When do we know which way is up? The time course of orientation perception

There is evidence that global visual context affects orientation perception in later stages of processing than local context. We measured the relative time courses of two orientation illusions: The Tilt Illusion (TI) involves dense gratings in close proximity; the Rod and Frame Illusion (RFI) involves a solitary bar surrounded by a distant frame. We also varied whether the context was flashed briefly (Experiment 1) or remained visible (Experiment 2). Results showed that the TI (but not the RFI) occurs when the context is briefly flashed in advance of the test, that both illusions are strongest when the context and inducer appear simultaneously, and that the RFI frame must be visible for at least 800 ms to induce an illusion with asynchronous displays. Experiment 3 confirmed these patterns held for measures of illusion magnitude and discriminability. Results are consistent with an earlier effect of local spatial context and a later effect of global spatial context on orientation perception.