What can motion perception do?
- help break camouflage
- help attract attention
- help segregate objects from background
(e.g. seeing a moving animal) - help us interpret events (e.g. to see relationships)
- help us determine structure of objects (kinetic depth effect - allows us to see 3D shape)
- help us determine what actions people are performing
how does motion help us interpret actions?
- static poses might be ambiguous
- when someone moves, their actions & intentions are made clear
- made clear using point-light walkers
How are point-light walkers created?
- Point-light walkers are created by placing lights on
- person’s joints and having them perform an
action (e.g. walking!) - They are videoed so that only the lights can be
seen. - Humans are so good at processing motion stimuli, that they can guess what action the person was performing.
Condition where u cannot perceive motion
akinetopsia
- Sometime due to either disease or trauma, a patient will suffer damage to a part of the brain responsible for motion perception.
e.g. LM could see that things had moved but couldn’t see them MOVING
When do we perceive motion?
- real motion (something actually moving)
- illusory motion (nothing actually moving)
- motion aftereffects
- static image
- apparent motion
- induced motion (moving background or moving object causes stationary objects to appear to move)
Rotating snake illusion
the percept of motion is caused by the
contrast between the colours – but it is unclear why these contrast difference cause the percept of motion
apparent motion
occurs when a series of stationary images are presented in succession to give the impression of motion
- only works if dots are sufficiently close together
- colour can be used to disambiguate ambiguous apparent motion
for apparent motion to occur, as separation increases. . .
alternation rate needs to decrease
Korte’s Third Law of Apparent Motion
What is motion aftereffect?
After staring at the waterfall
for several minutes, he then
shifted his gaze to the
rockface which appeared to
move upwards.
what is induced motion?
A nearby object (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 induced blindness? (Bonneh et al, 2001)
Although motion can make things more visible (e.g. breaking camouflage), it can also cause things to disappear, as in motion induced blindness
What is motion induced change blindness? (Suchow & Alvarez, 2011)
- motion can make it harder to notice changes
- In the following demo, when the dots are stationary, it is very easy to notice them changing colour.
- When they move, it is much more difficult to notice that they change colour
How does motion induced change blindness work?
- Normally colour changes attract attention because of the transient signals associated with the change (i.e. the “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.
What does the Footsteps illusions show?
it shows that contrast affects motion perception as higher contrast objects appear to move faster and lower contrast objects appear to move more slowly
- could explain why people often underestimate the speed of cars moving through fog
What is the Aperture problem?
- if you can’t see the ends of a line, the movement of a line is ambiguous
- the motion of the line viewed through an aperture is ambiguous & is ‘captured’ by the movement of the terminators (the points where the line joins the aperture)
Barber Pole illusion
- In this illusion, lines curve around a cylinder creating virtual terminators that move vertically.
- Thus, the lines themselves appear to move vertically.
What is the function of colour perception?
- colour vision evolved to help search for things
- colour perception can also make judgments
- attracting mates
- spot poisonous animals
What is an opaque object?
an object that light cannot pass through - it is not at all transparent
- The colour of an opaque object is determined by the light that it reflects
What determines the colour of a transparent object?
- it is determined by the colour it transmits
- if an object absorbs blue but transmits red, then it will appear red
blue paint aborbs. . .
red light but REFLECTS blue & green light
yellow paint absorbs. . .
blue light but reflects red & green light
mixing blue & yellow paint results in. . .
a mixture that absorbs both red & blue light but REFLECTS green light - so looks green
mixing red & green light makes. . .
yellow light
mixing blue light with yellow light makes. . .
light that contains blue, green, & red – this light will look WHITE
