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
