Two stationary lights flashed in rapid succession produce an impression of motion between them.
Wertheimer's 1912 paper on the seeing of motion reported that at certain intervals observers saw pure movement between two flashing lines, without seeing an object travel. The finding is usually taken as the starting point of Gestalt psychology, because the motion is in the experience of the whole and in neither stimulus. The related beta movement, in which an object seems to travel from one position to the next, is what makes film and animation appear to move.
In practice
- A chain of bulbs on a cinema marquee lit in sequence looks like light running round the sign.
- A loading icon of eight dots appears to spin although no dot moves.
- Possible explanations
- The explanation favoured today is that the visual system infers the most likely event from sparse samples, and successive flashes are most plausibly one thing moving. Wertheimer took it as evidence that perception organises wholes rather than adding up parts.
- In its favour
- Apparent motion is among the most robust findings in perception, and it underlies every moving-image medium from flip books to video.
- Against it
- Historians such as Robert Steinman and colleagues argue that Wertheimer's pure phi is a different and harder-to-reproduce effect from beta movement, and that the two have been conflated in textbooks, which complicates the claim that phi explains cinema.
- Standing
- The perceptual basis that the Persistence of Vision story was later invoked to explain. Compare the Law of Common Fate (Gestalt).
- Further reading
- Wikipedia
- Subject
- Psychology & Behavior
- Essay
- A century and a half of measuring the mind