A visual illusion where perception alternates between two possibilities. A pair of shapes, either of which taken alone would be seen as an object of some kind, share a common border-line. What happens is that, when joined, each shape competes with the other. One is at first relegated to mere background and is hardly seen, while the other dominates as an object. Then the reverse happens: the object fades perceptually away to become for a time mere background in its turn.

from: R. Gregory, Intelligent Eye, p15-16.
