If two stimuli are briefly flashed in rapid succession, observers will sometimes report seeing motion between the two stimuli. Reports of this type of apparent motion were noted near the beginning of the 20th century by Gestalt psychologists (most notably Westheimer). They noticed, among other things, that the timing of the flashes was important in determining whether or not motion was seen. If the time between the offset of the first stimulus and the onset of the second stimulus (called the interstimulus interval or ISI) was very short, observers simply see two dots presented nearly simultaneously. If the ISI is very long observers see one dot flash one and off and then the other flash on and off. For intermediate ISIs the first dot seems to turn off, move through the space between the dots and appear at the position of the second dot.

Korte (1915) noted that the ISI thresholds for seeing motion versus being too short varied with the spatial separation of the stimuli. The farther apart the stimuli were, the longer the ISI needed to be for a good motion percept. This finding, and others, were formulated in a set of "Korte's laws" that have been used to describe aspects of apparent motion.

The relationship between spatial separation and ISI threshold is consistent with a variety of theories that hypothesize the visual system builds a motion percept. For larger separation, the stimulus must "move" a farther distance, which presumably requires a greater length of time.

Apparent motion is, by the way, the basis of movement in all television, movies, and computer animation. The screen actually shows a rapid succession of still images, the perceived motion is entirely "apparent" and illusory.

This lab allows you see apparent motion in one of its simplest forms and to demonstrate Korte's law for yourself.


To run the experiment press the button below. A window will fill the entire screen. Press the space bar to start a trial.

After pressing the space bar, the screen go dark and a small fixation square will appear in the middle of the screen. Fixate this square. It will disappear after one and a half seconds. Half a second letter a dot will appear on the left, disappear, and then a dot will appear on the right and disappear. These stimuli will then cycle back and forth repeatedly. Each part of the cycle (left stimulus, blank, right stimulus, blank) is presented for 150 milliseconds.

Your task is to vary the ISI until the motion percept is as strong as you can make it. You can increase the ISI by 20 milliseconds by pressing the "i" key. You can decrease the ISI by 20 milliseconds by pressing the "k" key. The ISI can never become negative.

After you are satisfied that the motion percept is as strong as you can make it, press the space bar to start the next trial. The spacing of the dots will vary from trial to trial. There are 6 spacings and 5 replications for each spacing for a total of 30 trials.

At the end of the experiment a graph will appear that plots the average Best ISI (in milliseconds) as a function of the center-to-center separation (in millimeters) of the dots. You should find that the Best ISI increases with dot separation.

This page constructed and maintained by A. Stevenson, last revised October 24, 1998
©1998 Purdue University