Apparent Motion

The perception of movement is a complex process in which your brain detects a pattern of change in the input, and interprets this as movement. For example, when you watch a movie, you are actually watching a series of still images, shown in rapid succession. Your brain, however, fills in the missing information, allowing you to see fluid movement.

In a movie, an object appears here in one frame and there in the next. If the spacing and timing are right, the brain interprets this as movement from here to there. This same process of interpretation can be seen in simpler situations. For example, if two lights flash on and off alternately — and if the timing of the flashes is right — then your brain will fill in the missing information so that a single light appears to move across your field of vision. This experience is referred to as "apparent movement." In a sense, this is an illusion of motion, but the illusion is quite compelling: if the timing is properly adjusted, observers cannot tell the difference between apparent movement and a presentation in which a single light really is moving back and forth.

In most apparent movement displays, one light is on, then both lights are off for a moment, then the second light comes on, then both are off again, and then the cycle repeats. The time period between the two lights — the time in which both lights are off — is known as the interstimulus interval (ISI). This crucial variable determines how the display is perceived: if the ISI is too long or too short, you will see no movement at all.

Depending on the ISI, two lights flashing on and off can lead to several different perceptions. If the ISI is very short, the lights appear to flicker, and no motion is perceived. If the ISI is very long, then the lights appear to be stationary and blinking alternately (as indeed they are).

The Experiment

You can use the buttons on this screen to adjust the ISI and to adjust the distance between the two lights. Set the lights relatively close together. At what ISI do you get a compelling illusion of movement? Now, without changing the ISI, move the lights further apart. What does this do you the apparent movement?

Try other combinations of distance and ISI to see if you can determine the relationship between these two factors.


Were you able to perceive the movement illusion?

For your information, here are the ISI durations that normally produce the perception of illusory movement.

< 30 milliseconds


Strobing, no movement

30 - 60 milliseconds


Partial movement

60 - 200 milliseconds


Illusory movement

> 200 milliseconds


Successive flashing, no movement

Note that these numbers represent AVERAGE times as reported by many individuals; your experience may not correspond exactly with these numbers.

What is the effect of increasing the distance between the two lights? For most people, any increase in the distance between the lights demands a corresponding increase in the ISI. In essence, some mechanism within the visual system seems to register the fact that the lights are "traveling" a greater distance, and so need more time to make this journey. Therefore, the ISI that produces apparent movement at a 150 pixel distance is generally half that needed for apparent movement at a 300 pixel distance.

Your Results

If your instructor has asked you to submit your responses by e-mail, refer to your observations in the experiment above and complete the form below. Press "Send" to send your estimates of the "optimal" ISI for apparent movement at these two distances.

150 pixels:






300 pixels:







Student's Name


Student's Email


Professor's Email