lecture 8 Flashcards
evolutionary importance of movement
- probably evolved very early
- movement= life
- predators that can detect movement of prey is more likely to catch it
- prey that can detect the movement of predators more likely to survive
- many animals have poor shape, depth and colour perception
- none lack the ability to perceive movement
what are the functions of movement?
1) attracts our attention (active or passive)
2) movement of an object relative to an observer provides information about the objects 3D shape
3) movement provides information that helps us segregate figure from ground and perceptual organisation (common fate)
4) movement breaks camouflage (freeze reflex)
5) movement provides information that enables us to actively interact with the environment
6) informs of your heading and time to collision, your movement as well as other objects
5 ways to make a spot of light move
1) real movement
2) apparent movement
3) induced movement
4) autokinetic movement
5) movement after effects
what problem is highlighted by random dot kinematograms, what does this suggest?
correspondence problem
suggest that motion detection is direct, as we can not imagine a visual system matching point for point over time in these displays
how do random dot kinematograms work? what do we perceive?
instead of presenting each simultaneously to each eye, we now present the first and then the second after a short time lag
this causes us to perceive movement of a stimulus, even though we can not perceive a stimulus in either frame alone
real movement
we perceive movement when the eyes are stationary, so that the image moves across the retina
what happens when an image moves across the retina
it stimulates a series of receptors
what do some neurons in the visual system respond best to?
when a stimulus moves in a particular direction and speed
movement detectors
receptors that are stimulated when an image moves across the retina
what interact to create a cell that only responds to movement from right to left
excitation and inhibition
what happens if you change the order of delay in a Reichardt detector
it becomes specified to movement in the other direction
Reichardt detector
neural circuit that detects movement viewed by a stationary eye , proposed by Werner Reichardt
what does the Reichardt detector consist of
two neurons, A and B
where do neurons A and B (Reichardt detector) send their signal?
to an output unit
what is the function of the output detector?
compares the signals it receives from neurons A and B
It multiplies the responses from A and B to create the movement signal that results in the perception of motion
what is the purpose of delay unit in Reichardt detector?
to slow down the signals from A as they travel towards the output unit
when will a signal be produced by the Reichardt detector?
when the signal passes A first and travels at the right speed, so that the signals from A and B reach the output unit at the same time
what stops a response being produced in the Reichardt detector?
1) when something moves in the wrong direction
2) when something goes at the wrong speed
what happens if you change the spacing of the detectors in a Reichardt detector?
they respond to different speeds
bigger separation detects faster motion
where have Reichardt detectors been found?
insects and frogs
do we have Reichardt detectors? if so where?
we have similar cells in the cortex, which are sensitive to different orientations, speed and direction of movement
what does the aperture problem mean ?
output of all detectors must be integrated at some stage
what is the aperture problem?
that the neurons receptive fields function like an aperture, only revealing a small portion of the scene
can we determine direction of movement through our ‘aperture’?
yes, as correlated movement of random dots increases, neurons in the middle temporal area fire more
monkey experiment on medial temporal lobe
let monkey to learn to point to the direction of movement (press up button when 100% correlated up in aperture)
stimulate monkeys MT neurons without showing visual stimuli
monkey still presses the up button
velocity transposition
show to images moving across the visual field at different speeds can be perceived to moving at the same speed
what is threshold for detecting movement dependent on?
an object and its surroundings
how can you lower the threshold for movement detection?
add vertical lines in space between point A and pint B
what is perception of velocity effected by?
surroundings
the size of the moving object and the framework through which it moves
when can movement detectors not explain motion perception?
when there is no movement on the retina
when you perceive no movement when there is movement on the retina
how there be motion when there is no movement on the retina?
as you follow a moving object with your eyes the eye movements keep the objects image stationary on you fovea
how can there be no movement when an object is moving on your retina?
when you move your eyes or look at different parts of a visual scene/ walk through a scene
what is Helmholtz outflow theory?
if afferent signal (movement signal from retina) cancels out efferent signal (signal to eye muscles), no movement of has occurred. If they do not match, movement is perceived
evidence for Helmholtz outflow theory
1) after images move when our eyes move,
eye muscle movement signal, no retinal movement
2) the world moves when we passively wobble our eyes
retinal movement, no eye muscle movement signal
3) immobilising eyeball results in attempted eye movement, leading to apparent movement of world in opposite direction
eye movement signal, no retinal movement
another name for apparent movement
stroboscopic movement
what is apparent movement?
illusion of movement between two lights by flashing one light on and off, waiting 40-200msec and then flashing another light on and off
what media relies on apparent movement
film: series of static images perceived as movement
apparent movement when delay between images is: less than 30 msec
no movement, simultaneous
apparent movement when delay between images is: above 30- 60 msec
partial movement
apparent movement when delay between images is: about 60msec
optimum movement
apparent movement when delay between images is: about 60-200 msec
beta and phi movement
beta movement
while movement appears to occur between the two lights, it is difficult to actually perceive an object moving across the space between them
phi movement
perceive an object between
apparent movement when delay between images is: above 200msec
no movement, successive
what changes need to be maid to maintain same perception of apparent movement as distance between two lights increases?
time interval or intensity of flashes must be increased
induced movement
surround spot with another object, then move the other object
everyday example of induced movement
sitting on a train, feeling it move backwards, only to realise your train is still and the one next to you is moving forward
autokinetic movement
when the surrounding framework of a room is not visible, the small stationary light appears to move, usually in an erratic path
sherif (1935) autokinetic study results
individually: dot moved from 0.8 inches to 7.4 inches
group: all reported dot moved 4 inches
suggested cause of autokinetic effect
control of eye movements not completely stable in the dark?
movement after effects
if an observer first views a pattern moving in one direction, and then views the spot of light, teh spot and the surroundings will appear to move in the opposite direction
waterfall illusion
anstis & gregory (1964)
depends on movement of stripes across the retina
supports the idea of movement detectors, which respond only to movement across the retina
ratio hypothesis
sutherland (1961) argued that motion after-effects arose from an imbalance in the ratio of the activities form two sets of directionally-tuned receptors, each sensitive to the opposite directions of motion
who produced evidence to support the ratio hypothesis? when?
Barlow & Hill (1963)
event perception
movement provides information about 3D shape, helps us to segregate figure from ground an interact with the environmet
motion blindness
when a global moving pattern surrounds a high contrast stationary target stimulus , the target disappears and reappears alternately for durations of several seconds
