Local and Global Motion

Background:

The perception of motion involves both local and global motion cues. Local motion involves motion over a small part of the retina while global motion involves motion over a much larger part of the retina. As demonstrated in the aperature problem, local motion can be ambiguous -- movement over a small part of the retina can be perceived in ways that are very different from the global motion.

Local motion can affect the way global motion is perceived especially when the motion occurs in the periphery of the retina. In such cases, the visual system seems to integrate the two sets of motion cues, local and global, and the global motion perception is the sum of local and global motion, even if that is not veridical. That integration of local and global cues is demontrated in the following activity.

The Activity:

Look directly at the moving ball. Notice the path of its motion. When looking directly at the moving ball, most people veridically perceive it as moving straight down. Now look look directly at the stationary blue ball, but notice the path of the moving ball. Does the moving ball appear to move straight down? If not, in which direction does it appear to move -- left or right? In which direction is the pattern on the moving ball moving -- left or right?

What should happen to the perceived motion of the moving ball if you change the direction of motion of the pattern on the moving ball from right to left to left to right? After you make your prediction, try it (you can either type the letter L (left to right) or R (right to left) or click on the appropriate radio button). Again, notice the perceive direction of the moving ball when you look directly at it and when you are looking directly at the stationary blue ball.

What should happen to the perceived motion of the moving ball if you slow down the speed of motion of the pattern on the moving ball? What should happen to the perceived motion of the moving ball if you speed up the speed of motion of the pattern on the moving ball? After you make your prediction, try it (you can type the letter S (slow), M (medium) or F (fast) or click on the appropriate radio button). Again, notice the perceive direction of the moving ball when you look directly at it and when you are looking directly at the stationary blue ball.

What should happen to the perceived motion of the moving ball if you move it closer or farther from the stationary blue ball? After you make your prediction, try it (you can type the character < to move the ball to the left or > to move the ball to the right or slide the range control). Again, notice the perceive direction of the moving ball when you look directly at it and when you are looking directly at the stationary blue ball.

Ball texture animation properties (keyboard shortcuts in bold):

Slow Med Fast
Left to Right Right to Left
X axis position of ball: <>