The Tiger Moving Through the Tall Grass: Spontaneous Perception and Tracking of Change-Defined Objects from Second-Order Features

The most important objects - from predators to projectiles - are often those that are moving. Accordingly, visual systems are specialized for object tracking, and many models of such processing involve continually re-identifying surface features across space and time (as you might follow a tiger by keeping track of its orange stripes). Here, in contrast, we show across four experiments - and in phenomenologically compelling demonstrations - that people also spontaneously perceive and track change-defined objects, with no enduring surface properties from moment to moment. Observers viewed regular grids filled with hundreds of small elements (e.g. randomly oriented crosses). Change-defined objects were implemented by having an element change (e.g., from one random orientation to another random orientation), with these changes propagating through space and time. Observers still detected the 'second-order' motion in such displays - despite the lack of persisting features, and without being able to identify the object in any static frame. And beyond motion detection, observers also spontaneously perceived and tracked persisting objects - and were even able to perform multiple-object tracking. We also generalized this phenomenon in several ways, showing that it occurs with other types of changes (e.g., to brightness or shape), and even when different types of changes are haphazardly interleaved (e.g. with a random orientation change to one element followed by a random brightness change to its neighbor, etc.). In essence, these stimuli show how observers can track a tiger that is always fully hidden while moving through tall grass, by tracking variable local changes to the grass itself.