Motion Flashcards
where is motion processed?
in V1, and the dorsal stream
why is it important to understand motion?
important to understand how we perceive motion, as this is a survival necessity important in evolution and modern environments
where does motion processing begin?
in the retina, into the LGN, and then into the cortex
each successive area processes more complex motion patterns
motion processing in V1/2
small receptive fields.
cells respond to simple stimuli and linear motion, but are not tuned or specialised.
motion processing in V3
larger receptive fields and specialised for motion of complex stimuli, e.g., texture.
motion processing in V5/MT
large receptive fields that respond to motion of anything.
tuned for complex direction and speed, and respond to vestibular cues.
motion processing in V6
responds to self-induced motion
motion processing in MST
identical to V5, but also respond to static images that imply motion, rather than just simple patterns of motion.
what does differentiating between static and moving objects suggest?
we can incorporate information about our own movements when interpreting information in the environment
led to two theories generated
sherrington’s eye muscle signal theory
incorporating actual muscle movement information
helmholtz’s brain signal theory
intention to move based on outflow signals sent to muscles
how are object motions perceived?
relative to the motion of our body, and copies of self-motion signals are sent to motion processing areas of the brain
what are time delays required for?
to detect motion happening at the same time
what does combining information between areas allow?
for a sense of relative motion – this is called an opponent-motion circuit
what do opponent-motion detectors respond to?
the balance between motion cells preferring opposite directions
apparent motion
interpreting a set of still images as smooth motion, as long as the time interval and physical displacement are not too large between points A and B
wagon wheel illusion demonstrates…
how increases in speed cause changes in the perception of repetitive motion, as larger displacements between faster movements make them harder to link
this results in movements being connected to the closest one, which can generate counterclockwise
when does motion sensitivity develop?
10-12 weeks, however certain types of motion, e.g., looming stimuli (things that might collide with the child) are detected even earlier around birth – however these may be later lost and have to be relearnt
damage to MT
akinetopsia (motion blindness)
integration of smooth motion breaks down
MT neurons are sensitive to specific direction
damage to V1
functionally blind
damage to V3
impedes motion perception
damage to MST/V6
inhibits navigation
MT and speed
MT has specific speed-sensitive neurons, excited for different degrees of speed
what can affect estimates of speed?
physical distance from the object being recorded
how do neurons measure speed?
degrees per second, rather than actual movement of the item in the environment
waterfall illusion (addams, 1834) suggests…
the systems used to identify and encode positions are separate to the systems used for motio
what do motion cells in V1 respond to?
image contrast and speed of stimulus
what are motion after-effects?
visual illusion experienced after viewing a moving stimulus, and then fixating on a stationary stimulus, which appears to move in the opposite direction to the original
how do motion after-effects work?
- perceived motion is encoded by a population of cells to generate a neural code
- after prolonged adaptation to a certain direction, these cells reduce their output
- the neural code for a subsequent static stimulus will be biased in favour of cells that normally respond to the opposite direction of that movement
evidence of adaptation in perceptions of speed over time
RGC neurons increase dramatically when motion first occurs, but then drops down in excitement
once motion is finished, the neuron inhibits below initial baseline activity
evidence of adaptation in perceptions of speed over space
fovea is sensitive to changes in speed – important for processing information, however after 10s of movement in the periphery, we stop seeing this motion entirely
when are speed judgements more accurate?
before adaptation, than afterwards
what happens to motion in the periphery?
fades over time
principle of univariance
cells cannot differentiate between speed and contrast
since motion cells can only have one output, they cannot signal two different things, e.g., contrast and motion
how can PoU affect perceptions of speed?
if neurons are driven by more than one input, they are unable to differentiate between high-low or low-high speed and contrast
can perceived speed reduce with reduced contrast?
for some stimuli, e.g., looming and receding objects
motion contrast in MT
their centre-surround cells activate to motion in the centre, but are inhibited if the same motion occurs in the periphery
therefore, cells are only excited when there is the same direction of motion
snow blind illusion
shows how motion-contrast cells can affect speed and motion perception, as motionless stripes covering background excite the motion-contrast cells and suggest higher speed
when is induced motion and vection common?
when motion is slight and acceleration is smooth, by creating uncertainty as to whether it is us moving or the environment
drum experiment
shows the sickness reflex is caused by vection due to conflict between visual and vestibular (used for balance) cues
illusory motion (leviant, 1981)
the sensation of motion in a static stimulus
MT cells are active when viewing the image stripes which over-excite the orientation cells and pass a motion signal
aperture problem
involves only seeing motion at right angles to the edge, when viewing a moving stimulus through a small window (aperture)
distorts the true direction of motion perception
what do motion cells in V1 only respond to?
short edges
meaning they can only signal motion at right angles to their preferred orientation
cells in MT must integrate these signals to derive the true motion, but they can be fooled.
