Motion perception- week 10

MBB1

Break Camouflage, Attract Attention, Segregate Objects

• A camouflaged animal can be virtually invisible – until it moves.

• When it moves, attention is attracted to it and its camouflage is broken!

• So, motion attracts attention…

• …and help us segregate objects from the background.

Interpret Events

• Motion can also allow us to interpret events

• By seeing how objects interact, you can infer causality relationships and even social relationships.

• In the following video a large triangle gets into an argument with a small triangle and a small circle.

• The large triangle chases off the other two shapes before destroying the house.

• The entire story is conveyed using motion info

Structure From Motion

• Motion can also allow us to infer structure

• It can help us determine the shape of a moving object

• This is sometimes referred to as the “kinetic depth effect”

• In the following video the 3D shape of a number of objects will be revealed by their motion.

• Note that when these objects are stationary, their 3D shape information is unclear.

• Motion allows us to infer 3D shape

Interpreting Actions

• Motion can also allow us to interpret actions

• While static poses are often ambiguous…

• …when someone moves, their actions and intentions are often made clear.

• This phenomenon is typically demonstrated using point-light walkers

Point-Light “Walkers”

• Point-light walkers are created by placing lights on a 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

Summary: Function of Motion Perception

• Motion perception can

  • Help break camouflage

  • Help attract attention

  • Help segregate objects from the background.

  • Help us interpret events.

  • Help us determine the structure of objects

  • Help us determine what actions people are performing

Life Without Motion Perception

• Sometime due to either disease or trauma, a patient will suffer damage to a part of the brain responsible for motion perception.

• Consequently, the person may no longer be able to perceive motion.

• This condition is known as akinetopsia.

Life Without Motion Perception- akinetopsia

• Patient L.M. had akinetopsia

• Consequently, L.M. had difficulties

  • Pouring a cup of tea

  • Crossing the street

  • Following speech

• Essentially, L.M. could see that things had moved but couldn’t see them moving

Life Without Motion Perception - phenomena

• While this may seem strange, we have all experienced this phenomenon.

• For example, at the beach, you may have seen a ship on the horizon.

• It may be so far away that it appears not to be moving.

• But when you glance up a few minutes later, you are surprised to see that it has moved.

• This is how L.M. experienced everyday life – she constantly discovered that things had moved but could not perceive them to moving

When Do We Perceive Motion?

• Real motion (something actually moving)

• Illusory motion (nothing actually moving)

  • Static image (e.g. rotating snakes illusion - Kitaoka & Ashida, 2003)

• Apparent motion

• Motion aftereffects

• Induced motion

  • Moving background (or a moving object) causes a stationary object to appear to move

Rotating Snake Illusion

• We don’t really know why this particular static illusion gives the impression of motion

• Clearly, 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

• For our current best guess, see Faubert, J. & Herbert, A.M. (1999). The peripheral drift illusion: A motion illusion in the visual periphery. Perception, 28, 617– 622

Apparent Motion

• Apparent motion occurs when a series of stationary images are presented in succession to give the impression of motion.

• For example, if I alternate between the two slides below, you wouldn’t see two separate dots.

• Instead, you would see a single dot moving from left to right

• Apparent motion works only if the dots are sufficiently close together

• If they are too far apart and the alternation rate is too fast, motion perception ceases to occur.

• Instead, the percept will be of two separate flashing dots

• For apparent motion to occur, as separation increases alternation rate needs to decrease.

• This is known as Korte’s Third Law of Apparent Motion (Korte, 1915)

• Curiously, apparent motion is mostly insensitive to colour changes

• However, colour can be used to disambiguate ambiguous apparent motion

• In the following demonstration the green dot will appear to move from top-right to bottom left and back again

Motion Aftereffect

• Image of Fall of Foyers near Loch Ness in Scotland where Robert Adams (1834) first experienced the waterfall illusion.

• After staring at the waterfall for several minutes, he then shifted his gaze to the rockface which appeared to move upwards

Induced Motion

• 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.

Motion Induced Blindness

• Although motion can make things more visible (e.g. breaking camouflage), it can also cause things to disappear, as in motion induced blindness (Bonneh, Cooperman & Sagi, 2001)

• Another motion-related phenomenon we don’t have a good explanation for

• Motion can also 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

Motion induced blindness - how it works

• 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.

Motion Illusions

• Motion illusions can inform us of the processes underlying motion perception

• For example, the Footsteps Illusion shows us that contrast affects motion perception.

• As the yellow and blue rectangles traverse the striped background, they appear to speed up and slow down asynchronously

Footsteps Illusion

• Contrast at leading edge of yellow rectangle is low

• Contrast at leading edge of blue rectangle is high

• Contrast at leading edge of yellow rectangle is high

• Contrast at leading edge of blue rectangle is low

Footsteps illusion shows us

• The Footsteps Illusion shows us that contrast affects motion perception.

• Higher contrast objects appear to move faster and lower contrast objects appear to move more slowly

• This could explain why people often underestimate the speed of cars moving through fog…

• …a phenomenon that sometimes causes accidents

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)

• When the terminators move horizontally, the line appears to move horizontally.

• When the terminators move vertically, the line appears to move vertically.

• When we can see that actual ends of the line, we can see its

Barber Pole Illusion

• Same principle applies in the 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