Motion in the world is created by the continual change in position of an object relative to some frame of reference. That is, we watch the cat run across the kitchen tiles. We watch the leaves of the trees bend back and forth in the wind. We watch the ducks swim across the pond. This is real motion. However, human beings also perceive a number of forms of illusory motion, that is, situations in which we perceive motion when none actually occurs. One form of this illusory motion is apparent motion. Apparent motion is the appearance of real motion from a sequence of still images. Apparent motion occurs whenever stimuli separated by time and location are actually perceived as a single stimulus moving from one location to another. Apparent motion is the basis of our sense of motion in watching videography and animation.
There are different types of apparent motion. Beta motion is also called optimal motion and is indistighishable from real motion. Phi motion, or 'objectless motion' is where you gain a sense of motion but you know the underlying elements do not move, like a marquee on a theater. The major difference in the types of motion is how long there is between the two images, called the interstimulus interval (ISI). Each type of motion is illustrated below. Notice how in the beta motion the dot appears to move continuously, while in the phi motion, there is the perception of motion, but the dot appears to blink out, thus the objectless of objectless motion.
|Beta Motion||Phi Motion|
In this activity, you can manipulate variables to see how it impacts apparent motion and to try to create both beta and phi motions.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On the Illustration tab, you can start, stop and alter the motion of squares to experience optic flow.
Below is a list of the ways that you can alter the illustration. The settings include the following:
Move: Start and stop the motion of the wheel. If the wheel is set to flicker it
will only flicker when moving
Dot Duration: change how long the dots are visible (msec).
ISI (or blank interval): change the duration of the blank interval between the dots (msec).
Dot Positon: move the dots closer or farther apart.
Show Barrier: add or remove a barrier between the two dots.
Contrast: change the contrast between the dots and background.
Pressing this button restores the settings to their default values.