Ponzo's illusion, illustration. The vertical line to the right of the image appears to be smaller than the one to the left. In fact they are the same length, but the brain perceives them differently due to the converging lines. This illusion exploits the brain's understanding of perspective: imagine the converging lines are a representation of a wall going into the distance. The vertical line to the left then appears to span a greater width of what the brain interprets as parallel lines. Hence it concludes it is larger.