Perceptual learning without perception

The brain is able to adapt rapidly and continually to the surrounding envir onment, becoming increasingly sensitive to important and frequently encount ered stimuli(1-4). It is often claimed that this adaptive learning is highl y task-specific, that is, we become more sensitive to the critical signals in the tasks we attend to(5-15). Here, we show a new type of perceptual lea rning, which occurs without attention, without awareness and without any ta sk relevance. Subjects were repeatedly presented with a background motion s ignal so weak that its direction was not visible; the invisible motion was an irrelevant background to the central task that engaged the subject's att ention. Despite being below the threshold of visibility and being irrelevan t to the central task, the repetitive exposure improved performance specifi cally for the direction of the exposed motion when tested in a subsequent s uprathreshold test. These results suggest that a frequently presented featu re sensitizes the visual system merely owing to its frequency, not its rele vance or salience.