/w red & green
Tritanopia lack . .
S cones
- cant distinguish b/w blue & green
What are unilateral dichromats?
people have normal trichromatic vision in one eye but dichromatic vision in the other - they can give us insight into what dichromats actually experience
What colour is the world for protanopes & deuteranopes?
shades of blue and yellowish-green
What colour is the world for tritanopes?
shades of blue & red
If you were to mix red & green light, what would you get?
yellow light
If you were to mix blue & yellow light what would you get?
white light
Opponent-Process Theory of Colour Vision
what colour we perceived is determined not only by the cones in our retina
signals from these cones are processed by the cortex where they are combined into 3 colour opponent channels:
- red-green
- blue-yellow
- white-black
Psychophysical evidence for opponent process theory of colour vision
- afterimages
- impossible colours
What does opponent processing explain?
the existence of afterimages and why some colour combinations are impossible
Colour constancy
the light reflected by an object is determined by the product of its reflectance and the illumination
reflectance x illumination = reflected light
Why does a green sweater still look green when we place it under red light?
- part of the explanation: we habituate to the colour –> become less sensitive to that colour
- DISCOUNTING THE ILLUMINANT
