How repeated training or practice leads to long-term performance improvements is one of the fundamental questions in skill acquisition. Although long-term benefits of perceptual learning have been observed in a wide range of perceptual tasks, studies also documented some very interesting short-term phenomena during the learning process. However, most existing studies have focused on the average performance at the session-by-session time scale that typically involves hundreds of trials and may have missed some important short-term processes that only manifest at a finer time scale.