Two phases of offline learning in contour integration.

We demonstrate daytime and overnight offline modulations of perceptual learning in a visual integration task. We employed a contour integration task, which requires longer range spatial integration than the more commonly used texture discrimination task, yet, still addresses the earliest cortical processing levels. In order to dissociate the effect of daytime and overnight offline modulations on perceptual learning, we introduced a 12-h shift between the practice times of two groups of subjects. Throughout the five practice sessions, the 12-h shift resulted in stepwise modulation of a typical learning curve, with a phase shift between the two groups. Between sessions (offline) improvement during the day was relatively small and only occurred in the first few sessions, while it was always significant after a night of sleep. Our results extend the body of evidence on the potential role of sleep in perceptual learning and generalize it to integrative visual processes. We have clearly distinguished two phases of learning: both daytime and overnight improvements in the initial phase, and only overnight improvements in the later phase.