Changing expectations about speed alters perceived motion direction

Our perceptions are fundamentally altered by our knowledge of the world. When cloud-gazing, for example, we spontaneously tend 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 (typically .5 deg/s, equivalent to ~1km/h viewed at 30m [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 (e.g. 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. Here, we show that systematic exposure to high speed stimuli can lead to a reversal of this 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.