Lecture 3 Notes: Motion Perception

Welcome to Lecture 3, focusing on motion perception.
The lecture begins with a review of the answers to questions from the previous lecture to reinforce active learning.

Definition: Cues that cause us to segregate part of an image and perceive it as a figure.
List of Four Figural Cues:
- In Front: The part of the image that appears in front is usually seen as the figure.
- Bottom: Elements at the bottom of an image tend to be perceived as the figure.
- Convexity: Convex shapes (bulging outwards) are more likely to be seen as figures than concave shapes (bulging inwards).
- Recognizability: Recognizable objects or patterns are more likely to be seen as figures.
The more of these cues that act together, the more likely something is to be seen as a figure.

Definition: The vague overall impression of what's going on in a scene when it's flashed rapidly.
Even if you can't identify all objects, you might get an impression like "crowded cafe," "indoor scene," or "bridge."
Masking: A mask is used after flashing an image to stop the iconic memory and control the amount of time an image is seen.
- Iconic Memory: The retina transduces light into electrical signals and continues firing for up to 200 milliseconds after the image is removed.
- Masking scrambles these retinal signals to prevent the brain from continuing to "see" the image.
At 40 milliseconds, people start to get something that could be described as a gist.
The gist is not detailed.

A camouflaged fish is hard to see when still.
The minute the fish moves, its camouflage is broken, attention is attracted, and it's segregated from the background.

Motion can convey narratives and meaning, even without other cues like sound or color.
A video with a big triangle, little triangle, and a disc demonstrates this.
Viewers can interpret emotions such as anger and fear, and understand the narrative of an interaction between shapes solely based on their movements.

Motion allows us to determine an object's shape, especially in low visibility conditions.
This is the kinetic depth effect.
Objects are shown as silhouettes and then set in motion.
Viewers can often identify the object once it moves.

Static poses can be ambiguous, but motion clarifies actions and intentions.
Demonstrated using point light walkers.
Creation of Point Light Walker Videos:
- Infrared diodes are placed on a person's joints.
- An infrared camera captures only the movement of these diodes.
- Viewers can interpret actions from these moving dots.

Akinetopsia is a condition resulting from brain damage where a patient can no longer perceive motion.
Patients with akinetopsia have trouble with everyday tasks:
- Pouring a cup of tea
  - Because they can't estimate how quickly the liquid is rising and don't know when to stop to avoid spillage.
- Crossing the street
  - They can't perceive the motion of oncoming cars.
- Following speech
  - They have difficulty understanding how people move their mouths, which actually helps people parse words.
People with akinetopsia can see that things have moved, but cannot see them moving, making it difficult to predict where they're going to be.

As eyes move around, the snakes seem to rotate.
Explanation: Complex and not completely understood, related to how different parts of the image stimulate motion detectors in the brain.

Two stationary discs in alternation can create the perception of one disc moving back and forth.
Apparent motion only works when the discs are sufficiently close together. The brain assumes that objects won't move above a particular speed, and if the object would need to exceed this speed, it will not appear to be a single disc.
Cortez's Third Law of Apparent Motion:
- If discs are far apart and flickering rapidly, you see two stationary discs rather than one moving back and forth, unless alternation is slowed.
Apparent motion is mostly insensitive to color changes.
Color can be used to disambiguate ambiguous apparent motion.

Staring at motion in one direction and then looking at a stationary object causes the stationary object to appear to move in the opposite direction.
Example: Waterfall illusion (Falls of Foyers, Scotland).
Mechanism: You have opponent motion cells in the brain. If I show you no net motion, the two cells each fire a little bit, giving no net signal. If you tire the one cell type, then the other cells dominate giving you motion. This will be discussed more when doing color after effects.

The perceived motion of an object is affected by the motion of a nearby object or background.
Demo: Flies moving opposite directions, with a background moving with one fly to make this fly appear to have smaller circle than the other.
A nearby object (usually a large one) either affects the perceived motion of a second object (usually a small one) or causes the second object to appear to move.

Motion can make it harder to notice changes.
Change blindness doesn't typically happen because whenever a change occurs, motion transits draw your attention to that change.
When things are already moving, they are not drawing your attention preferentially to the change, so you're not going to notice the change.

Striped background with two rectangles (yellow and blue).
The rectangles are moving in unison at a constant rate, but they appear to speed up and slow down asynchronously.
Fixating on one of the fixation crosses instead of one of the rectangles helps create the illusion.
The yellow rectangle is astride the black stripe, the blue rectangle is astride thee black stripe. This means the leading and trailing edge of the yellow rectangle is above white stripes and this hard to see, and vice versa for the blue rectangle.
Contrast affects apparent speed. High contrast objects appear to move faster than low contrast objects.
In fog, everything becomes low contrast, and so you might perceive things as moving more slowly than they really are.

If you can't see the ends of a line, its motion can be ambiguous.
With a long identical line, any motion which is parallel to itself is hard to see.
To see how a line is really moving, you need to see the ends.
If you occlude these ends, you assume your brain assumes that the bits of the line you can see is moving in that direction.
Barber Pole Illusion
- Lines on a rotating cylinder appear to move upwards.
- You don't see the actual ends of this line, you see the ends of the visible lines, and these ends move vertically.
- The motion is captured by the visible ends.

Review of topics covered: function of motion perception, life without it, when we do and don't perceive motion, motion-induced blindness and change blindness, and motion illusions.
Preview of next lecture: color perception.