The brain demonstrates an amazing ability to become increasingly sensitive to important stimuli. It is often claimed that we become more sensitive only to the critical signals in the tasks we attend to. However, our recent series of experiments have shown that perceptual learning occurs with little attention. First, mere exposure to sub-threshold and task-irrelevant motion coherence signals led to enhancement in sensitivity to the motion direction. This finding indicates that attention is not necessary for perceptual learning (Watanabe, Nanez & Sasaki, 2001). Second, exposure to two types of taskñirrelevant motion that are processed at different levels of >visual processing improved sensitivity only at the lowerñlevel. These results suggest that task-irrelevant perceptual learning occurs at a very low-level (Watanabe et al, 2002). Third, we addressed the question as to whether such task-irrelevant learning occurs purely passively (caused by stimulus-exposure). During exposure, we presented four different directions of motion an equal number of times, but the direction of interest (DOI) was paired with the task targets. If learning is purely passive, thresholds should improve equally for all the presented directions. Surprisingly, the threshold improved only for the DOI. These results show that learning of a task-irrelevant and sub-threshold feature is not purely passive, but it occurs only when the feature is correlated with a task target (Seitz & Watanabe,2003). Based on these findings, we propose a model in which diffuse reinforcement learning signals perform an important role, complementary to focused attention in perceptual learning.

Watanabe, T., Sasaki, Y. & Nanez, J. (2001). Perceptual learning without perception, Nature,413, 844-848.
Watanabe, T. et al (2002). Greater plasticity in lowerñlevel than higherñlevel visual processing revealed by passive perceptual learning, Nature Neuroscience, 5, 1002-1009.
Seitz, A R. & Watanabe, T. (2003). Is subliminal learning really passive? Nature. 2003, 422, 36-37.