In the first block of trials, two dots appeared at the same points on the screen on every trial for the same length of time, but the inter-stimulus interval (ISI), or the time from the offset of the dot on the left to the onset of the dot on the right, was varied from trial to trial. Sometimes the ISI was very short, other times it was very long. The following is the data from the first portion of the experiment. Typically when ISIs are very short or very long, two dots are perceived. When the ISI is moderate, apparent motion is perceived.