In a bisection discrimination task, 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. Performance in this task improves strongly with training. This perceptual learning is usually assumed to be caused by synaptic changes which are mainly driven by the presentation of the stimuli. Here, we show that perceptual learning can also occur in the absence of physical stimulation via mental imagery. 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. Control experiments show that this improvement cannot be explained by unspecific aspects, such as adaptation to the experimental conditions. Hence, perceptual learning can occur without proper physical stimulation, driven by mental imagery