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Lecture notes sensation & perception - part 2
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Part 2 from the lecture notes from S&P, from lecture 8 on.
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Lecture notes 2/2Lecture 8: Motion perception
March 15th 2021
Motion sensitivity in humans:
- Very special that there are rarely deficits in this.
- Because colour blindness (8%) and deficit depth vision occur in 7-10%.
- Might be because there are many areas sensitive to motion (V1,V2,V3,V3a,MT,MST)
- Some said this comes from cells in the retina, though there are plenty animals that
show sensitivity but the mechanisms are very different.
Motion sensitivity is very common and robust. But the mechanisms are different in different
animals.
Sometimes objects are defined by their motion, like a ball of little light dots.
Motion processing is independent of object processing.
Often based on gestalt principles, like illusory rotation.
Motion aftereffect = seeing motion in not moving things after looking at motion.
Reichart detector two points that get input, which send it to the same cell. These inputs are
not at the same time when things move.
- Velocity = span/delay.
- In figure 2 = if there is no movement there is inhibition.
- Speed selectivity = some cells respond more to a certain speed.
o Because the delay is longer
o Or the span is made longer.
o If it is the right speed, the signals come to the collector cell at the same time
and thus the signal is given through.
- Same for direction: if the signals come in at the same time the direction is good.
- Small differences from preferred speed or direction will still work.
- If signals cannot be matched it doesn’t work as well, like change in luminance and
movement.
Detector failure = failure to see motion suggest we actually use a mechanism like the
Reichart detector.
For some things we cannot use a Reichart detector
tho.
Reichart detector is based on 1st order motion, based
on luminance and colour.
When something is more based on texture/spatial
frequency it is not the luminance that changes and
thus it is 2nd order motion.
Reichart detector is solely based on luminance and so
doesn’t work for texture changes.
,There is a double dissociation between first and second order motion.
There is less interocular transfer of 2nd compared to 1st order motion.
Different mechanisms with 2nd order processing originate late in the visual hierarchy.
Local motion = V1, small receptive fields → very small movements.
- V1 receives input from the LGN, V2, MT (and maybe much more from later visual
areas as well as non-visual areas)
- Known for it’s orientation selectivity
- About 20% show direction selectivity.
- So where does the motion processing come from?
Local motion in V2 & V3
- Many direction selective responses
- Receive much of their input from V1
- Direction selective properties are similar to V1.
o This can also be send through V1 partly.
Global motion = MT with large receptive fields → integrates local motion parts together.
- MT is also known as hV5, detected in monkeys.
- Input from the superior colliculus, pulvinar and V1-4.
- Perception of motion
- 90% direction selective.
- Integrates local motion input
Complex motion = MST
- Rotation, contraction and expansion for example.
- Much input from MT
- Integrates global motion input.
- Biological motion for example.
o Aids you in object recognition
o Helps you identify activity, species, gender, emotional state and person
identity.
o Is called spontaneous sometimes, but how spontaneous can this be?
o Baby chickens = preference for type of motion, not for chicken form per se.
o This is an innate preference for biological motion, takes years in humans
since objects need to be learned.
Many areas are sensitive to motion, but this is sensation.
Where is the perception?
MT =
- Global motion, by combining local motion signals.
- There is no global motion perception after MT lesions.
- Electrical stimulation of MT-cells → feedback from MT to V1 is crucial
- Microstimulation of MT biases direction judgements.
, o Amount of dots predict the response
o Stimulation leads to more preference for motion coherence.
Patient LM has bilateral damage to area MT. He cannot perceive motion in a normal way, he
sees snapshots from different times.
→ only this problem when you have damage to both
sides like he has.
Ratio model:
- Adaptation to stimulus going off, motion after
effect.
- Combined activity of two cells will cause
perception.
- Ratio of up/down favours up, so after the lil dip,
relative increase.
- Relative increase causes perception.
- Opposites (direction preference) are linked
together and highest activity determine
perception. Ratio between motion signals.
(you want to watch back this part of the lecture or read the book, because these notes
aren’t all there is to it)
MT cells are tuned to the direction of the adaptor show a decrease in activity but oppositely
tuned to not show an increase in activity.
There was some evidence that you did increase activity for oppositely tuned cells, but this
increase in MT activity is actually an attentional effect.
Disinhibition model:
- There is inhibition from one to other, because the opposite cell of the active cell is
inhibited.
- Release from inhibition later will show a lil peak.
o Absolute increase.
o Because down goes below baselines
o Less inhibition from down-cell to up-cell
- In V1 cells tuned to an adaptor show a decrease in response after adaptation,
direction selective cells tuned to the anti-preferred stimulus become depolarized
- Response enhancement after adaptation.
Only disinhibition model includes direct inhibition between motion cells with opposing
direction processing.
… + add picture
Both integrate motion signals from opposite directions.
Look into these models and make sure you can tell what model will predict what motion!!!
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