Lectures Sensation and Perception Chapter 8 - 15
Lecture 8 – Motion perception & processing
Today
• Motion sensitivity
o In humans and other animals
• What is motion?
o Do we need objects?
• Motion detectors
o How to detect motion
• Motion types
o Different motion types processed in different areas
• Motion perception
o Ratio model
o Disinhibition model
o Distribution Shift model
Motion sensitivity
• Every visually guided organism detects motion, but not:
o Color
o Stereo (depth)
o Form
It is quite special.
Motion sensitivity in humans
• In humans
o 8% of male population congenitally R/G color blind (due to limitations in their
photoreceptors)
o Depth (stereo vision) does not work in about 7-10% of people
▪ Most people become aware of their ‘deficit’ only when explicitly tested for
stereo vision, because there are so many other cues for perceiving depth
(shadows, size on the retina).
o It is really rare to not perceive motion. Only a few people have it.
• Many areas are motion sensitive
Motion sensitivity is: A cell shows a preference for something
that moves compared to something that does not move. To
be motion sensitive, it needs to respond to a particular
direction, it has to have a preference for a particular
direction. More response than to a static image. A lot of cells
will fire when there is a sudden onset.
Motion sensitivity in different species
• Retina:
o Rabbits
o Frogs
o Turtles
o Birds
o Squirrels
▪ Not all same mechanisms
▪ Not in humans
,Motion sensitivity
• Motion sensitivity is very common
• It is hard to destroy it; it is robust
• Different mechanisms in different animals
o So how does it work in humans?
▪ What is motion anyways?
What is motion?
Motion & Objects
• Change in position of an object?
You only see an object when it is moving?
(They are moving in the video). You don’t need an object. Motion
processing proceeds object processing (or is independent of it).
• Illusory rotation (also a video, they are moving towards the
center and back to the edges.
So what is motion? Video
We don’t infer motion. If you can adapt to it, there is a cell to it.
Motion & Objects
• Objects can be based on motion
• Motion perception is not necessarily based on objects
o So what is it based on?
,Reichart detector
We have two points of input to cells that receive input and they
converge their information together in a collective cell (X in this
image). In this example it responds to light on the left and later light
on the right (relative to its center). But, the upper left cell signals
towards the summation site are delayed in time. This means that
activation that is triggered from here ends up at this location at the
same time as information from this one, because there is a delay
mechanisms.
So basically, we have two components: we have the distance between the two points of the input
and the amount of the delay. With these parameters, we get a direction selectivity and a speed
selectivity.
This is a simplified version of a Reichart detector.
• Inhibition if it doesn’t move
This one above is a more extended version of a Reichart detector. A
Reichart detector would also respond to a static stimuli. If you take the
more complete model, you see that when you have an input here, which is
normally delayed, information from the left to the right. This one would be
delayed, and this one would not be delayed. It would activate the cell.
However, point 1 also goes over here and inhibits the cell (the summation
site). So, if the stimulus that triggered the first activation that is going to be
delayed stays in the same place, it would also inhibit the output.
Speed selectivity
Any cell that likes motion will have a preferred speed, to which is can respond and the other way
around. Two ways to achieve selectivity for a particular speed:
1. Change the delay. If there is a longer delay, there is more time to travel the same distance for the
stimuli, so it is moving slower. It would be preferring slower speeds.
2. Change the span. If the distance between the two input sites is increased, but the delay is the
same, more distance can be traveled in the same time. It would be preferring higher speeds.
Speed selectivity
This is an animation. If it is the wrong speed, it will show up on
different times. Than the membrane potential is restored by the
sodium potassium pump. You can’t get any response anymore,
the signal fizzles.
, Direction selectivity
When it is in the right direction, the delays are in the right location, they end up in the same position
at the same time. But when it is in the wrong direction, they end up in different times and the signal
is lost.
Reichart detector tuning
• The Signal need to reach the collector site before sodium / potassium pump restores the
membrane potential
o Small differences from the preferred speed or direction will still work, will still trigger
a respond in the collector site
o Tuned responses
Detector Failure
The Reichart detector is based on luminance, and it matches to
the inputs. Now if the inputs change over time, it can’t match
them. We use a concept like this Reichart detector.
Detector Failure
• The failure to see motion (to some degree) suggests we actually use a mechanism like the
Reichart detector, it is very sensitive to having a particular luminance signal and at a second
point in the time. If the input changes, this is a problem, because the second selector won’t
pick up the signal.
Detector Problems
• The ability to see motion suggests we actually use
additional mechanisms beyond the Reichart detector.
This is called Second Order Motion.
We can still see it, because the spatial frequency in this
background is different from this spatial frequency in this
object. This is referred as second order motion.
