Abstract
In our previous studies (Watanabe et al., 2001, Nature; 2002, Nat Neurosci), subjects performed an attentionally demanding letter identification task in the fovea while sub-threshold coherent motion was presented in the periphery. Repetitive exposure improved performance specifically for the exposed sub-threshold motion direction. This showed that perceptual learning occurs as a result of exposure to a subliminal feature, and therefore, without attention directed to the feature.
This raises an important question: Does such subliminal learning occur only as a result of purely passive processing of a stimulus? To address this question, 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. On the other hand, if learning is formed only on a feature to which attention is directed, no improvement should be found for any presented direction. Contrary to these hypotheses, the threshold improved for the DOI, which was paired with the task targets, but not for the other directions, which were paired with distractors.
While the results are at odds with the passive learning hypothesis as well as the focused attention hypothesis, they are consistent with classic conditioning data, in which arbitrary features are learned when paired with rewarding or noxious stimuli. We propose a model in which diffuse reinforcement learning signals perform an important role, complementary to focused attention, in subliminal perceptual learning. In reinforcement learning a reward signal allows the correlation of the neural response with task performance to be learned. In contrast, focussed attention allows knowledge to bias the neural response and involves cortical processing.