Our perceptions are fundamentally altered by our knowledge of the world. When cloud-gazing, for example, we tend spontaneously to recognize known objects in the random configurations of evaporated moisture. How our brains acquire such knowledge and how it impacts our perceptions is a matter of heated discussion. A topic of recent debate has concerned the hypothesis that our visual system ?assumes? that objects are static or move slowly [1] rather than more quickly [1?3]. This hypothesis, or ?prior on slow speeds?, was postulated because it could elegantly explain a number of perceptual biases observed in situations of uncertainty [2]. Interestingly, those biases affect not only the perception of speed, but also the direction of motion. For example, the direction of a line whose endpoints are hidden (as in the ?aperture problem?) or poorly visible (for example, at low contrast or for short presentations) is more often perceived as being perpendicular to the line than it really is ? an illusion consistent with expecting that the line moves more slowly than it really does. How this ?prior on slow speeds? is shaped by experience and whether it remains malleable in adults is unclear. Here, we show that systematic exposure to high-speed stimuli can lead to a reversal of this direction illusion. This suggests that the shaping of the brain's prior expectations of even the most basic properties of the environment is a continuous process.
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