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Abigail Lee, Justin Ales, Julie Harris; Speed discrimination for real-world motion in depth. Journal of Vision 2017;17(10):416. doi: https://doi.org/10.1167/17.10.416.
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© ARVO (1962-2015); The Authors (2016-present)
Several studies have shown that precise speed discrimination is possible for both lateral motion and motion in depth. Typically, studies ask observers to discriminate between different constant retinal speeds. However, in everyday life, when an object approaches us in depth at a constant speed in the world, the image of that object on the retina accelerates as the object grows closer. Here, we compare naïve observers' ability to discriminate a change in speed using lateral motion or motion in depth, constant retinal speed or constant world speed, and 'slow' (4.5 - 48 arcmin/second) or 'fast' (7.5 - 121 arcmin/second) speed conditions. Importantly, measuring speed discrimination can be confounded by distance and duration cues in the stimulus. For example, if the duration of the stimulus is fixed, faster objects within the stimulus will travel further. We therefore developed a novel experimental design allowing us to control against the use of distance and duration cues. We found no significant difference between thresholds for constant real-world and constant retinal speed conditions, with average speed change thresholds of around 32% for both. However, we found that there was a significant difference in thresholds between the lateral motion and motion in depth conditions. Thresholds for motion in depth were worse than those for lateral motion (around 37% and 28% respectively). We also found that the average threshold for fast speed conditions was significantly better than for the slow conditions (around 30% and 35% respectively). These results show that naïve observers are as precise in judging the speeds of stimuli with constant retinal and constant world speed. To our knowledge, this is the first study to show for 3D motion that observers can be as sensitive to changes in real-world speed (that result in accelerating retinal motion) as changes in the speed of constant retinal motion.
Meeting abstract presented at VSS 2017
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