When observers move through an environment, they are immersed in a sea of motions that guide their further movements. The horizontal relative motions of all possible pairs of stationary objects fall into three classes: They converge, diverge and slow down, or diverge with increasing velocity. Conjoined with ordinal depth information, the first two motions reveal nominal invariants, constraining heading to one side of the visual field. When two object pairs yield invariants on opposing sides of the heading, they can constrain judgments to a narrow region. Distributional analyses of responses in an experiment involving simulated observer movement suggest that observers follow these constraints.
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