In perceptual learning, performance improves when stimuli are presented over and over again. In a bisection task, for example, two vertical outer lines delineate an interval which is bisected by a centre line. Observers indicate whether this centre line is closer to the left or right outer line. Perceptual learning in this task is usually assumed to be mediated by smooth synaptic changes driven by the stimuli. Here, we show that perceptual learning can also occur via mental imagery, ie in the absence of physical stimulation. We presented only the two outer lines of the bisection stimulus and asked observers to imagine the centre line to be closer to the left or right outer line. Surprisingly, performance improved also in this task. Control experiments showed that this improvement of performance cannot be explained by unspecific aspects, such as adaptation to the experimental conditions, etc Hence, perceptual learning can occur without proper physical stimulation, possibly driven by top ^ down processing only.