Persistent fear responses in rhesus monkeys to the optical stimulus of "looming".

The approach of an object corresponds with a spatiotemporal optical stimulus consisting of a symmetrical expansion of a closed contour in the field of view. The visual equivalent of impending collision was isolated and compared with its sequential inversion. Infant and adult rhesus monkeys manifested persistent avoidance responses to "looming" but not to the inverse. This visual stimulus alone is a strong exciter of avoidance, and the response appears early in life.