SENSATION AND PERCEPTION LECTURES PART 2
LECTURE 8 MOTION PERCEPTION
Every visual guided organism detects motion, but not color, depth and form. Why are there so few motion
perception related problems? Because many areas of the brain are motion sensitive.
- V1
- V2
- V3
- V3a
- MT
- MST
Therefore you need quite a lot of damage to lose the ability to perceive motion. Is there one motion system?
Do all motion sensitive animals process motion via the same mechanism? No you don’t. So this system seems
to be evolved in multiple ways. So motion perception is common, but different mechanisms in different
animals.
SO how does it work in humans?
What is motion anyway? a change of an object in time. In the real
world this change is continuous. Objects can be defined by motion.
We have input from one location but later in time in another location.
But, this will also respond to a stationary object.
Reichart detector:
- Inhibition if it doesn’t move
- Inhibition for flicker (on/off)
- Signal cancels out due to inhibition when the stimulus doesn’t move
Speed selectivity:
- Change the delay: a longer delay means more time for the signal to reach the summation point. This
also means that the model is tuned to lower speeds.
- Change the span.
Detector failure:
, - Signals can’t be matched based on luminance/color anymore.
- The failure to see motion suggests that we actually use a mechanism like the Reichart detector.
- The ability to see this kind of motion suggests we use additional mechanisms beyond the reichart
detector (video of black and white moving bar).
o Second order motion!
1st order: motion based on luminance
2nd order: not based on luminance
- Double dissociation between first and second order motion
- More interocular transfer of 2nd compared to 1st order motion.
o Note: later brain structure show more interocular transfer
- Different mechanisms with 2nd order processing originating later in the visual hierarchy
Local motion requires small receptive fields.
Local motion in V1:
- Receives input from LGN, V2, MT
- Known for it’s orientation selectivity
- About 20% show direction selectivity.
Local motion in V2-V2
- Many direction selective responses
- Receives much of their input from V1
- Direction selective properties similar to V1
Aperture problem: viewed through a small aperture, motion signals can be ambiguous.
Large receptive fields capture global motion signal. Small receptive fields provide local information. Local
motion signal differs from global motion signal.
Global motion:
- MT
- Large receptive fields
The idea is that we integrate al the local motion together to perceive global motion.
Global motion in MT:
- Input from superior colliculus, pulvinar and V1-4
- 90% of the cells are direction selective (motion processing cells)
- Large receptive fields
o Integration of small receptive fields
- Associated with the perception of motion
Complex motion : medial superior temporal area. (MST)
Expansion and contraction are thought to be processed relatively late in the system. Rotation is thought to be
processed in the same areas as expansion and contraction.
Complex motion in MST;
- Medial superior temporal area
, - Rotation, expansion, contraction
- Integration of local and global motion signals
o With different directions and speeds.
Biological motion:
- Aids objects recognition
o Identify the activity
o The species
o The gender
o Emotional stat
o Person’s identity
- Innate preference for biological motion
o Objects need to be learned sensitivity to motion defined form and biological motion may take
years
Biological motion perception from Point-Lights is a spontaneous and universal perceptual ability, occurring
both inside and outside traditional laboratory.
Chicks prefer biological motion, even if form information is removed form it. But they do not use their ability to
detect biological motion very wisely. They also chose biological motion of cats over non-biological motion.
MT and motion perception:
- Global motion:
o Combine local motion signals to code the global direction of motion
- Motion perception:
o No global motion perception after MT lesions
Motion sensitivity in humans:
- Patient L.M.
- Bilateral damage to area MT
- LM sees static images, instead of motion perception.
The motion after effect shows you pure motion signals. There is no shift in
position over time and still we perceive motion. The waterfall illusion,
Ratio model: the motion direction perceived is thought to reflect the
ratios of firing rates of differently tuned motion detectors. After
adaptation, decreased responsiveness of cells coding the adaptors’
direction would change the spiking ratios in factor of the opposite
direction when viewing a stationary pattern. Note that none of the
direction selective cells are active above baseline. The percept of motion
is thus thought to involve only a decrease in responsiveness.
Support for ratio model: MT cells tuned to the direction of the adaptor show a
decrease in activity but oppositely tuned cell do not show an increase in activity.
An alternative explanation involves disinhibition. This theory suggests motion- tuned
cells for opposite directions inhibit each other. Consequently, the attenuation of the
baseline-spiking rate of neurons tuned to an adaptors’ direction results in release of
inhibition cells tuned to the opposite direction of motion. In this theory, it is then
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