Motion Perception Flashcards
What is the evolutionary importance of motion perception?
- Movement = life
- Predators that can detect movement of prey more likely to catch it
- Prey that can detect movement of predators are more likely to survive
- Many animals have poor depth, shape and colour perception
- NONE lack the ability to perceive movement
What are the functions of motion?
- Movements attracts our attention (wave) (active or passive)
- Movement of an object relative to an observer provides information object’s 3D shape
- Movement provides information that helps segregate figure from ground and perceptual organisation (common fate)
- Movement breaks camouflage (freeze reflex)
- Movement provides information that enables us to actively interact with the environment. Ball games
- Informs of your heading and time to collision, your movement as well as other objects
Do we need to be able to recognise an object in order to see it move and do we match edges and contours between successive views of an object? What provides evidence for this?
- Random dot kinematograms suggest not (the above) – motion analogs of Random dot stereograms
- Instead of presented each simultaneously to the right and left eye, we now present the first and then the second after a short time lag
- Here we perceive a central square to move rightward, even though we cannot perceive a square in either frame alone
- The correspondence problem highlighted by RDKs suggest that motion detection is direct
What are 5 ways to make a spot of light move?
- Real movement
- Apparent movement
- Induced movement
- Autokinetic movement
- Movement aftereffects
What is real movement?
- Light physically moves, i.e. from place A to place B
- We perceive movement when the eyes are stationary, so that the image moves across the retina
- When an image moves across the retina, it stimulates a series of receptors
- There are neurons in the visual system that respond best when a stimulus moves in a particular direction
What are movement detectors?
When an image moves across the retina, it stimulates a series of receptors
Excitation and inhibition interact to create a cell that responds only to movement from right to left but does not respond to movement from left to right
- Receptor cells can also detect movement of a specified direction and speed – if it receives two signals in close enough succession it will fire
Give features of the speed movement detectors
When something moves in the wrong direction there will be no response
Bigger separation of receptors detects faster motion
Detectors such as these have been found in insect sand frogs
We have something similar
Cells in the cortex are sensitive to different orientations, speed and direction of movement
What is the aperture problem?
means output of all detectors must be integrated at some stage. This is a problem as there are lots of types of motion that can cause the same receptors to be activated. Brain must integrate activity of all the detection receptors.
what is the threshold for detecting movement in real movement and what does it depend on?
- How quickly an object needs to be moving for you to be able to detect it
- Depends on object and surrounding
What is the perception of velocity in real movement affected by?
Affected by surrounding plus the size of both the moving object and the framework through which it moves
What can’t movement detectors explain?
- Cannot explain movement perception when:
1. There is no movement on the retina – as when you follow a moving object with your eyes so your eye movements keep the object’s image stationary on fovea
2. When you perceive no movement when these is movement on retina – as when you move your eyes to look at different parts of the scene or as you walk through a scene
What is Helmholtz’s outflow theory?
- Efferent (outgoing) signal to eye muscles from brain
- Efferent copy of eye muscle commands
- Afferent (ingoing) movement signal from retina
- Brain comparator: if afferent signal cancels out efferent signal ‘no movement’ of the world has occurred; if there is a mismatch, movement is perceived.
- If there is a difference between muscle movement command and movement of image across the retina then we perceive movement
How does the outflow theory work in regards to tracking the motion of a car and when we look around the world?
- when tracking car, eyes move but retinal signal remains stationary, therefore perceive movement of the car
- When we look around the world, eye movement command and retinal image movement are equal so we perceive NO movement
Give evidence for Helmholtz’s outflow theory
- Afterimages move when we move our eyes (eye muscle movement signal no retinal movement)
- The world moves when we passively wobble our eyes (retinal movement, no eye muscle movement signal)
- Immobilizing eye-ball results in attempted eye-movement leading to apparent movement of world in opposite direction (eye movement signal, no retinal movement)
What is apparent movement?
- Illusion of movement between two light by flashing one light on and off, waiting between 40 & 299 msec, then flashing other light on and off
- Perception of movement in film = series of static images
- Less than 30 msec = no movement, simultaneously
- Above 30=60 msec = partial movement
- About 60 msec = optimum movement
- About 60-200 msec = beta and phi movement (phenomenon) phi: while movement appears to occur between the two lights, it is difficult to actually perceive an object moving across the space between them. Beta perceive an object between
- Above about 200 msec = no movement, successive
- Slow apparent motion can be ambiguous
