Psychologists Paul Kolers and Michael von Grunau investigated the phi phenomenon in response to a question from the philosopher Nelson Goodman. Goodman wondered what would happen if the circles changed color, going from red to green and vice versa. Would the sense of motion persist, or would the phenomenon disappear? What would happen to the color of the circle in between?
Subjects often report being able to see the circle change color in midstream. If so, how can this occur if the second circle is yet to be perceived? This question can be raised for any version of the phenomenon. How can the brain "fill in" the gap between the first circle and the second before consciousness of the second circle has occurred? This question is addressed in several accounts of the phi phenomenon.